Using the GNU Compiler Collection (GCC): Instrumentation Options

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### 3.12 Program Instrumentation Options

GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes.

`-p`
`-pg`

  Generate extra code to write profile information suitable for the analysis program `prof` (for `-p`) or `gprof` (for `-pg`). You must use this option when compiling the source files you want data about, and you must also use it when linking.

  You can use the function attribute `no_instrument_function` to suppress profiling of individual functions when compiling with these options. See Common Function Attributes.

`-fprofile-arcs`

  Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable.

  When the compiled program exits it saves this data to a file called `auxname.gcda` for each source file. The data may be used for profile-directed optimizations (`-fbranch-probabilities`), or for test coverage analysis (`-ftest-coverage`). Each object file’s auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. `foo.gcda` for input file `dir/foo.c`, or `dir/foo.gcda` for output file specified as `-o dir/foo.o`).

  Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. `gcc a.c b.c -o binary` would generate `binary-a.gcda` and `binary-b.gcda` files.

  See Cross-profiling.

`--coverage`

  This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for `-fprofile-arcs` `-ftest-coverage` (when compiling) and `-lgcov` (when linking). See the documentation for those options for more details.

  - Compile the source files with `-fprofile-arcs` plus optimization and code generation options. For test coverage analysis, use the additional `-ftest-coverage` option. You do not need to profile every source file in a program.
  - Compile the source files additionally with `-fprofile-abs-path` to create absolute path names in the `.gcno` files. This allows `gcov` to find the correct sources in projects where compilations occur with different working directories.
  - Link your object files with `-lgcov` or `-fprofile-arcs` (the latter implies the former).
  - Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, `fork` calls are detected and correctly handled without double counting.

    Moreover, an object file can be recompiled multiple times and the corresponding `.gcda` file merges as long as the source file and the compiler options are unchanged.

  - For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus `-fbranch-probabilities` (see Options that Control Optimization).
  - For test coverage analysis, use `gcov` to produce human readable information from the `.gcno` and `.gcda` files. Refer to the `gcov` documentation for further information.

  With `-fprofile-arcs`, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

`-ftest-coverage`

  Produce a notes file that the `gcov` code-coverage utility (see `gcov`—a Test Coverage Program) can use to show program coverage. Each source file’s note file is called `auxname.gcno`. Refer to the `-fprofile-arcs` option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize.

`-fprofile-abs-path`

  Automatically convert relative source file names to absolute path names in the `.gcno` files. This allows `gcov` to find the correct sources in projects where compilations occur with different working directories.

`-fprofile-dir=path`

  Set the directory to search for the profile data files in to path. This option affects only the profile data generated by `-fprofile-generate`, `-ftest-coverage`, `-fprofile-arcs` and used by `-fprofile-use` and `-fbranch-probabilities` and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the `sourcename.gcda` file and use it as the file name of a `.gcda` file. See details about the file naming in `-fprofile-arcs`. See similar option `-fprofile-note`.

  When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:

  `%p`

    process ID.

  `%q{VAR}`

    value of environment variable VAR

`-fprofile-generate`
`-fprofile-generate=path`

  Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use `-fprofile-generate` both when compiling and when linking your program.

  The following options are enabled: `-fprofile-arcs`, `-fprofile-values`, `-finline-functions`, and `-fipa-bit-cp`.

  If path is specified, GCC looks at the path to find the profile feedback data files. See `-fprofile-dir`.

  To optimize the program based on the collected profile information, use `-fprofile-use`. See Optimize Options, for more information.

`-fprofile-info-section`
`-fprofile-info-section=name`

  Register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to `.gcov_info`. A pointer to the profile information generated by `-fprofile-arcs` is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets free-standing environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.

  The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:

        .gcov_info      :
        {
          PROVIDE (__gcov_info_start = .);
          KEEP (*(.gcov_info))
          PROVIDE (__gcov_info_end = .);
        }

  The program could dump the profiling information registered in this linker set for example like this:

      #include <gcov.h>
      #include <stdio.h>
      #include <stdlib.h>

      extern const struct gcov_info *__gcov_info_start[];
      extern const struct gcov_info *__gcov_info_end[];

      static void
      filename (const char *f, void *arg)
      {
        puts (f);
      }

      static void
      dump (const void *d, unsigned n, void *arg)
      {
        const unsigned char *c = d;

        for (unsigned i = 0; i < n; ++i)
          printf ("%02x", c[i]);
      }

      static void *
      allocate (unsigned length, void *arg)
      {
        return malloc (length);
      }

      static void
      dump_gcov_info (void)
      {
        const struct gcov_info **info = __gcov_info_start;
        const struct gcov_info **end = __gcov_info_end;

