[SDAG] Now that we have a way to communicate the exact bit on sdiv use it to simplify sdiv by a constant.

We had a hack in SDAGBuilder in place to work around this but now we
can avoid that. Call BuildExactSDIV from BuildSDIV so DAGCombiner can
perform this trick automatically.

The added check in DAGCombiner is necessary to prevent exact sdiv by pow2
from regressing as the target-specific pow2 lowering is not aware of
exact bits yet.

This is mostly covered by existing tests. One side effect is that we
get the better lowering for exact vector sdivs now too :)

llvm-svn: 240891

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2c4ab8b9f8f..29f9df6425e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2178,7 +2178,11 @@ SDValue DAGCombiner::visitSDIV(SDNode *N) {
   }
 
   // fold (sdiv X, pow2) -> simple ops after legalize
+  // FIXME: We check for the exact bit here because the generic lowering gives
+  // better results in that case. The target-specific lowering should learn how
+  // to handle exact sdivs efficiently.
   if (N1C && !N1C->isNullValue() && !N1C->isOpaque() &&
+      !cast<BinaryWithFlagsSDNode>(N)->Flags.hasExact() &&
       (N1C->getAPIntValue().isPowerOf2() ||
        (-N1C->getAPIntValue()).isPowerOf2())) {
     // If dividing by powers of two is cheap, then don't perform the following