https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68106
--- Comment #1 from Vladimir Makarov <vmakarov at gcc dot gnu.org> --- (In reply to Zdenek Sojka from comment #0) > Created attachment 36594 [details] > reduced testcase > > The testcase fails at aarch64 at both trunk and 5-branch with -O > -flra-remat. I haven't managed to generate wrong code with -O2, which > enables -flra-remat. I am using qemu userspace emulation to run the testcase. > > $ gcc -O -flra-remat testcase.c > $ ./a.out > qemu: uncaught target signal 6 (Aborted) - core dumped > Aborted > > $ gcc -O -flra-remat testcase.c -S > $ gcc -O -fno-lra-remat testcase.c -S -o testcase-no-lra-remat.s > $ diff -u testcase.s testcase-no-lra-remat.s > ... > @@ -104,20 +104,21 @@ > .L25: > lsl x20, x23, 56 > add x19, x20, x26 > + str x19, [x29, 128] > asr x22, x19, 63 > adds x27, x21, x19 > adc x0, x24, x22 > - str x0, [x29, 120] > - add x2, x29, 140 > + str x0, [x29, 136] > + add x2, x29, 156 > mov x1, x19 > mov x0, x25 > bl upseu > cmp x0, x27 > bne .L14 > - adc x0, x24, x22 > + ldr x0, [x29, 136] > cmp x0, xzr > cset w1, ne > - ldr w0, [x29, 140] > + ldr w0, [x29, 156] > cmp w1, w0 > bne .L14 > subs x1, x21, x20 > ... > > If I am reading the assembly correctly, the important difference is: > ... > adds x27, x21, x19 > adc x0, x24, x22 > - str x0, [x29, 120] > - add x2, x29, 140 > + str x0, [x29, 136] > + add x2, x29, 156 > ... > - adc x0, x24, x22 > + ldr x0, [x29, 136] > ... > > Normally, without lra-remat, the result of "adc x0, x24, x22" is stored > to the stack and then reloaded. > With -flra-remat, the value is stored as well, but later, "adc" is used to > recompute the value again - that saves one access to the stack, but cpsr has > changed in the meantime, so it is using wrong value of the C bit. > > Tested revisions: > r229293 - FAIL > 5-branch r229305 - FAIL > 4_9-branch - doesn't know -flra-remat I was not able to reproduce it on the current trunk. But I've reproduced it on r229293. I've been working on it and I am planning to submit a patch for the trunk today.