        /* Obfuscate variable to prevent compiler optimizations.  */
        __asm__ ("" : "+r" (info));

        while (info != end)
        {
          void *arg = NULL;
          __gcov_info_to_gcda (*info, filename, dump, allocate, arg);
          putchar ('\n');
          ++info;
        }
      }

      int
      main()
      {
        dump_gcov_info();
        return 0;
      }

`-fprofile-note=path`

  If path is specified, GCC saves `.gcno` file into path location. If you combine the option with multiple source files, the `.gcno` file will be overwritten.

`-fprofile-prefix-path=path`

  This option can be used in combination with `profile-generate=`profile_dir and `profile-use=`profile_dir to inform GCC where is the base directory of built source tree. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. This is not desirable when directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback because the profile data will not be found during the optimized build. In such setups `-fprofile-prefix-path=`path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory.

`-fprofile-prefix-map=old=new`

  When compiling files residing in directory `old`, record profiling information (with `--coverage`) describing them as if the files resided in directory `new` instead. See also `-ffile-prefix-map`.

`-fprofile-update=method`

  Alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of ‘`single`’, ‘`atomic`’ or ‘`prefer-atomic`’. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code.

  Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted.

  Using ‘`prefer-atomic`’ would be transformed either to ‘`atomic`’, when supported by a target, or to ‘`single`’ otherwise. The GCC driver automatically selects ‘`prefer-atomic`’ when `-pthread` is present in the command line.

`-fprofile-filter-files=regex`

  Instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons).

  For example, `-fprofile-filter-files=main\.c;module.*\.c` will instrument only `main.c` and all C files starting with ’module’.

`-fprofile-exclude-files=regex`

  Instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons).

  For example, `-fprofile-exclude-files=/usr/.*` will prevent instrumentation of all files that are located in the `/usr/` folder.

`-fprofile-reproducible=[multithreaded|parallel-runs|serial]`

  Control level of reproducibility of profile gathered by `-fprofile-generate`. This makes it possible to rebuild program with same outcome which is useful, for example, for distribution packages.

  With `-fprofile-reproducible=serial` the profile gathered by `-fprofile-generate` is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before `fork` function is invoked.

  Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by `no_instrument_function` function attribute. `gcov-dump` with `-l` can be used to dump gathered data and verify that they are indeed reproducible.

  With `-fprofile-reproducible=parallel-runs` collected profile stays reproducible regardless the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in parallel (such as with `make -j`). This reduces quality of gathered data, in particular of indirect call profiling.

`-fsanitize=address`

  Enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables `-fsanitize-address-use-after-scope`. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the `ASAN_OPTIONS` environment variable. When set to `help=1`, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. The option cannot be combined with `-fsanitize=thread` or `-fsanitize=hwaddress`. Note that the only target `-fsanitize=hwaddress` is currently supported on is AArch64.

`-fsanitize=kernel-address`

  Enable AddressSanitizer for Linux kernel. See https://github.com/google/kasan for more details.

`-fsanitize=hwaddress`

  Enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to allow the detection of memory errors with a low memory overhead. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables `-fsanitize-address-use-after-scope`. See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html for more details. The run-time behavior can be influenced using the `HWASAN_OPTIONS` environment variable. When set to `help=1`, the available options are shown at startup of the instrumented program. The option cannot be combined with `-fsanitize=thread` or `-fsanitize=address`, and is currently only available on AArch64.

`-fsanitize=kernel-hwaddress`

  Enable Hardware-assisted AddressSanitizer for compilation of the Linux kernel. Similar to `-fsanitize=kernel-address` but using an alternate instrumentation method, and similar to `-fsanitize=hwaddress` but with instrumentation differences necessary for compiling the Linux kernel. These differences are to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.

  Note: This option has different defaults to the `-fsanitize=hwaddress`. Instrumenting the stack and alloca calls are not on by default but are still possible by specifying the command-line options `--param hwasan-instrument-stack=1` and `--param hwasan-instrument-allocas=1` respectively. Using a random frame tag is not implemented for kernel instrumentation.

