[llvm] [SimplifyLibCalls] Simplify cabs libcall if real or imaginary part of input is zero (PR #97976)

[Matt Arsenault via llvm-commits]

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@@ -1880,25 +1881,45 @@ static Value *optimizeBinaryDoubleFP(CallInst *CI, IRBuilderBase &B,
 
 // cabs(z) -> sqrt((creal(z)*creal(z)) + (cimag(z)*cimag(z)))
 Value *LibCallSimplifier::optimizeCAbs(CallInst *CI, IRBuilderBase &B) {
-  if (!CI->isFast())
-    return nullptr;
-
-  // Propagate fast-math flags from the existing call to new instructions.
-  IRBuilderBase::FastMathFlagGuard Guard(B);
-  B.setFastMathFlags(CI->getFastMathFlags());
-
   Value *Real, *Imag;
+
   if (CI->arg_size() == 1) {
     Value *Op = CI->getArgOperand(0);
     assert(Op->getType()->isArrayTy() && "Unexpected signature for cabs!");
     Real = B.CreateExtractValue(Op, 0, "real");
     Imag = B.CreateExtractValue(Op, 1, "imag");
+
   } else {
     assert(CI->arg_size() == 2 && "Unexpected signature for cabs!");
     Real = CI->getArgOperand(0);
     Imag = CI->getArgOperand(1);
   }
 
+  // if real or imaginary part is zero, simplify to abs(cimag(z))
+  // or abs(creal(z))
+  if (ConstantFP *ConstReal = dyn_cast<ConstantFP>(Real)) {
+    if (ConstReal->isZeroValue()) {
+      IRBuilderBase::FastMathFlagGuard Guard(B);
+      B.setFastMathFlags(CI->getFastMathFlags());
+      return copyFlags(
+          *CI, B.CreateUnaryIntrinsic(Intrinsic::fabs, Imag, nullptr, "cabs"));
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arsenm wrote:

There's no reason to ever use a fabs libcall. Just always use the fabs intrinsic 

https://github.com/llvm/llvm-project/pull/97976