#4211: LLVM: Stack alignment on OSX
------------------------+---------------------------------------------------
Reporter: dterei | Owner: dterei
Type: task | Status: new
Priority: normal | Component: Compiler (LLVM)
Version: 6.13 | Keywords:
Os: MacOS X | Testcase:
Architecture: x86 | Failure: None/Unknown
------------------------+---------------------------------------------------
On OSX the ABI requires that the stack is 16 byte aligned when making
function calls. (Although this only really needs to be obeyed when making
calls that will go through the dynamic linker, so FFI calls). Since the
stack is 16 byte aligned at the site of the call, on entry to a function
most compilers (both llvm and gcc) expect the stack to now be aligned to
16n - 4, since 4 bytes should have been pushed for the return address as
part of the call instruction. GHC though since it uses jumps everywhere
keeps that stack at 16 byte aligned on function entrance. This means that
LLVM generates incorrect stack alignment code, always off by 4.
For the moment I have handled this by using the LLvm Mangler (which is
only needed on OS X already for TNTC) to simply correctly fix up the stack
alignment code.
E.g Asm generated by LLVM:
{{{
_func:
subl $12, %esp
...
call _sin
...
addl $12, %esp
}}}
The mangler will change this to:
{{{
_func:
subl $16, %esp
...
call _sin
...
addl $16, %esp
}}}
The better solution would be to change GHC to keep the stack at 16n - 4
alignment on function. This will require changing the RTS (StgCRun.hs) to
set the stack properly before calling into Stg land and also fixing up the
NCG to align code properly. There may also be a problem with the C backend
as currently all function prolouge and epilouge code is stripped out,
which means all the stack manipulation code generated by GCC is removed.
This works fine now since the stack is already 16 byte aligned on entry,
but if it is now 16n - 4 byte aligned then some stack manipulation will be
required.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4211>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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