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



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