[llvm] [DAG] computeKnownBits - abs(x) will be zero in the upper bits if x is sign-extended (PR #94382)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Simon Pilgrim (RKSimon)

<details>
<summary>Changes</summary>

As reported on https://github.com/llvm/llvm-project/issues/94344 - if x has more than one signbit, then the upper bits of its absolute value are guaranteed to be zero

Alive2: https://alive2.llvm.org/ce/z/a87fHU

---
Full diff: https://github.com/llvm/llvm-project/pull/94382.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (+2) 
- (modified) llvm/test/CodeGen/X86/combine-abs.ll (+44) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 414c724b94f7b..6c9b64810c33b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4051,6 +4051,8 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
   case ISD::ABS: {
     Known2 = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
     Known = Known2.abs();
+    Known.Zero.setHighBits(
+        ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1) - 1);
     break;
   }
   case ISD::USUBSAT: {
diff --git a/llvm/test/CodeGen/X86/combine-abs.ll b/llvm/test/CodeGen/X86/combine-abs.ll
index 202c88109eaeb..76ee02e798707 100644
--- a/llvm/test/CodeGen/X86/combine-abs.ll
+++ b/llvm/test/CodeGen/X86/combine-abs.ll
@@ -201,6 +201,50 @@ define <8 x i32> @combine_v8i32_abs_pos(<8 x i32> %a) {
   ret <8 x i32> %2
 }
 
+; (abs x) upper bits are known zero if x has extra sign bits
+define i32 @combine_i32_abs_zerosign(i32 %a) {
+; CHECK-LABEL: combine_i32_abs_zerosign:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    retq
+  %1 = ashr i32 %a, 15
+  %2 = call i32 @llvm.abs.i32(i32 %1, i1 false)
+  %3 = and i32 %2, -524288 ; 0xFFF80000
+  ret i32 %3
+}
+
+define <8 x i16> @combine_v8i16_abs_zerosign(<8 x i16> %a) {
+; SSE-LABEL: combine_v8i16_abs_zerosign:
+; SSE:       # %bb.0:
+; SSE-NEXT:    xorps %xmm0, %xmm0
+; SSE-NEXT:    retq
+;
+; AVX-LABEL: combine_v8i16_abs_zerosign:
+; AVX:       # %bb.0:
+; AVX-NEXT:    vxorps %xmm0, %xmm0, %xmm0
+; AVX-NEXT:    retq
+  %1 = ashr <8 x i16> %a, <i16 7, i16 8, i16 9, i16 10, i16 11, i16 12, i16 13, i16 14>
+  %2 = call <8 x i16> @llvm.abs.v8i16(<8 x i16> %1, i1 false)
+  %3 = and <8 x i16> %2, <i16 32768, i16 32768, i16 32768, i16 32768, i16 32768, i16 32768, i16 32768, i16 32768>
+  ret <8 x i16> %3
+}
+
+; negative test - mask extends beyond known zero bits
+define i32 @combine_i32_abs_zerosign_negative(i32 %a) {
+; CHECK-LABEL: combine_i32_abs_zerosign_negative:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    sarl $3, %edi
+; CHECK-NEXT:    movl %edi, %eax
+; CHECK-NEXT:    negl %eax
+; CHECK-NEXT:    cmovsl %edi, %eax
+; CHECK-NEXT:    andl $536346624, %eax # imm = 0x1FF80000
+; CHECK-NEXT:    retq
+  %1 = ashr i32 %a, 3
+  %2 = call i32 @llvm.abs.i32(i32 %1, i1 false)
+  %3 = and i32 %2, -524288 ; 0xFFF80000
+  ret i32 %3
+}
+
 declare <16 x i8> @llvm.abs.v16i8(<16 x i8>, i1) nounwind readnone
 declare <4 x i32> @llvm.abs.v4i32(<4 x i32>, i1) nounwind readnone
 declare <8 x i16> @llvm.abs.v8i16(<8 x i16>, i1) nounwind readnone

``````````

</details>


https://github.com/llvm/llvm-project/pull/94382