Multiply can be evaluated in a different type, so long as the target type has

a smaller bitwidth.

llvm-svn: 46244

2 files changed, 21 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/InstructionCombining.cpp b/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
index 2ad592a8f11..3e009ebce46 100644
--- a/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -6512,6 +6512,15 @@ static bool CanEvaluateInDifferentType(Value *V, const IntegerType *Ty,
            CanEvaluateInDifferentType(I->getOperand(1), Ty, CastOpc,
                                       NumCastsRemoved);
 
+  case Instruction::Mul:
+    break;
+    // A multiply can be truncated by truncating its operands.
+    return Ty->getBitWidth() < OrigTy->getBitWidth() && 
+           CanEvaluateInDifferentType(I->getOperand(0), Ty, CastOpc,
+                                      NumCastsRemoved) &&
+           CanEvaluateInDifferentType(I->getOperand(1), Ty, CastOpc,
+                                      NumCastsRemoved);
+
   case Instruction::Shl:
     // If we are truncating the result of this SHL, and if it's a shift of a
     // constant amount, we can always perform a SHL in a smaller type.
@@ -6571,6 +6580,7 @@ Value *InstCombiner::EvaluateInDifferentType(Value *V, const Type *Ty,
   switch (I->getOpcode()) {
   case Instruction::Add:
   case Instruction::Sub:
+  case Instruction::Mul:
   case Instruction::And:
   case Instruction::Or:
   case Instruction::Xor:
diff --git a/llvm/test/Transforms/InstCombine/2008-01-21-MulTrunc.ll b/llvm/test/Transforms/InstCombine/2008-01-21-MulTrunc.ll
new file mode 100644
index 00000000000..8de0959bf75
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/2008-01-21-MulTrunc.ll
@@ -0,0 +1,11 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | notcast
+
+define i16 @test1(i16 %a) {
+        %tmp = zext i16 %a to i32               ; <i32> [#uses=2]
+        %tmp21 = lshr i32 %tmp, 8               ; <i32> [#uses=1]
+        %tmp5 = mul i32 %tmp, 5         ; <i32> [#uses=1]
+        %tmp.upgrd.32 = or i32 %tmp21, %tmp5            ; <i32> [#uses=1]
+        %tmp.upgrd.3 = trunc i32 %tmp.upgrd.32 to i16           ; <i16> [#uses=1]
+        ret i16 %tmp.upgrd.3
+}
+