On Mon, Jan 11, 2016 at 5:11 PM, Bin Cheng <bin.ch...@arm.com> wrote: > Hi, > A wrong code bug is reported in PR68911, which GCC generates infinite loop > for below example after loop niter analysis changes. After that change, > scev_probably_wraps_p identifies that e_1 in below case never overflow/wrap: > <bb 8>: > e_15 = e_1 + 1; > > <bb 13>: > # e_1 = PHI <e_2(3), e_15(8), e_12(7)> > if (e_1 <= 93) > goto <bb 8>; > else > goto <bb 10>; > > The loop niter analysis gives us: > Analyzing # of iterations of loop 2 > exit condition [e_8, + , 1] <= 93 > bounds on difference of bases: -4294967202 ... 93 > result: > zero if e_8 > 94 > # of iterations 94 - e_8, bounded by 94 > > I think the analysis is good. When scev_probably_wraps_p returns false, it > may imply two possible cases. > CASE 1) If loop's latch gets executed, then we know the SCEV doesn't > overflow/wrap during loop execution. > CASE 2) If loop's latch isn't executed, i.e., the loop exits immediately at > its first check on exit condition. In this case the SCEV doesn't > overflow/wrap because it isn't increased at all. > > The real problem I think is VRP only checks scev_probably_wraps_p then > assumes SCEV won't overflow/wrap after niter.bound iterations. This is not > true for CASE 2). If we have a large enough starting value for e_1, for > example, 0xfffffff8 in this example, e_1 is guaranteed not overflow/wrap only > because the loop exits immediately, not after niter.bound interations. Here > VRP assuming "e_1 + niter.bound" doesn't overflow/wrap is wrong. > > This patch fixes the issue by adding overflow check in range information > computed for "e_1 + niter.bound". It catches overflow/wrap of the expression > when loop may exit immediately. > > With this change, actually I think it may be possible for us to remove the > call to scev_probably_wraps_p, though I didn't do that in this patch. > > Bootstrap and test on x86_64 and AArch64. Is it OK?
Ok. Thanks, Richard. > Thanks, > bin > > 2016-01-10 Bin Cheng <bin.ch...@arm.com> > > PR tree-optimization/68911 > * tree-vrp.c (adjust_range_with_scev): Check overflow in range > information computed for expression "init + nit * step". > > gcc/testsuite/ChangeLog > 2016-01-10 Bin Cheng <bin.ch...@arm.com> > > PR tree-optimization/68911 > * gcc.c-torture/execute/pr68911.c: New test. > >
