Re: BUG: GCC-4.4.x changes the function frame on some functions

[Andrew Haley <aph@redhat.com>]

Thomas Gleixner wrote:
> On Thu, 19 Nov 2009, Thomas Gleixner wrote:
> 
> Can the GCC folks please shed some light on this:
> 
> standard function start:
> 
> 	 push	%ebp
> 	 mov	%esp, %ebp
> 	 ....
> 	 call   mcount
> 
> modified function start on a handful of functions only seen with gcc
> 4.4.x on x86 32 bit:
> 
> 	push   %edi
> 	lea    0x8(%esp),%edi
> 	and    $0xfffffff0,%esp
> 	pushl  -0x4(%edi)
> 	push   %ebp
> 	mov    %esp,%ebp
> 	...
> 	call   mcount
> 
> This modification leads to a hard to solve problem in the kernel
> function graph tracer which assumes that the stack looks like:
> 
>        return address
>        saved  ebp
> 
> With the modified function start sequence this is not longer true and
> the manipulation of the return address on the stack fails silently.
> 
> Neither gcc 4.3 nor gcc 3.4 are generating such function frames, so it
> looks like a gcc 4.4.x feature.
> 
> There is no real obvious reason why the edi magic needs to be done
> _before_ 
> 
> 	push   %ebp
> 	mov    %esp,%ebp

OK, I found it.  There is a struct defined as

struct entry {
 ...
} __attribute__((__aligned__((1 << (4)))));

and then in timer_stats_update_stats you have a local variable of type
struct entry:

void timer_stats_update_stats()
{
 spinlock_t *lock;
 struct entry *entry, input;

So, gcc has to 16-align the stack pointer to satisfy the alignment
for struct entry.

Andrew.