From eda7a86d42ff6f62b93fd1c4d963385d436dfc08 Mon Sep 17 00:00:00 2001 From: Sanjoy Das Date: Tue, 26 Sep 2017 21:54:27 +0000 Subject: [BypassSlowDivision] Improve our handling of divisions by constants Summary: Don't bail out on constant divisors for divisions that can be narrowed without introducing control flow . This gives us a 32 bit multiply instead of an emulated 64 bit multiply in the generated PTX assembly. Reviewers: jlebar Subscribers: jholewinski, mcrosier, llvm-commits Differential Revision: https://reviews.llvm.org/D38265 llvm-svn: 314253 --- llvm/lib/Transforms/Utils/BypassSlowDivision.cpp | 20 +++++++++++++------- 1 file changed, 13 insertions(+), 7 deletions(-) (limited to 'llvm/lib/Transforms/Utils') diff --git a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp index d6c31f282e8..4aed897d641 100644 --- a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp +++ b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp @@ -339,11 +339,6 @@ Optional FastDivInsertionTask::insertFastDivAndRem() { Value *Dividend = SlowDivOrRem->getOperand(0); Value *Divisor = SlowDivOrRem->getOperand(1); - if (isa(Divisor)) { - // Keep division by a constant for DAGCombiner. - return None; - } - VisitedSetTy SetL; ValueRange DividendRange = getValueRange(Dividend, SetL); if (DividendRange == VALRNG_LIKELY_LONG) @@ -359,7 +354,9 @@ Optional FastDivInsertionTask::insertFastDivAndRem() { if (DividendShort && DivisorShort) { // If both operands are known to be short then just replace the long - // division with a short one in-place. + // division with a short one in-place. Since we're not introducing control + // flow in this case, narrowing the division is always a win, even if the + // divisor is a constant (and will later get replaced by a multiplication). IRBuilder<> Builder(SlowDivOrRem); Value *TruncDividend = Builder.CreateTrunc(Dividend, BypassType); @@ -369,7 +366,16 @@ Optional FastDivInsertionTask::insertFastDivAndRem() { Value *ExtDiv = Builder.CreateZExt(TruncDiv, getSlowType()); Value *ExtRem = Builder.CreateZExt(TruncRem, getSlowType()); return QuotRemPair(ExtDiv, ExtRem); - } else if (DividendShort && !isSignedOp()) { + } + + if (isa(Divisor)) { + // If the divisor is not a constant, DAGCombiner will convert it to a + // multiplication by a magic constant. It isn't clear if it is worth + // introducing control flow to get a narrower multiply. + return None; + } + + if (DividendShort && !isSignedOp()) { // If the division is unsigned and Dividend is known to be short, then // either // 1) Divisor is less or equal to Dividend, and the result can be computed -- cgit v1.2.3