`-fsanitize=pointer-compare`

  Instrument comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either `-fsanitize=kernel-address` or `-fsanitize=address` The option cannot be combined with `-fsanitize=thread`. Note: By default the check is disabled at run time. To enable it, add `detect_invalid_pointer_pairs=2` to the environment variable `ASAN_OPTIONS`. Using `detect_invalid_pointer_pairs=1` detects invalid operation only when both pointers are non-null.

`-fsanitize=pointer-subtract`

  Instrument subtraction with pointer operands. The option must be combined with either `-fsanitize=kernel-address` or `-fsanitize=address` The option cannot be combined with `-fsanitize=thread`. Note: By default the check is disabled at run time. To enable it, add `detect_invalid_pointer_pairs=2` to the environment variable `ASAN_OPTIONS`. Using `detect_invalid_pointer_pairs=1` detects invalid operation only when both pointers are non-null.

`-fsanitize=shadow-call-stack`

  Enable ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (e.g. stack buffer overflows.) It works by saving a function’s return address to a separately allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the stack.

  Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For the user space programs, runtime support is not currently provided in libc and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime. It should be noted that this may cause the ABI rules to be broken.

  On aarch64, the instrumentation makes use of the platform register `x18`. This generally means that any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag `-ffixed-x18`, otherwise functions compiled without `-ffixed-x18` might clobber `x18` and so corrupt the shadow stack pointer.

  Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use `-fno-exceptions` to turn off exceptions.

  See https://clang.llvm.org/docs/ShadowCallStack.html for more details.

`-fsanitize=thread`

  Enable ThreadSanitizer, a fast data race detector. Memory access instructions are instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the `TSAN_OPTIONS` environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. The option cannot be combined with `-fsanitize=address`, `-fsanitize=leak`.

  Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (`-fnon-call-exceptions`).

`-fsanitize=leak`

  Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables and the executable is linked against a library that overrides `malloc` and other allocator functions. See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the `LSAN_OPTIONS` environment variable. The option cannot be combined with `-fsanitize=thread`.

`-fsanitize=undefined`

  Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime. See https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html for more details. The run-time behavior can be influenced using the `UBSAN_OPTIONS` environment variable. Current suboptions are:

  `-fsanitize=shift`

    This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, `-fsanitize=shift-base` and `-fsanitize=shift-exponent`.

  `-fsanitize=shift-exponent`

    This option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.

  `-fsanitize=shift-base`

    If the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.

  `-fsanitize=integer-divide-by-zero`

    Detect integer division by zero.

  `-fsanitize=unreachable`

    With this option, the compiler turns the `__builtin_unreachable` call into a diagnostics message call instead. When reaching the `__builtin_unreachable` call, the behavior is undefined.

  `-fsanitize=vla-bound`

    This option instructs the compiler to check that the size of a variable length array is positive.

  `-fsanitize=null`

    This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.

  `-fsanitize=return`

    This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.

  `-fsanitize=signed-integer-overflow`

    This option enables signed integer overflow checking. We check that the result of `+`, `*`, and both unary and binary `-` does not overflow in the signed arithmetics. This also detects `INT_MIN / -1` signed division. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:

        signed char a = SCHAR_MAX;
        a++;

  `-fsanitize=bounds`

    This option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented.

  `-fsanitize=bounds-strict`

    This option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are not instrumented.

  `-fsanitize=alignment`

    This option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object.

  `-fsanitize=object-size`

    This option enables instrumentation of memory references using the `__builtin_object_size` function. Various out of bounds pointer accesses are detected.

  `-fsanitize=float-divide-by-zero`

    Detect floating-point division by zero. Unlike other similar options, `-fsanitize=float-divide-by-zero` is not enabled by `-fsanitize=undefined`, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs.

  `-fsanitize=float-cast-overflow`

    This option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options, `-fsanitize=float-cast-overflow` is not enabled by `-fsanitize=undefined`. This option does not work well with `FE_INVALID` exceptions enabled.

  `-fsanitize=nonnull-attribute`

    This option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the `nonnull` function attribute.

  `-fsanitize=returns-nonnull-attribute`

    This option enables instrumentation of return statements in functions marked with `returns_nonnull` function attribute, to detect returning of null values from such functions.

  `-fsanitize=bool`

    This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued.

  `-fsanitize=enum`

    This option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued.

  `-fsanitize=vptr`

    This option enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type.

  `-fsanitize=pointer-overflow`

    This option enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued.

  `-fsanitize=builtin`

    This option enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to `__builtin_ctz` or `__builtin_clz` invokes undefined behavior and is diagnosed by this option.

