GNU Info
(gcc-300.info)Trampolines
Trampolines for Nested Functions
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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 GCC 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.
If you do not define this macro, it means no template is needed
for the target. Do not define this macro 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.
`TRAMPOLINE_SECTION'
The name of a subroutine to switch to the section in which the
trampoline template is to be placed (Note: 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.
`TRAMPOLINE_ADJUST_ADDRESS (ADDR)'
A C statement that should perform any machine-specific adjustment
in the address of the trampoline. Its argument contains the
address that was passed to `INITIALIZE_TRAMPOLINE'. In case the
address to be used for a function call should be different from
the address in which the template was stored, the different
address should be assigned to ADDR. If this macro is not defined,
ADDR will be used for function calls.
`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 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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