[InstCombine] include 'sub' in the list of narrow-able binops

      // trunc (binop X, C) --> binop (trunc X, C')
      // trunc (binop (ext X), Y) --> binop X, (trunc Y)

I'm grouping sub with the other binops  because that makes the code simpler
and the transforms are valid:
https://rise4fun.com/Alive/UeF
...so even though we don't expect a sub with constant Op1 or any of the
other opcodes with constant Op0 due to canonicalization rules, we might as
well handle those situations if non-canonical code somehow reaches this
point (it should just make instcombine more efficient in reaching its
end goal).

This should solve the problem that later manifests in the vectorizers in 
PR35295:
https://bugs.llvm.org/show_bug.cgi?id=35295

llvm-svn: 318404

1 files changed, 7 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index c2fe661c467..178c8eaf250 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -552,8 +552,15 @@ Instruction *InstCombiner::narrowBinOp(TruncInst &Trunc) {
   case Instruction::Or:
   case Instruction::Xor:
   case Instruction::Add:
+  case Instruction::Sub:
   case Instruction::Mul: {
     Constant *C;
+    if (match(BinOp0, m_Constant(C))) {
+      // trunc (binop C, X) --> binop (trunc C', X)
+      Constant *NarrowC = ConstantExpr::getTrunc(C, DestTy);
+      Value *TruncX = Builder.CreateTrunc(BinOp1, DestTy);
+      return BinaryOperator::Create(BinOp->getOpcode(), NarrowC, TruncX);
+    }
     if (match(BinOp1, m_Constant(C))) {
       // trunc (binop X, C) --> binop (trunc X, C')
       Constant *NarrowC = ConstantExpr::getTrunc(C, DestTy);
@@ -573,16 +580,6 @@ Instruction *InstCombiner::narrowBinOp(TruncInst &Trunc) {
     }
     break;
   }
-  case Instruction::Sub: {
-    Constant *C;
-    if (match(BinOp0, m_Constant(C))) {
-      // trunc (binop C, X) --> binop (trunc C', X)
-      Constant *NarrowC = ConstantExpr::getTrunc(C, DestTy);
-      Value *TruncX = Builder.CreateTrunc(BinOp1, DestTy);
-      return BinaryOperator::Create(BinOp->getOpcode(), NarrowC, TruncX);
-    }
-    break;
-  }
 
   default: break;
   }