[llvm-commits] patch: fix broken code generation with r125471 and newer when preferred alignment of some data type is is bigger than stack alignment

[Chris Lattner]

On Feb 14, 2011, at 11:11 AM, Heikki Kultala wrote:

> <stackalign.patch>

Hi Heikki,

This patch isn't correct.  If a stack slot claims to be (e.g.) 32-byte aligned, then InferPtrAlignment can return 32 bytes.  It is up to the target to provide this guarantee (by doing dynamic stack realignment) otherwise other things will fail.

You can see this at work on X86 (for example):

$ cat t.c
void foo() {
  int x __attribute__((aligned(128)));
  bar(&x);
}

$ clang t.c -S -o - -m32 -O3 
_foo:                                   ## @foo
## BB#0:                                ## %entry
	pushl	%ebp
	movl	%esp, %ebp
	andl	$-128, %esp
	subl	$256, %esp              ## imm = 0x100
	leal	128(%esp), %eax
	movl	%eax, (%esp)
	calll	_bar
	movl	%ebp, %esp
	popl	%ebp
	ret

If your target isn't aligning things properly, then attribute(aligned) is broken.

-Chris