[PATCH] D87184: [InstCombine] Fold abs with dominating condition

[Nikita Popov via Phabricator via llvm-commits]

nikic created this revision.
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Similar to D87168 <https://reviews.llvm.org/D87168>, but for abs. If we have a dominating x >= 0 condition, then we know that abs(x) is x. This fold is in InstCombine, because we need to create a sub instruction for the x < 0 case.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D87184

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/test/Transforms/InstCombine/abs-intrinsic.ll


Index: llvm/test/Transforms/InstCombine/abs-intrinsic.ll
===================================================================
--- llvm/test/Transforms/InstCombine/abs-intrinsic.ll
+++ llvm/test/Transforms/InstCombine/abs-intrinsic.ll
@@ -132,10 +132,9 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[X:%.*]], -1
 ; CHECK-NEXT:    br i1 [[CMP]], label [[TRUE:%.*]], label [[FALSE:%.*]]
 ; CHECK:       true:
-; CHECK-NEXT:    [[A1:%.*]] = call i32 @llvm.abs.i32(i32 [[X]], i1 false)
-; CHECK-NEXT:    ret i32 [[A1]]
+; CHECK-NEXT:    ret i32 [[X]]
 ; CHECK:       false:
-; CHECK-NEXT:    [[A2:%.*]] = call i32 @llvm.abs.i32(i32 [[X]], i1 false)
+; CHECK-NEXT:    [[A2:%.*]] = sub i32 0, [[X]]
 ; CHECK-NEXT:    ret i32 [[A2]]
 ;
   %cmp = icmp sge i32 %x, 0
@@ -155,10 +154,9 @@
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[X:%.*]], -1
 ; CHECK-NEXT:    br i1 [[CMP]], label [[TRUE:%.*]], label [[FALSE:%.*]]
 ; CHECK:       true:
-; CHECK-NEXT:    [[A1:%.*]] = call i32 @llvm.abs.i32(i32 [[X]], i1 true)
-; CHECK-NEXT:    ret i32 [[A1]]
+; CHECK-NEXT:    ret i32 [[X]]
 ; CHECK:       false:
-; CHECK-NEXT:    [[A2:%.*]] = call i32 @llvm.abs.i32(i32 [[X]], i1 true)
+; CHECK-NEXT:    [[A2:%.*]] = sub nsw i32 0, [[X]]
 ; CHECK-NEXT:    ret i32 [[A2]]
 ;
   %cmp = icmp sge i32 %x, 0
Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -776,6 +776,8 @@
     return nullptr;
   case Intrinsic::abs: {
     Value *IIOperand = II->getArgOperand(0);
+    bool IntMinIsPoison = cast<Constant>(II->getArgOperand(1))->isOneValue();
+
     // abs(-x) -> abs(x)
     // TODO: Copy nsw if it was present on the neg?
     Value *X;
@@ -786,6 +788,19 @@
     if (match(IIOperand, m_Select(m_Value(), m_Neg(m_Value(X)), m_Deferred(X))))
       return replaceOperand(*II, 0, X);
 
+    if (Optional<bool> Imp = isImpliedByDomCondition(
+            ICmpInst::ICMP_SGE, IIOperand,
+            Constant::getNullValue(IIOperand->getType()), II, DL)) {
+      // abs(x) -> x if x >= 0
+      if (*Imp)
+        return replaceInstUsesWith(*II, IIOperand);
+
+      // abs(x) -> -x if x < 0
+      if (IntMinIsPoison)
+        return BinaryOperator::CreateNSWNeg(IIOperand);
+      return BinaryOperator::CreateNeg(IIOperand);
+    }
+
     break;
   }
   case Intrinsic::bswap: {


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