Revert "[BypassSlowDivision] Improve our handling of divisions by constants"

This reverts commit r314253.  It causes a miscompile on P100 in an internal
benchmark.  Reverting while I investigate.

llvm-svn: 314482

1 files changed, 7 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
index 4aed897d641..d6c31f282e8 100644
--- a/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
+++ b/llvm/lib/Transforms/Utils/BypassSlowDivision.cpp
@@ -339,6 +339,11 @@ 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)
@@ -354,9 +359,7 @@ 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.  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).
+    // division with a short one in-place.
 
     IRBuilder<> Builder(SlowDivOrRem);
     Value *TruncDividend = Builder.CreateTrunc(Dividend, BypassType);
@@ -366,16 +369,7 @@ Optional<QuotRemPair> FastDivInsertionTask::insertFastDivAndRem() {
     Value *ExtDiv = Builder.CreateZExt(TruncDiv, getSlowType());
     Value *ExtRem = Builder.CreateZExt(TruncRem, getSlowType());
     return QuotRemPair(ExtDiv, ExtRem);
-  }
-
-  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()) {
+  } else 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