On Mon, Apr 30, 2018 at 8:49 PM, Marc Glisse <marc.gli...@inria.fr> wrote: > On Fri, 12 Jan 2018, Wilco Dijkstra wrote: > >> Hi, >> >> Here is the updated version: >> >> This patch implements some of the optimizations discussed in >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71026. >> >> Simplify (C / x >= 0.0) into x >= 0.0 with -fno-signed-zeros >> and -ffinite-math-only. If C is negative the comparison is reversed. >> Only handle >= and <= for now since C / x can underflow if C is small. >> >> >> Simplify (x * C1) > C2 into x > (C2 / C1) with >> -funsafe-math-optimizations. >> If C1 is negative the comparison is reversed. >> >> OK for commit? >> >> ChangeLog >> 2018-01-10 Wilco Dijkstra <wdijk...@arm.com> >> Jackson Woodruff <jackson.woodr...@arm.com> >> >> gcc/ >> PR 71026/tree-optimization >> * match.pd: Simplify floating point comparisons. >> >> gcc/testsuite/ >> PR 71026/tree-optimization >> * gcc.dg/div-cmp-1.c: New test. >> * gcc.dg/div-cmp-2.c: New test. >> -- >> >> diff --git a/gcc/match.pd b/gcc/match.pd >> index >> 435125a317275527661fba011a9d26e507d293a6..8a6fee906de6a750201362119862f8326868f26b >> 100644 >> --- a/gcc/match.pd >> +++ b/gcc/match.pd >> @@ -376,6 +376,21 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) >> (rdiv @0 (negate @1)) >> (rdiv (negate @0) @1)) >> >> +/* Simplify (C / x op 0.0) to x op 0.0 for C != 0, C != Inf/Nan. >> + Only handle >= and <= since C / x may underflow to zero. */ >> +(for op (le ge) >> + res_op (lt ge) >> + neg_op (ge lt) >> + (simplify >> + (op (rdiv REAL_CST@0 @1) real_zerop@2) >> + (if (!HONOR_SIGNED_ZEROS (@1) && !HONOR_INFINITIES (@1)) >> + (switch >> + (if (real_less (&dconst0, TREE_REAL_CST_PTR (@0))) >> + (res_op @1 @2)) >> + /* For C < 0, use the inverted operator. */ >> + (if (real_less (TREE_REAL_CST_PTR (@0), &dconst0)) >> + (neg_op @1 @2)))))) > > > Let's try with C = DBL_MIN and x = 婊BL_MAX. I don't believe it involves > signed zeros or infinities, just an underflow. First, the result depends on > the rounding mode. And in the default round-to-nearest, both divisions give > 0, and thus compare the same with 0, but we replace that with a sign test on > x, where they clearly give opposite answers. > > What would be the proper flag to test to check if we care about underflow?
We have none specific so this makes it flag_unsafe_math_optimizations. Richard. > -- > Marc Glisse