  While `-ftrapv` causes traps for signed overflows to be emitted, `-fsanitize=undefined` gives a diagnostic message. This currently works only for the C family of languages.

`-fno-sanitize=all`

  This option disables all previously enabled sanitizers. `-fsanitize=all` is not allowed, as some sanitizers cannot be used together.

`-fasan-shadow-offset=number`

  This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.

`-fsanitize-sections=s1,s2,...`

  Sanitize global variables in selected user-defined sections. si may contain wildcards.

`-fsanitize-recover[=opts]`

  `-fsanitize-recover=` controls error recovery mode for sanitizers mentioned in comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The `-fno-sanitize-recover=` option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code.

  Currently this feature only works for `-fsanitize=undefined` (and its suboptions except for `-fsanitize=unreachable` and `-fsanitize=return`), `-fsanitize=float-cast-overflow`, `-fsanitize=float-divide-by-zero`, `-fsanitize=bounds-strict`, `-fsanitize=kernel-address` and `-fsanitize=address`. For these sanitizers error recovery is turned on by default, except `-fsanitize=address`, for which this feature is experimental. `-fsanitize-recover=all` and `-fno-sanitize-recover=all` is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

  Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to `halt_on_error=0` for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is `halt_on_error=1`. This can be overridden through setting the `halt_on_error` flag in the corresponding environment variable.

  Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:

      undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

`-fsanitize-address-use-after-scope`

  Enable sanitization of local variables to detect use-after-scope bugs. The option sets `-fstack-reuse` to ‘`none`’.

`-fsanitize-undefined-trap-on-error`

  The `-fsanitize-undefined-trap-on-error` option instructs the compiler to report undefined behavior using `__builtin_trap` rather than a `libubsan` library routine. The advantage of this is that the `libubsan` library is not needed and is not linked in, so this is usable even in freestanding environments.

`-fsanitize-coverage=trace-pc`

  Enable coverage-guided fuzzing code instrumentation. Inserts a call to `__sanitizer_cov_trace_pc` into every basic block.

`-fsanitize-coverage=trace-cmp`

  Enable dataflow guided fuzzing code instrumentation. Inserts a call to `__sanitizer_cov_trace_cmp1`, `__sanitizer_cov_trace_cmp2`, `__sanitizer_cov_trace_cmp4` or `__sanitizer_cov_trace_cmp8` for integral comparison with both operands variable or `__sanitizer_cov_trace_const_cmp1`, `__sanitizer_cov_trace_const_cmp2`, `__sanitizer_cov_trace_const_cmp4` or `__sanitizer_cov_trace_const_cmp8` for integral comparison with one operand constant, `__sanitizer_cov_trace_cmpf` or `__sanitizer_cov_trace_cmpd` for float or double comparisons and `__sanitizer_cov_trace_switch` for switch statements.

`-fcf-protection=[full|branch|return|none|check]`

  Enable code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP).

  The value `branch` tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. The value `return` implements checking of validity at the point of returning from a function. The value `full` is an alias for specifying both `branch` and `return`. The value `none` turns off instrumentation.

  The value `check` is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different `-fcf-protection` values. The value `check` is ignored at the compile time.

  The macro `__CET__` is defined when `-fcf-protection` is used. The first bit of `__CET__` is set to 1 for the value `branch` and the second bit of `__CET__` is set to 1 for the `return`.

  You can also use the `nocf_check` attribute to identify which functions and calls should be skipped from instrumentation (see Function Attributes).

  Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer.

`-fharden-compares`

  For every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call `__builtin_trap` if the results do not match. Use with ‘`-fharden-conditional-branches`’ to cover all conditionals.

`-fharden-conditional-branches`

  For every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call `__builtin_trap` if the result is unexpected. Use with ‘`-fharden-compares`’ to cover all conditionals.

`-fstack-protector`

  Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call `alloca`, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don’t count.

`-fstack-protector-all`

  Like `-fstack-protector` except that all functions are protected.

`-fstack-protector-strong`

  Like `-fstack-protector` but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don’t count.

`-fstack-protector-explicit`

  Like `-fstack-protector` but only protects those functions which have the `stack_protect` attribute.

`-fstack-check`

  Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

  Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.

  You can additionally specify a string parameter: ‘`no`’ means no checking, ‘`generic`’ means force the use of old-style checking, ‘`specific`’ means use the best checking method and is equivalent to bare `-fstack-check`.

  Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

  1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
  2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
  3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.

  Note that old-style stack checking is also the fallback method for ‘`specific`’ if no target support has been added in the compiler.

