On Wed, 29 Mar 2017, Jeff Law wrote: > On 03/29/2017 04:05 AM, Richard Biener wrote: > > > > After quite some pondering over this and other related bugs I propose > > the following for GCC 7 which tames down PRE a bit (back to levels > > of GCC 6). Technically it's the wrong place to fix this, we do > > have measures in place during elimination but they are not in effect > > at -O2. For GCC 8 I'd like to be more aggressive there but that > > would require to enable predictive commoning at -O2 (with some > > limits to its unrolling) to not lose optimization opportunities. > > > > The other option is to ignore this issue and postpone the solution > > to GCC 8. > > > > Bootstrapped / tested on x86_64-unknown-linux-gnu. > > > > Any preference? > > > > Thanks, > > Richard. > > > > 2017-03-29 Richard Biener <rguent...@suse.de> > > > > PR tree-optimization/77498 > > * tree-ssa-pre.c (phi_translate_1): Do not allow simplifications > > to non-constants over backedges. > > > > * gfortran.dg/pr77498.f: New testcase. > I've got a slight preference for this patch. > > If you had a good start on the real fix then I'd lean more towards postponing.
I wouldn't it yet call "real fix" but just an idea (where I tested the PRE side already, with the expected testsuite regressions). I've done no benchmarking at all for that. For the proposed patch the situation is that we're only going to remove some PRE that was done additionally over GCC 6 because of the rev. that caused the regression, so I have confidence that it won't make things worse when comparing to GCC 6. Thus I've now applied the patch. Richard.
