[InstCombine] Fold abs with dominating condition

[nikic]

Summary

Similar to 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.

[spatel]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
   Intrinsic::ID IID = II->getIntrinsicID();
   switch (IID) {
   case Intrinsic::objectsize:
     if (Value *V = lowerObjectSizeCall(II, DL, &TLI, /*MustSucceed=*/false))
       return replaceInstUsesWith(CI, V);
     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;
     if (match(IIOperand, m_Neg(m_Value(X))))
       return replaceOperand(*II, 0, X);
     if (match(IIOperand, m_Select(m_Value(), m_Value(X), m_Neg(m_Deferred(X)))))
       return replaceOperand(*II, 0, X);
     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: {
     Value *IIOperand = II->getArgOperand(0);
     Value *X = nullptr;
 
     // bswap(trunc(bswap(x))) -> trunc(lshr(x, c))
     if (match(IIOperand, m_Trunc(m_BSwap(m_Value(X))))) {

llvm/test/Transforms/InstCombine/abs-intrinsic.ll

;
   ret <4 x i32> %abs
 }
 
 define i32 @abs_dom_cond_nopoison(i32 %x) {
 ; CHECK-LABEL: @abs_dom_cond_nopoison(
 ; 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
   br i1 %cmp, label %true, label %false
 
 true:
   %a1 = call i32 @llvm.abs.i32(i32 %x, i1 false)
   ret i32 %a1
 
 false:
   %a2 = call i32 @llvm.abs.i32(i32 %x, i1 false)
   ret i32 %a2
 }
 
 define i32 @abs_dom_cond_poison(i32 %x) {
 ; CHECK-LABEL: @abs_dom_cond_poison(
 ; 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
   br i1 %cmp, label %true, label %false
 
 true:
   %a1 = call i32 @llvm.abs.i32(i32 %x, i1 true)
   ret i32 %a1
 
 false:
   %a2 = call i32 @llvm.abs.i32(i32 %x, i1 true)
   ret i32 %a2
 }