Bug 489290

The confidence comes from?  Could be strict aliasing violation, other kind of undefined code.  In any case, try with __attribute__((__optimize__(2))) vs.
__attribute__((__optimize__(3))) on the function you suspect, if that makes a difference, find out with what arguments it is being called when it misbehaves and what the result difference is, try to create a self-contained testcase that
calls that function with those arguments and checks for the outcome.
Disassembly isn't very useful when you don't know what to look for.

__attribute__((__optimize__(2))) works fine, __attribute__((__optimize__(3))) makes it crash.  The crash happens when a local variable is written to.  This is the beginning of the function:

__attribute__((__optimize__(3)))
static FcPattern *
FcFontSetMatchInternal (FcConfig    *config,
                        FcFontSet   **sets,
                        int         nsets,
                        FcPattern   *p,
                        FcResult    *result)
{
    double          score[NUM_MATCH_VALUES], bestscore[NUM_MATCH_VALUES];
    int             f;
    FcFontSet       *s;
    FcPattern       *best;
    int             i;
    int             set;

    for (i = 0; i < NUM_MATCH_VALUES; i++)
        bestscore[i] = 0;


And the crash happens when bestscore is written to.  If I move the bestscore assignment further down, it crashes then.  Means, I can't really test the function much.  I'm mostly out of ideas on how to debug this.  Still investigating though.

Is the function inlined into another one or not?  Use
__attribute__((__optimize__(3),__noinline__))
to find out.  Does it also fail if you drop the static from the function (and adjust a prototype as well if any)?
Do the arguments matter for the crash or not (e.g. if you call FnFontSetMatchInternal from the debugger early on with 0, 0, 0, 0, 0, does it crash in the same spot?

In any case, I'll need a preprocessed source and the complete set of command line options used to trigger it.

Humm, I think I see what's wrong now:

The code causing this is the SSE2 instruction:

0x00442f31 <FcFontSetMatchInternal+33>:	movapd %xmm0,0x40(%esp)

However, %esp has the value:

esp            0xbfffc554	0xbfffc554

which is not ALIGN16'ed and hence cannot be used with movapd operation as far as my googling skills suggest.

(Ok, yes, forgot to mention that I'm specifying command-line options for pentium-m)

Then can you please find out what misaligns the stack pointer?  Normally on i?86 and x86_64 %esp/%rsp should be 16-byte aligned (in particular, the return address slot (what's pushed by call insn) on the stack should be 16 byte aligned).
Are you calling code from -mpreferred-stack-boundary=2 compiled functions or something similar?

The crash only happens with thunderbird and firefox, so I suspect they do something funky with the stack.  %esp is unaligned at the beginning of that function call.  It's unaligned some 20 function calls earlier also.

With rawhide:

- run "thunderbird -g"
- in gdb do:

    b gtk_entry_new
    r
    c
    info registers

Observe that esp is unaligned.

Should be reassigned to gecko'ish?

Should be reassigned to whatever component misaligned the stack.
Just do a bt
and then go up one by one and look at what address the next frame eip is stored at, I guess main will be ok, probably a couple of functions below it in the call trace and then something that is misaligned.

Doing "info frame" suggests that all frames up to 99 are unaligned.  Trying it on frame 100 causes a gdb assertion failure.  Reassigning to thunderbird for further inspection.  Thanks for the help Jakub.

This bug appears to have been reported against 'rawhide' during the Fedora 11 development cycle.
Changing version to '11'.

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