[InstCombine] Fold abs(a * abs(b)) --> abs(a * b)

[elhewaty]

• [InstCombine] Add test coverage for abs(b * abs(a)) --> abs(a * b) (NFC)
• [InstCombine] Fold abs(b * abs(a)) --> abs(a * b)
• alive2: https://alive2.llvm.org/ce/z/hfbEra
• Fixes: abs(b * abs(a)) --> abs(a * b) #73211

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: None (elhewaty)

Changes

• [InstCombine] Add test coverage for abs(b * abs(a)) --> abs(a * b) (NFC)
• [InstCombine] Fold abs(b * abs(a)) --> abs(a * b)
• alive2: https://alive2.llvm.org/ce/z/cXQqGd
• Fixes: abs(b * abs(a)) --> abs(a * b) #73211

---

Full diff: https://github.com/llvm/llvm-project/pull/78110.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp (+8)
• (modified) llvm/test/Transforms/InstCombine/abs-intrinsic.ll (+49)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 64fbd5543a9e203..978a7c1e256cef6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1601,6 +1601,14 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
       return CastInst::Create(Instruction::ZExt, NarrowAbs, II->getType());
     }
 
+    // abs(a * abs(b)) --> abs(a * b)
+    Value *A, *B;
+    if (match(IIOperand,
+              m_c_Mul(m_Value(A), m_Intrinsic<Intrinsic::abs>(m_Value(B))))) {
+      Value *New = Builder.CreateMul(A, B);
+      return replaceOperand(*II, 0, New);
+    }
+
     // Match a complicated way to check if a number is odd/even:
     // abs (srem X, 2) --> and X, 1
     const APInt *C;
diff --git a/llvm/test/Transforms/InstCombine/abs-intrinsic.ll b/llvm/test/Transforms/InstCombine/abs-intrinsic.ll
index 7fe34d923764857..8f31cb58dc8f14b 100644
--- a/llvm/test/Transforms/InstCombine/abs-intrinsic.ll
+++ b/llvm/test/Transforms/InstCombine/abs-intrinsic.ll
@@ -5,8 +5,57 @@ declare i8 @llvm.abs.i8(i8, i1)
 declare i32 @llvm.abs.i32(i32, i1)
 declare <4 x i32> @llvm.abs.v4i32(<4 x i32>, i1)
 declare <3 x i82> @llvm.abs.v3i82(<3 x i82>, i1)
+declare <2 x i8> @llvm.abs.v2i8(<2 x i8>, i1)
 declare void @llvm.assume(i1)
 
+define i8 @test_abs_abs_a_mul_b_i8(i8 %a, i8 %b) {
+; CHECK-LABEL: @test_abs_abs_a_mul_b_i8(
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i8 [[B:%.*]], [[A:%.*]]
+; CHECK-NEXT:    [[ABS2:%.*]] = call i8 @llvm.abs.i8(i8 [[TMP1]], i1 true)
+; CHECK-NEXT:    ret i8 [[ABS2]]
+;
+  %abs1 = call i8 @llvm.abs.i8(i8 %a, i1 true)
+  %mul = mul i8 %abs1, %b
+  %abs2 = call i8 @llvm.abs.i8(i8 %mul, i1 true)
+  ret i8 %abs2
+}
+
+define i8 @test_abs_a_mul_abs_b_i8(i8 %a, i8 %b) {
+; CHECK-LABEL: @test_abs_a_mul_abs_b_i8(
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i8 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[ABS2:%.*]] = call i8 @llvm.abs.i8(i8 [[TMP1]], i1 true)
+; CHECK-NEXT:    ret i8 [[ABS2]]
+;
+  %abs1 = call i8 @llvm.abs.i8(i8 %b, i1 true)
+  %mul = mul i8 %abs1, %a
+  %abs2 = call i8 @llvm.abs.i8(i8 %mul, i1 true)
+  ret i8 %abs2
+}
+
+define i32 @test_abs_abs_a_mul_b_i16(i32 %a, i32 %b) {
+; CHECK-LABEL: @test_abs_abs_a_mul_b_i16(
+; CHECK-NEXT:    [[TMP1:%.*]] = mul i32 [[B:%.*]], [[A:%.*]]
+; CHECK-NEXT:    [[ABS2:%.*]] = call i32 @llvm.abs.i32(i32 [[TMP1]], i1 true)
+; CHECK-NEXT:    ret i32 [[ABS2]]
+;
+  %abs1 = call i32 @llvm.abs.i32(i32 %a, i1 true)
+  %mul = mul i32 %abs1, %b
+  %abs2 = call i32 @llvm.abs.i32(i32 %mul, i1 true)
+  ret i32 %abs2
+}
+
+define <2 x i8> @test_abs_abs_a_mul_b_vector_i8(<2 x i8> %a, <2 x i8> %b) {
+; CHECK-LABEL: @test_abs_abs_a_mul_b_vector_i8(
+; CHECK-NEXT:    [[TMP1:%.*]] = mul <2 x i8> [[B:%.*]], [[A:%.*]]
+; CHECK-NEXT:    [[ABS2:%.*]] = call <2 x i8> @llvm.abs.v2i8(<2 x i8> [[TMP1]], i1 true)
+; CHECK-NEXT:    ret <2 x i8> [[ABS2]]
+;
+  %abs = call <2 x i8> @llvm.abs.v2i8(<2 x i8> %a, i1 true)
+  %mul = mul <2 x i8> %abs, %b
+  %abs2 = call <2 x i8> @llvm.abs.v2i8(<2 x i8> %mul, i1 true)
+  ret <2 x i8> %abs2
+}
+
 ; abs preserves trailing zeros so the second and is unneeded
 define i32 @abs_trailing_zeros(i32 %x) {
 ; CHECK-LABEL: @abs_trailing_zeros(

[elhewaty]

cc @dtcxzyw @goldsteinn

[elhewaty]

@nikic Can you please review this again?
and what is wrong with the build?

[elhewaty]

@dtcxzyw, please share your thoughts.

[elhewaty]

@nikic @goldsteinn ping

[elhewaty]

@dtcxzyw ping

[nikic]

LGTM