[DAGCombiner] fold select-of-constants to shift

This reverses the scalar canonicalization proposed in D63382.

Pre: isPowerOf2(C1)
%r = select i1 %cond, i32 C1, i32 0
=>
%z = zext i1 %cond to i32
%r = shl i32 %z, log2(C1)

https://rise4fun.com/Alive/Z50

x86 already tries to fold this pattern, but it isn't done
uniformly, so we still see a diff. AArch64 probably should
enable the TLI hook to benefit too, but that's a follow-on.

llvm-svn: 374397

1 files changed, 12 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 377c6086b1a..41303921d87 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8221,10 +8221,11 @@ SDValue DAGCombiner::foldSelectOfConstants(SDNode *N) {
       return Cond;
     }
 
-    // For any constants that differ by 1, we can transform the select into an
-    // extend and add. Use a target hook because some targets may prefer to
-    // transform in the other direction.
+    // Use a target hook because some targets may prefer to transform in the
+    // other direction.
     if (TLI.convertSelectOfConstantsToMath(VT)) {
+      // For any constants that differ by 1, we can transform the select into an
+      // extend and add.
       const APInt &C1Val = C1->getAPIntValue();
       const APInt &C2Val = C2->getAPIntValue();
       if (C1Val - 1 == C2Val) {
@@ -8239,6 +8240,14 @@ SDValue DAGCombiner::foldSelectOfConstants(SDNode *N) {
           Cond = DAG.getNode(ISD::SIGN_EXTEND, DL, VT, Cond);
         return DAG.getNode(ISD::ADD, DL, VT, Cond, N2);
       }
+
+      // select Cond, Pow2, 0 --> (zext Cond) << log2(Pow2)
+      if (C1Val.isPowerOf2() && C2Val.isNullValue()) {
+        if (VT != MVT::i1)
+          Cond = DAG.getNode(ISD::ZERO_EXTEND, DL, VT, Cond);
+        SDValue ShAmtC = DAG.getConstant(C1Val.exactLogBase2(), DL, VT);
+        return DAG.getNode(ISD::SHL, DL, VT, Cond, ShAmtC);
+      }
     }
 
     return SDValue();