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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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