https://llvm.org/bugs/show_bug.cgi?id=26662
Bug ID: 26662
Summary: clang: x86: __force_align_arg_pointer__ assumes that
stack arguments are aligned
Product: clang
Version: trunk
Hardware: Macintosh
OS: MacOS X
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: h...@codeweavers.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
When accessing stack arguments in a function marked with the
__force_align_arg_pointer__ attibute, the generated code assumes that the stack
args are 16-byte aligned. The point of re-aligning the stack with this
attribute is that the stack may not be so aligned.
This is of importance to Wine, where the caller is win32 code with 4-byte
alignment, yet the code is running on a OS/X / Linux host with 16-byte
alignment.
When the function below is compiled with:
clang -O1 -m32 -fno-stack-protector -c ~/test.c
(stack protection disabled just to simplify disassembly)
struct sixteen
{
char b[16];
};
void __attribute__((__force_align_arg_pointer__)) __attribute__((__stdcall__))
test_fn( struct sixteen s )
{
volatile struct sixteen d = s;
}
We get:
_test_fn:
00000000 pushl %ebp
00000001 movl %esp, %ebp
00000003 andl $-0x10, %esp
00000006 subl $0x20, %esp
00000009 movaps 0x8(%ebp), %xmm0
0000000d movaps %xmm0, (%esp)
00000011 movl %ebp, %esp
00000013 popl %ebp
00000014 ret $0x10
Note that while the stack is successfully re-aligned (andl $-0x10, %esp), the
movaps 0x8(%ebp), %xmm0 assumes that the stack was originally 16-byte aligned.
It should not make that assumption.
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