linux x86-32 stack not kept 16-byte aligned

[llvmbot]

Bugzilla Link	8969
Resolution	FIXED
Resolved on	Jan 23, 2011 15:38
Version	trunk
OS	Linux
Attachments	testcase
Reporter	LLVM Bugzilla Contributor
CC	@asl,@echristo,@sunfishcode

Extended Description

On 32-bit Linux, the stack is supposed to be 16-byte aligned. LLVM doesn't actually do that by default.

The attached .bc file demonstrates:

nlewycky@ducttape:~$ llvm/Debug+Asserts/bin/llc -O0 pp.bc -o -
.file "pp.bc"
.text
.hidden prologProcessor
.globl prologProcessor
.align 16, 0x90
.type prologProcessor,@function
prologProcessor: # @​prologProcessor

BB#0:

      pushl   %ebp
      pushl   %ebx
      pushl   %edi
      pushl   %esi
      subl    $68, %esp

which is 8-byte aligned, not 16-byte aligned. This isn't ABI compliant.

[llvmbot]

assigned to @echristo

[echristo]

Looks like the default alignment has changed in recent years. We can update it in llvm :)

[echristo]

Maybe fixed in r123367.

[asl]

FWIW: stack is supposed to be 16-bytes aligned not in Linux, but due to gcc. According to the latest ABI available it's still 4-bytes aligned :) So, some problems might be expected with legacy code, etc.

[echristo]

Anton: Yeah, I figured. It looks like gcc is setting it to that by default these days so we'll go with compatibility or something...

[llvmbot]

I couldn't find any document stating the stack must be 16 bytes aligned, but with a ubuntu 4.4.4-1 compiling

void g(void*);
void f(long long int x) {
g(alloca(x));
}

I get

    andl    $-16, %eax
    subl    %eax, %esp
    leal    15(%esp), %eax
    subl    $12, %esp
    andl    $-16, %eax
    pushl   %eax
    call    g

I don't get what gcc is doing aligning twice, but yes, it is 16 bytes aligned.

[llvmbot]

Fixed! Thanks Eric!