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Re: fix allocno comparison function misbehavior on overflow


Olivier Hainque <hainque@adacore.com> writes:

> Compiling a big proprietary Ada source base with gcc 4.5 on x86-solaris
> exposed misbehavior like
>
>     received signal SIGSEGV, Segmentation fault.
>     in allocno_priority_compare_func (v1p=0x21b0ce4, v2p=0x21b0cec)
>     at ../../src/gcc/ira-color.c:1745
>     1745      pri1 = allocno_priorities[ALLOCNO_NUM (a1)];
>
> Very similar to what was reported as PR/44763 on existing testcases.
>
> This turned out to be caused by a glitch with respect to possible overflows in
> allocno_priority_compare_func:
>
>   <<  pri1 = allocno_priorities[ALLOCNO_NUM (a1)];
>       pri2 = allocno_priorities[ALLOCNO_NUM (a2)];
>       if (pri2 - pri1)
>         return pri2 - pri1;
>   >>
>
> The case we had ended up here with
>
>   p2 = 247786560, p1 = -1982292480
>
> so p2 > p1, but the function computed otherwise (p2-p1 = -2064888256).
>
> This, in turn, lead to inconsistencies in the ordering relationship,
> which the x86-solaris qsort implementation dislikes.
>
> The attached patch is a proposal to address this by replacing the difference
> computation by a comparison, factored out in a macro to allow reuse in other
> places (not in this patch) and provide a clean spot for a general comment.
>
> This fixed the observed SEGV with 4.5 and bootstrapped on i386-solaris. Was
> also bootstrapped and regression tested for mainline on both sparc-solaris
> and x86_64-linux.

This will most likely fix PR middle-end/44763.  I'll give it a try.

Thanks.
	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University


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