[ConstantFold] Special case atan +/-0.0 (llvm#143962)

C's Annex F specifies that atan +/-0.0 returns the input value;
however, this behavior is optional and host C libraries may behave
differently. This change applies the Annex F behavior to constant
folding by LLVM.

Ref:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/atan.html

Author: lei137

llvm/lib/Analysis/ConstantFolding.cpp

@@ -2553,6 +2553,9 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
       case Intrinsic::cosh:
         return ConstantFoldFP(cosh, APF, Ty);
       case Intrinsic::atan:
+        // Implement optional behavior from C's Annex F for +/-0.0.
+        if (U.isZero())
+          return ConstantFP::get(Ty->getContext(), U);
         return ConstantFoldFP(atan, APF, Ty);
       case Intrinsic::sqrt:
         return ConstantFoldFP(sqrt, APF, Ty);
@@ -2606,6 +2609,9 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
       break;
     case LibFunc_atan:
     case LibFunc_atanf:
+      // Implement optional behavior from C's Annex F for +/-0.0.
+      if (U.isZero())
+        return ConstantFP::get(Ty->getContext(), U);
       if (TLI->has(Func))
         return ConstantFoldFP(atan, APF, Ty);
       break;

llvm/test/Transforms/InstSimplify/ConstProp/atan-intrinsic.ll

@@ -1,6 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
 ; RUN: opt -S -passes=instsimplify < %s | FileCheck %s
-; XFAIL: target={{.*}}-aix{{.*}}
 
 define double @test_atan_0() {
 ; CHECK-LABEL: define double @test_atan_0() {

llvm/test/Transforms/InstSimplify/ConstProp/calls.ll

@@ -202,5 +202,17 @@ entry:
   ret float %0
 }
 
+define float @test_atan_negzero() nounwind uwtable ssp {
+entry:
+; CHECK-LABEL: @test_atan_negzero(
+; CHECK:    ret float -0.000000e+00
+;
+; FNOBUILTIN-LABEL: @test_atan_negzero(
+; FNOBUILTIN:  ret float -0.000000e+00
+;
+  %1 = call float @atanf(float -0.0)
+  ret float %1
+}
+
 declare double @llvm.pow.f64(double, double) nounwind readonly
 declare float @llvm.pow.f32(float, float) nounwind readonly