diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 139a0b81e299b..b7a1d75e408fa 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -2549,6 +2549,9 @@ 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);
@@ -2602,6 +2605,9 @@ 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;
diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/atan-intrinsic.ll b/llvm/test/Transforms/InstSimplify/ConstProp/atan-intrinsic.ll
index c5c17d65524c2..d824d6d35643d 100644
--- a/llvm/test/Transforms/InstSimplify/ConstProp/atan-intrinsic.ll
+++ b/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() {
diff --git a/llvm/test/Transforms/InstSimplify/ConstProp/calls.ll b/llvm/test/Transforms/InstSimplify/ConstProp/calls.ll
index 61a30c781c0f4..26fb8c0d7a1c6 100644
--- a/llvm/test/Transforms/InstSimplify/ConstProp/calls.ll
+++ b/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