[Bug target/35659] [4.3/4.4 Regression] Miscompiled code with -O2 (but not with -O2 -funroll-loops) on ia64

["jakub at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #19 from jakub at gcc dot gnu dot org  2008-06-26 08:41 -------
To be more precise, the problem is in speculating conditional store:
        ld4.a r18 = [r44]
        st4 [r59] = r14, -60
...
        cmp4.ge p6, p7 = r22, r18
        (p7) ld4 r14 = [r62]
...
        (p7) st4 [r77] = r14
        chk.a.clr r18, .L69
.L70:
...
.L69:
        ld4 r18 = [r44]
        ;;
        cmp4.ge p6, p7 = r22, r18
        ;;
        (p7) ld4 r14 = [r62]
        ;;
        (p7) st4 [r77] = r14
        br .L70

Now, ld4.a returns the stale value in r18 (0x20202020), $r22 is 2 and so in the
code before chk.a.clr p7 is 1, so [r62] is loaded into r14 and stored into
[r77]
(i.e. MR = M in IF     (K.GT.M1)                 MR = M is executed).
Then chk.a.clr realizes the [r44] memory changed, so branches to .L69 to do the
speculated stuff again.  This time r18 is 1, so p7 will be 0 and MR = M is not
done.  But this really doesn't and can't undo the st4 [r77] = r14 store that
already happened before chk.a.clr and wasn't supposed to happen.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35659

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