[BypassSlowDivision] Improve our handling of divisions by constants

(This reapplies r314253.  r314253 was reverted on r314482 because of a
correctness regression on P100, but that regression was identified to be
something else.)

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: 319677

1 files changed, 13 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
index e9c14c93a9a..f711b192f60 100644
--- a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
+++ b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
@@ -352,11 +352,6 @@ Optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
   Value *Dividend = SlowDivOrRem->getOperand(0);
   Value *Divisor = SlowDivOrRem->getOperand(1);
 
-  if (isa<ConstantInt>(Divisor)) {
-    // Keep division by a constant for DAGCombiner.
-    return None;
-  }
-
   VisitedSetTy SetL;
   ValueRange DividendRange = getValueRange(Dividend, SetL);
   if (DividendRange == VALRNG_LIKELY_LONG)
@@ -372,7 +367,9 @@ Optional<QuotRemPair> 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);
@@ -382,7 +379,16 @@ Optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
     Value *ExtDiv = Builder.CreateZExt(TruncDiv, getSlowType());
     Value *ExtRem = Builder.CreateZExt(TruncRem, getSlowType());
     return QuotRemPair(ExtDiv, ExtRem);
-  } else if (DividendShort && !isSignedOp()) {
+  }
+
+  if (isa<ConstantInt>(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