[llvm] [InstCombine] Fold `(X * (Y << K)) u>> K -> X * Y` when highbits are not demanded (PR #111151)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Alive2: https://alive2.llvm.org/ce/z/Z7QgjH


---
Full diff: https://github.com/llvm/llvm-project/pull/111151.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (+9) 
- (modified) llvm/test/Transforms/InstCombine/lshr.ll (+62) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index ee6b60f7f70d68..b330a8ad9f8a6c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/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.
diff --git a/llvm/test/Transforms/InstCombine/lshr.ll b/llvm/test/Transforms/InstCombine/lshr.ll
index 4360714c78caa6..ccc2b61b2989af 100644
--- a/llvm/test/Transforms/InstCombine/lshr.ll
+++ b/llvm/test/Transforms/InstCombine/lshr.ll
@@ -1523,3 +1523,65 @@ define <2 x i8> @bool_add_lshr_vec_wrong_shift_amt(<2 x i1> %a, <2 x i1> %b) {
   %lshr = lshr <2 x i8> %add, <i8 1, i8 2>
   ret <2 x i8> %lshr
 }
+
+define i32 @lowbits_of_lshr_mul(i64 %x) {
+; CHECK-LABEL: @lowbits_of_lshr_mul(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = trunc i64 [[X:%.*]] to i32
+; CHECK-NEXT:    [[CONV:%.*]] = mul i32 [[TMP0]], 15
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %mul = mul i64 %x, 64424509440
+  %shift = lshr i64 %mul, 32
+  %conv = trunc i64 %shift to i32
+  ret i32 %conv
+}
+
+define i32 @lowbits_of_lshr_mul_mask(i32 %x) {
+; CHECK-LABEL: @lowbits_of_lshr_mul_mask(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = mul i32 [[X:%.*]], 1600
+; CHECK-NEXT:    [[CONV:%.*]] = and i32 [[TMP0]], 32704
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %mul = mul i32 %x, 104857600
+  %shift = lshr i32 %mul, 16
+  %conv = and i32 %shift, 32767
+  ret i32 %conv
+}
+
+; Negative tests
+
+define i32 @lowbits_of_lshr_mul_mask_multiuse(i32 %x) {
+; CHECK-LABEL: @lowbits_of_lshr_mul_mask_multiuse(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[MUL:%.*]] = mul i32 [[X:%.*]], 104857600
+; CHECK-NEXT:    call void @use(i32 [[MUL]])
+; CHECK-NEXT:    [[SHIFT:%.*]] = lshr exact i32 [[MUL]], 16
+; CHECK-NEXT:    [[CONV:%.*]] = and i32 [[SHIFT]], 32704
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %mul = mul i32 %x, 104857600
+  call void @use(i32 %mul)
+  %shift = lshr i32 %mul, 16
+  %conv = and i32 %shift, 32767
+  ret i32 %conv
+}
+
+define i32 @lowbits_of_lshr_mul_mask_indivisible(i32 %x) {
+; CHECK-LABEL: @lowbits_of_lshr_mul_mask_indivisible(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[MUL:%.*]] = mul i32 [[X:%.*]], 25600
+; CHECK-NEXT:    [[SHIFT:%.*]] = lshr i32 [[MUL]], 16
+; CHECK-NEXT:    [[CONV:%.*]] = and i32 [[SHIFT]], 32767
+; CHECK-NEXT:    ret i32 [[CONV]]
+;
+entry:
+  %mul = mul i32 %x, 25600
+  %shift = lshr i32 %mul, 16
+  %conv = and i32 %shift, 32767
+  ret i32 %conv
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/111151