| Message ID | orboydg3q9.fsf@livre.localdomain |
|---|---|
| State | New |
| Headers | show |
On Sat, Jun 4, 2011 at 2:50 PM, Alexandre Oliva <aoliva@redhat.com> wrote: > If cprop regards changes to debug insns as “changed”, it will perform > cfg optimizations and more, even if no non-debug insns were changed, > causing divergence between -g and -g0 compilations. > > This was observed during bootstrap-debug-lib of the SSA coalesce patch, > building a-strsea.adb with -fcompare-debug. Because of different CFGs, > cse2 chose different equivalent pseudos for an insn. -fcompare-debug > detected the difference in REG notes, even though the executable code > turned out to be the same. We can't count on this luck, though: other > optimization passes could apply different transformations to the > different CFGs, producing different executable code, which must never > happen. > > This patch fixes the problem, disregarding changes to debug insns as > “changed”. Regstrapped on x86_64-linux-gnu and i686-linux-gnu. Ok to > install? Looks OK to me, although I can't approve it. I'm curious, though: What CFG changes or other transformations are performed without this patch? It could be a sign of a missed optimization before CPROP. Have you looked at that too? Ciao! Steven
> This patch fixes the problem, disregarding changes to debug insns as > “changed”. Regstrapped on x86_64-linux-gnu and i686-linux-gnu. Ok to > install? Yes, thanks.
On Jun 4, 2011, Steven Bosscher <stevenb.gcc@gmail.com> wrote: > I'm curious, though: What CFG changes or other transformations are > performed without this patch? It could be a sign of a missed > optimization before CPROP. Have you looked at that too? IIRC the transformation that was possible but that hadn't been performed yet involved merging two blocks (I remember block numbering was off after cprop, and that there were cfgoptimize notes in its -g dumps, but not in the non-g dumps), but I didn't look any further than that.
for gcc/ChangeLog from Alexandre Oliva <aoliva@redhat.com> * cprop.c (local_cprop_pass): Don't set changed for debug insns. Index: gcc/cprop.c =================================================================== --- gcc/cprop.c.orig 2011-06-04 05:09:24.414816329 -0300 +++ gcc/cprop.c 2011-06-04 05:09:25.954797626 -0300 @@ -1223,7 +1223,8 @@ local_cprop_pass (void) { if (do_local_cprop (reg_use_table[i], insn)) { - changed = true; + if (!DEBUG_INSN_P (insn)) + changed = true; break; } }