Using and Porting GNU CC

16.9: Trampolines for Nested Functions

A trampoline is a small piece of code that is created at run time when the address of a nested function is taken. It normally resides on the stack, in the stack frame of the containing function. These macros tell GNU CC how to generate code to allocate and initialize a trampoline.

The instructions in the 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.

TRAMPOLINE_TEMPLATE (file)

A C statement to output, on the stream file, 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.

TRAMPOLINE_SECTION

The name of a subroutine to switch to the section in which the trampoline template is to be placed (see Sections). The default is a value of `readonly_data_section', which places the trampoline in the section containing read-only data.

TRAMPOLINE_SIZE

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

TRAMPOLINE_ALIGNMENT

Alignment required for trampolines, in bits.

If you don't define this macro, the value of BIGGEST_ALIGNMENT is used for aligning trampolines.

INITIALIZE_TRAMPOLINE (addr, fnaddr, static_chain)

A C statement to initialize the variable parts of a trampoline. addr is an RTX for the address of the trampoline; fnaddr is an RTX for the address of the nested function; static_chain is an RTX for the static chain value that should be passed to the function when it is called.

ALLOCATE_TRAMPOLINE (fp)

A C expression to allocate run-time space for a trampoline. The expression value should be an RTX representing a memory reference to the space for the trampoline.

If this macro is not defined, by default the trampoline is allocated as a stack slot. This default is right for most machines. The exceptions are machines where it is impossible to execute instructions in the stack area. On such machines, you may have to implement a separate stack, using this macro in conjunction with FUNCTION_PROLOGUE and FUNCTION_EPILOGUE.

fp points to a data structure, a struct function, which describes the compilation status of the immediate containing function of the function which the trampoline is for. Normally (when ALLOCATE_TRAMPOLINE is not defined), the stack slot for the trampoline is in the stack frame of this containing function. Other allocation strategies probably must do something analogous with this information.

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 macros which describe the shape of the cache.

INSN_CACHE_SIZE

The total size in bytes of the cache.

INSN_CACHE_LINE_WIDTH

The length in bytes of each cache line. The cache is divided into cache lines which are disjoint slots, each holding a contiguous chunk of data fetched from memory. Each time data is brought into the cache, an entire line is read at once. The data loaded into a cache line is always aligned on a boundary equal to the line size.

INSN_CACHE_DEPTH

The number of alternative cache lines that can hold any particular memory location.

Alternatively, if the machine has system calls or instructions to clear the instruction cache directly, you can define the following macro.

CLEAR_INSN_CACHE (BEG, END)

If defined, expands to a C expression clearing the instruction cache in the specified interval. If it is not defined, and the macro INSN_CACHE_SIZE is defined, some generic code is generated to clear the cache. The definition of this macro would typically be a series of asm statements. Both BEG and END are both 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.

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 GNU CC. 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.