[InstCombine] Fold abs(-x) -> abs(x)

[craig.topper]

Summary

Negating the input doesn't matter. I left a FIXME to copy the nsw flag if its present on the neg but not on the abs.

[nikic]

LG. I was wondering if we can use any of our more powerful negation machinery here, but I don't think so, because it would end up looping infinitely.

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);
+    // 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);
+
+    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))))) {
       unsigned C = X->getType()->getPrimitiveSizeInBits() -
         IIOperand->getType()->getPrimitiveSizeInBits();

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

 ; CHECK-LABEL: @abs_known_not_int_min_vec(
 ; CHECK-NEXT:    ret <4 x i1> <i1 true, i1 true, i1 true, i1 true>
 ;
   %or = or <4 x i32> %x, <i32 1, i32 1, i32 1, i32 1>
   %abs = call <4 x i32> @llvm.abs.v4i32(<4 x i32> %or, i1 false)
   %c2 = icmp sge <4 x i32> %abs, zeroinitializer
   ret <4 x i1> %c2
 }
+
+define i32 @abs_of_neg(i32 %x) {
+; CHECK-LABEL: @abs_of_neg(
+; CHECK-NEXT:    [[B:%.*]] = call i32 @llvm.abs.i32(i32 [[X:%.*]], i1 false)
+; CHECK-NEXT:    ret i32 [[B]]
+;
+  %a = sub i32 0, %x
+  %b = call i32 @llvm.abs.i32(i32 %a, i1 false)
+  ret i32 %b
+}
+
+define <4 x i32> @abs_of_neg_vec(<4 x i32> %x) {
+; CHECK-LABEL: @abs_of_neg_vec(
+; CHECK-NEXT:    [[B:%.*]] = call <4 x i32> @llvm.abs.v4i32(<4 x i32> [[X:%.*]], i1 false)
+; CHECK-NEXT:    ret <4 x i32> [[B]]
+;
+  %a = sub nsw <4 x i32> zeroinitializer, %x
+  %b = call <4 x i32> @llvm.abs.v4i32(<4 x i32> %a, i1 false)
+  ret <4 x i32> %b
+}