Using and Porting GNU CC
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.
readonly_data_section', which places the trampoline in the section containing read-only data.
If you don't define this macro, the value of
is used for aligning trampolines.
INITIALIZE_TRAMPOLINE (addr, fnaddr, static_chain)
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
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
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.
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)
asmstatements. 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.
asmstatements which will be compiled with GNU CC. They go in a library function named
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
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.