[InstCombine] Fold (X * (Y << K)) u>> K -> X * Y when highbits are not demanded

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -770,6 +770,15 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Instruction *I,
             return InsertNewInstWith(Shl, I->getIterator());
           }
         }
+
+        const APInt *Factor;
+        if (match(I->getOperand(0),
+                  m_OneUse(m_Mul(m_Value(X), m_APInt(Factor)))) &&
+            Factor->countr_zero() >= ShiftAmt) {
+          BinaryOperator *Mul = BinaryOperator::CreateMul(
+              X, ConstantInt::get(X->getType(), Factor->lshr(ShiftAmt)));
+          return InsertNewInstWith(Mul, I->getIterator());
+        }
       }
 
       // Unsigned shift right.

llvm/test/Transforms/InstCombine/lshr.ll

@@ -1527,9 +1527,8 @@ define <2 x i8> @bool_add_lshr_vec_wrong_shift_amt(<2 x i1> %a, <2 x i1> %b) {
 define i32 @lowbits_of_lshr_mul(i64 %x) {
 ; CHECK-LABEL: @lowbits_of_lshr_mul(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[MUL:%.*]] = mul i64 [[X:%.*]], 64424509440
-; CHECK-NEXT:    [[SHIFT:%.*]] = lshr exact i64 [[MUL]], 32
-; CHECK-NEXT:    [[CONV:%.*]] = trunc nuw i64 [[SHIFT]] to i32
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[X:%.*]] to i32
+; CHECK-NEXT:    [[CONV:%.*]] = mul i32 [[TMP0]], 15
 ; CHECK-NEXT:    ret i32 [[CONV]]
 ;
 entry:
@@ -1542,9 +1541,8 @@ entry:
 define i32 @lowbits_of_lshr_mul_mask(i32 %x) {
 ; CHECK-LABEL: @lowbits_of_lshr_mul_mask(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[MUL:%.*]] = mul i32 [[X:%.*]], 104857600
-; CHECK-NEXT:    [[SHIFT:%.*]] = lshr exact i32 [[MUL]], 16
-; CHECK-NEXT:    [[CONV:%.*]] = and i32 [[SHIFT]], 32704
+; CHECK-NEXT:    [[TMP0:%.*]] = mul i32 [[X:%.*]], 1600
+; CHECK-NEXT:    [[CONV:%.*]] = and i32 [[TMP0]], 32704
 ; CHECK-NEXT:    ret i32 [[CONV]]
 ;
 entry: