[ConstantFold] Special case log1p +/-0.0 (llvm#114635)

C's Annex F specifies that log1p +/-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.

Author: hubert-reinterpretcast

llvm/lib/Analysis/ConstantFolding.cpp

@@ -2407,6 +2407,9 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
       break;
     case LibFunc_log1p:
     case LibFunc_log1pf:
+      // Implement optional behavior from C's Annex F for +/-0.0.
+      if (U.isZero())
+        return ConstantFP::get(Ty->getContext(), U);
       if (APF > APFloat::getOne(APF.getSemantics(), true) && TLI->has(Func))
         return ConstantFoldFP(log1p, APF, Ty);
       break;

llvm/test/Transforms/InstCombine/log1p.ll

@@ -1,4 +1,3 @@
-; XFAIL: target={{.*}}-aix{{.*}}
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
 ; RUN: opt < %s -passes=instcombine -S | FileCheck %s