  ‘`-fstack-check=`’ is designed for Ada’s needs to detect infinite recursion and stack overflows. ‘`specific`’ is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want ‘`-fstack-clash-protection`’.

`-fstack-clash-protection`

  Generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system.

  Most targets do not fully support stack clash protection. However, on those targets `-fstack-clash-protection` will protect dynamic stack allocations. `-fstack-clash-protection` may also provide limited protection for static stack allocations if the target supports `-fstack-check=specific`.

`-fstack-limit-register=reg`
`-fstack-limit-symbol=sym`
`-fno-stack-limit`

  Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions.

  For instance, if the stack starts at absolute address ‘`0x80000000`’ and grows downwards, you can use the flags `-fstack-limit-symbol=__stack_limit` and `-Wl,--defsym,__stack_limit=0x7ffe0000` to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

  You can locally override stack limit checking by using the `no_stack_limit` function attribute (see Function Attributes).

`-fsplit-stack`

  Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux.

  When code compiled with `-fsplit-stack` calls code compiled without `-fsplit-stack`, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with `-fsplit-stack` is not an option, then the linker can fix up these calls so that the code compiled without `-fsplit-stack` always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.

`-fvtable-verify=[std|preinit|none]`

  This option is only available when compiling C++ code. It turns on (or off, if using `-fvtable-verify=none`) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted.

  This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options ‘`std`’ and ‘`preinit`’ control the timing of when these data structures are built. In both cases the data structures are built before execution reaches `main`. Using `-fvtable-verify=std` causes the data structures to be built after shared libraries have been loaded and initialized. `-fvtable-verify=preinit` causes them to be built before shared libraries have been loaded and initialized.

  If this option appears multiple times in the command line with different values specified, ‘`none`’ takes highest priority over both ‘`std`’ and ‘`preinit`’; ‘`preinit`’ takes priority over ‘`std`’.

`-fvtv-debug`

  When used in conjunction with `-fvtable-verify=std` or `-fvtable-verify=preinit`, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named `vtv_set_ptr_data.log` in the directory named by the environment variable `VTV_LOGS_DIR` if that is defined or the current working directory otherwise.

  Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one.

`-fvtv-counts`

  This is a debugging flag. When used in conjunction with `-fvtable-verify=std` or `-fvtable-verify=preinit`, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named `vtv_count_data.log` in the directory named by the environment variable `VTV_LOGS_DIR` if that is defined or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to `vtv_class_set_sizes.log` in the same directory.

  Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.

`-finstrument-functions`

  Generate instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, `__builtin_return_address` does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

      void __cyg_profile_func_enter (void *this_fn,
                                     void *call_site);
      void __cyg_profile_func_exit  (void *this_fn,
                                     void *call_site);

  The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.

  This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use `extern inline` in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

  A function may be given the attribute `no_instrument_function`, in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). See Common Function Attributes.

`-finstrument-functions-exclude-file-list=file,file,…`

  Set the list of functions that are excluded from instrumentation (see the description of `-finstrument-functions`). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match.

  For example:

      -finstrument-functions-exclude-file-list=/bits/stl,include/sys

  excludes any inline function defined in files whose pathnames contain `/bits/stl` or `include/sys`.

  If, for some reason, you want to include letter ‘`,`’ in one of sym, write ‘`\,`’. For example, `-finstrument-functions-exclude-file-list='\,\,tmp'` (note the single quote surrounding the option).

`-finstrument-functions-exclude-function-list=sym,sym,…`

  This is similar to `-finstrument-functions-exclude-file-list`, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as `vector<int> blah(const vector<int> &)`, not the internal mangled name (e.g., `_Z4blahRSt6vectorIiSaIiEE`). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names.

`-fpatchable-function-entry=N[,M]`

  Generate N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted, it defaults to `0` so the function entry points to the address just at the first NOP. The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface `gen_nop`. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.

  For run-time identification, the starting addresses of these areas, which correspond to their respective function entries minus M, are additionally collected in the `__patchable_function_entries` section of the resulting binary.

  Note that the value of `__attribute__ ((patchable_function_entry (N,M)))` takes precedence over command-line option `-fpatchable-function-entry=N,M`. This can be used to increase the area size or to remove it completely on a single function. If `N=0`, no pad location is recorded.

  The NOP instructions are inserted at—and maybe before, depending on M—the function entry address, even before the prologue.

  The maximum value of N and M is 65535.

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