Commit 9c617747 authored by simonmar's avatar simonmar
Browse files

[project @ 2005-03-30 09:49:55 by simonmar]

Hack around a nasty bug on x86_64, caused by failing to align %rsp
correctly at a C call.  See the comment for details.
parent 8e30c1ff
......@@ -225,7 +225,41 @@ static void StgRunIsImplementedInAssembler(void)
"addq %0, %%rsp\n\t"
"retq"
: : "i"(RESERVED_C_STACK_BYTES+48 /*stack frame size*/));
: : "i"(RESERVED_C_STACK_BYTES+48+8 /*stack frame size*/));
/*
HACK alert!
The x86_64 ABI specifies that on a procedure call, %rsp is
aligned on a 16-byte boundary + 8. That is, the first
argument on the stack after the return address will be
16-byte aligned.
Which should be fine: RESERVED_C_STACK_BYTES+48 is a multiple
of 16 bytes.
BUT... when we do a C-call from STG land, gcc likes to put the
stack alignment adjustment in the prolog. eg. if we're calling
a function with arguments in regs, gcc will insert 'subq $8,%rsp'
in the prolog, to keep %rsp aligned (the return address is 8
bytes, remember). The mangler throws away the prolog, so we
lose the stack alignment.
The hack is to add this extra 8 bytes to our %rsp adjustment
here, so that throughout STG code, %rsp is 16-byte aligned,
ready for a C-call.
A quick way to see if this is wrong is to compile this code:
main = System.Exit.exitWith ExitSuccess
And run it with +RTS -sstderr. The stats code in the RTS, in
particular statsPrintf(), relies on the stack alignment because
it saves the %xmm regs on the stack, so it'll fall over if the
stack isn't aligned, and calling exitWith from Haskell invokes
shutdownHaskellAndExit using a C call.
Future gcc releases will almost certainly break this hack...
*/
}
#endif /* x86-64 */
......
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