ld2.acq - sign extension?

[Jes Sorensen <jes@sgi.com>]

Hi,

Trying to figure out if it's GCC being broken or what else could be
causing this problem.

I am chasing an issue where the console hangs when I boot my self
compiled kernels under KVM. I have tracked down the problem to a
single instruction change in the vgacon driver, however if I
understand the rules correctly this specific change should not cause
any difference in code execution whatsoever?

Basically if I apply the below diff my code no longer works. However I
am under the impression that ld2.acq does sign extension? If thats the
case, does anybody have an explanation for why my bode breaks if I
don't perform the zxt2 instruction on the previously loaded register
before the comparison? Note the mov r19=r17 at the end, which should
mean there is no reason why this change should affect code flow
further down the lin...

Any input would be greatly appreciated, I have been trying to track
this down for a couple of weeks and I still don't get why this is
going wrong.

Thanks,
Jes


--- /tmp/vgacon-good.S  2008-05-16 06:09:08.000000000 -0500
+++ /tmp/vgacon-test2.S 2008-05-19 03:54:45.000000000 -0500
@@ -113,22 +113,22 @@
        ld2.acq r21 = [r18], 2
        ;;
        ld2.acq r16 = [r18]
        nop 0
        .mmi
        st2.rel [r8] = r20
        st2.rel [r18] = r22
        zxt2 r15 = r21
        .mii
-       ld2.acq r19 = [r8]
+       ld2.acq r17 = [r8]
        nop 0
        ;;
-       zxt2 r17 = r19
+       zxt2 r19 = r19
        .mib
        nop 0
        zxt2 r16 = r16
        nop 0
        ;;
        .mib
        cmp4.ne p8, p9 = r11, r17
        mov r19 = r17
        (p8) br.cond.dpnt .L8