On Wed, Dec 03, 2014 at 05:31:15PM +0800, Bin.Cheng wrote: > Does your series of patches affect PR62151? I am going to revisit > that one to see if I can answer Jeff's questions there.
I don't think any of my recent patches affects it. r215789 does, however, so you'll need to revert that to show the problem again. To recap: We have five insns, call them A, B, C, D, E. A feeds B feeds C; C feeds both D and E. A and C set the same pseudo. D and E are simple moves. A+B+C would not combine because of a limitation that r215789 removed. Then combine tried combining A+B+C+D doing effectively the combination of A+B+C, and un-cse'ing the result to D. Then distribute_notes messes up with the REG_DEAD note of B (the result of A is dead); it takes it to mean the result of C is dead, but E still uses that. r215789 hides this in the cases where A+B+C in fact combines, which in practice is (nearly) all cases. Either we need to fix distribute_notes, or we should disallow A->B->C->D combos where the result of C is not dead at D, to fix this for real. I vote for the latter: 4-insn combine is expensive, and doesn't often improve anything, and it's even less likely to match anything in this case (which after all needs to keep the result of A+B+C around, even though that on its own did not combine!) Even if it matches, it in effect does an uncse which combine does not normally do; it would be better to do that in a separate pass (maybe piggyback it on fwprop, its sibling; or maybe do it separately. Something for the future, anyway). Segher
