GNU Compiler Collection (GCC) Internals: Trampolines

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### 18.11 Support for Nested Functions

Taking the address of a nested function requires special compiler handling to ensure that the static chain register is loaded when the function is invoked via an indirect call.

GCC has traditionally supported nested functions by creating an executable trampoline at run time when the address of a nested function is taken. This is a small piece of code which normally resides on the stack, in the stack frame of the containing function. The trampoline loads the static chain register and then jumps to the real address of the nested function.

The use of trampolines requires an executable stack, which is a security risk. To avoid this problem, GCC also supports another strategy: using descriptors for nested functions. Under this model, taking the address of a nested function results in a pointer to a non-executable function descriptor object. Initializing the static chain from the descriptor is handled at indirect call sites.

On some targets, including HPPA and IA-64, function descriptors may be mandated by the ABI or be otherwise handled in a target-specific way by the back end in its code generation strategy for indirect calls. GCC also provides its own generic descriptor implementation to support the `-fno-trampolines` option. In this case runtime detection of function descriptors at indirect call sites relies on descriptor pointers being tagged with a bit that is never set in bare function addresses. Since GCC’s generic function descriptors are not ABI-compliant, this option is typically used only on a per-language basis (notably by Ada) or when it can otherwise be applied to the whole program.

For languages other than Ada, the `-ftrampolines` and `-fno-trampolines` options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.

Define the following hook if your backend either implements ABI-specified descriptor support, or can use GCC’s generic descriptor implementation for nested functions.

Target Hook: int TARGET_CUSTOM_FUNCTION_DESCRIPTORS

  If the target can use GCC’s generic descriptor mechanism for nested functions, define this hook to a power of 2 representing an unused bit in function pointers which can be used to differentiate descriptors at run time. This value gives the number of bytes by which descriptor pointers are misaligned compared to function pointers. For example, on targets that require functions to be aligned to a 4-byte boundary, a value of either 1 or 2 is appropriate unless the architecture already reserves the bit for another purpose, such as on ARM.

  Define this hook to 0 if the target implements ABI support for function descriptors in its standard calling sequence, like for example HPPA or IA-64.

  Using descriptors for nested functions eliminates the need for trampolines that reside on the stack and require it to be made executable.

The following macros tell GCC how to generate code to allocate and initialize an executable trampoline. You can also use this interface if your back end needs to create ABI-specified non-executable descriptors; in this case the "trampoline" created is the descriptor containing data only.

The instructions in an executable trampoline must do two things: load a constant address into the static chain register, and jump to the real address of the nested function. On CISC machines such as the m68k, this requires two instructions, a move immediate and a jump. Then the two addresses exist in the trampoline as word-long immediate operands. On RISC machines, it is often necessary to load each address into a register in two parts. Then pieces of each address form separate immediate operands.

The code generated to initialize the trampoline must store the variable parts—the static chain value and the function address—into the immediate operands of the instructions. On a CISC machine, this is simply a matter of copying each address to a memory reference at the proper offset from the start of the trampoline. On a RISC machine, it may be necessary to take out pieces of the address and store them separately.

Target Hook: void TARGET_ASM_TRAMPOLINE_TEMPLATE (FILE *f)

  This hook is called by `assemble_trampoline_template` to output, on the stream f, assembler code for a block of data that contains the constant parts of a trampoline. This code should not include a label—the label is taken care of automatically.

  If you do not define this hook, it means no template is needed for the target. Do not define this hook on systems where the block move code to copy the trampoline into place would be larger than the code to generate it on the spot.

Macro: TRAMPOLINE_SECTION

  Return the section into which the trampoline template is to be placed (see Sections). The default value is `readonly_data_section`.

Macro: TRAMPOLINE_SIZE

  A C expression for the size in bytes of the trampoline, as an integer.

Macro: TRAMPOLINE_ALIGNMENT

  Alignment required for trampolines, in bits.

  If you don’t define this macro, the value of `FUNCTION_ALIGNMENT` is used for aligning trampolines.

Target Hook: void TARGET_TRAMPOLINE_INIT (rtx m_tramp, tree fndecl, rtx static_chain)

  This hook is called to initialize a trampoline. m_tramp is an RTX for the memory block for the trampoline; fndecl is the `FUNCTION_DECL` for the nested function; static_chain is an RTX for the static chain value that should be passed to the function when it is called.

  If the target defines `TARGET_ASM_TRAMPOLINE_TEMPLATE`, then the first thing this hook should do is emit a block move into m_tramp from the memory block returned by `assemble_trampoline_template`. Note that the block move need only cover the constant parts of the trampoline. If the target isolates the variable parts of the trampoline to the end, not all `TRAMPOLINE_SIZE` bytes need be copied.

  If the target requires any other actions, such as flushing caches (possibly calling function maybe_emit_call_builtin___clear_cache) or enabling stack execution, these actions should be performed after initializing the trampoline proper.

Target Hook: void TARGET_EMIT_CALL_BUILTIN___CLEAR_CACHE (rtx begin, rtx end)

  On targets that do not define a `clear_cache` insn expander, but that define the `CLEAR_CACHE_INSN` macro, maybe_emit_call_builtin___clear_cache relies on this target hook to clear an address range in the instruction cache.

  The default implementation calls the `__clear_cache` builtin, taking the assembler name from the builtin declaration. Overriding definitions may call alternate functions, with alternate calling conventions, or emit alternate RTX to perform the job.

Target Hook: rtx TARGET_TRAMPOLINE_ADJUST_ADDRESS (rtx addr)

  This hook should perform any machine-specific adjustment in the address of the trampoline. Its argument contains the address of the memory block that was passed to `TARGET_TRAMPOLINE_INIT`. In case the address to be used for a function call should be different from the address at which the template was stored, the different address should be returned; otherwise addr should be returned unchanged. If this hook is not defined, addr will be used for function calls.

Implementing trampolines is difficult on many machines because they have separate instruction and data caches. Writing into a stack location fails to clear the memory in the instruction cache, so when the program jumps to that location, it executes the old contents.

Here are two possible solutions. One is to clear the relevant parts of the instruction cache whenever a trampoline is set up. The other is to make all trampolines identical, by having them jump to a standard subroutine. The former technique makes trampoline execution faster; the latter makes initialization faster.

To clear the instruction cache when a trampoline is initialized, define the following macro.

Macro: CLEAR_INSN_CACHE (beg, end)

  If defined, expands to a C expression clearing the instruction cache in the specified interval. The definition of this macro would typically be a series of `asm` statements. Both beg and end are pointer expressions.

To use a standard subroutine, define the following macro. In addition, you must make sure that the instructions in a trampoline fill an entire cache line with identical instructions, or else ensure that the beginning of the trampoline code is always aligned at the same point in its cache line. Look in `m68k.h` as a guide.

Macro: TRANSFER_FROM_TRAMPOLINE

  Define this macro if trampolines need a special subroutine to do their work. The macro should expand to a series of `asm` statements which will be compiled with GCC. They go in a library function named `__transfer_from_trampoline`.

  If you need to avoid executing the ordinary prologue code of a compiled C function when you jump to the subroutine, you can do so by placing a special label of your own in the assembler code. Use one `asm` statement to generate an assembler label, and another to make the label global. Then trampolines can use that label to jump directly to your special assembler code.

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