[InstCombine] Teach ComputeNumSignBitsImpl to handle integer multiply…

… instruction.

Differential Revision: https://reviews.llvm.org/D36679

llvm-svn: 311206

llvm/lib/Analysis/ValueTracking.cpp

@@ -2196,6 +2196,17 @@ static unsigned ComputeNumSignBitsImpl(const Value *V, unsigned Depth,
     if (Tmp == 1) return 1;  // Early out.
     return std::min(Tmp, Tmp2)-1;
 
+  case Instruction::Mul: {
+    // The output of the Mul can be at most twice the valid bits in the inputs.
+    unsigned SignBitsOp0 = ComputeNumSignBits(U->getOperand(0), Depth + 1, Q);
+    if (SignBitsOp0 == 1) return 1;  // Early out.
+    unsigned SignBitsOp1 = ComputeNumSignBits(U->getOperand(1), Depth + 1, Q);
+    if (SignBitsOp1 == 1) return 1;
+    unsigned OutValidBits =
+        (TyBits - SignBitsOp0 + 1) + (TyBits - SignBitsOp1 + 1);
+    return OutValidBits > TyBits ? 1 : TyBits - OutValidBits + 1;
+  }
+
   case Instruction::PHI: {
     const PHINode *PN = cast<PHINode>(U);
     unsigned NumIncomingValues = PN->getNumIncomingValues();

llvm/test/Transforms/InstCombine/trunc.ll

@@ -89,6 +89,21 @@ define i32 @test6(i64 %A) {
   ret i32 %D
 }
 
+define i16 @ashr_mul(i8 %X, i8 %Y) {
+; CHECK-LABEL: @ashr_mul(
+; CHECK-NEXT:    [[A:%.*]] = sext i8 %X to i16
+; CHECK-NEXT:    [[B:%.*]] = sext i8 %Y to i16
+; CHECK-NEXT:    [[C:%.*]] = mul nsw i16 [[A]], [[B]]
+; CHECK-NEXT:    [[D:%.*]] = ashr i16 [[C]], 8
+; CHECK-NEXT:    ret i16 [[D]]
+  %A = sext i8 %X to i20
+  %B = sext i8 %Y to i20
+  %C = mul i20 %A, %B
+  %D = ashr i20 %C, 8
+  %E = trunc i20 %D to i16
+  ret i16 %E
+}
+
 define i32 @trunc_ashr(i32 %X) {
 ; CHECK-LABEL: @trunc_ashr(
 ; CHECK-NEXT:    [[B:%.*]] = or i32 [[X:%.*]], -2147483648