[clang] [CLANG] Add warning when comparing to INF or NAN in fast math mode. (PR #76873)

[Andy Kaylor via cfe-commits]

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@@ -13846,6 +13880,37 @@ Sema::CheckReturnValExpr(Expr *RetValExp, QualType lhsType,
     CheckPPCMMAType(RetValExp->getType(), ReturnLoc);
 }
 
+/// Diagnose comparison to NAN or INFINITY in fast math modes.
+/// The comparison to NaN or INFINITY is always false in
+/// fast modes: float evaluation will not result in inf or nan.
+void Sema::CheckInfNaNFloatComparison(SourceLocation Loc, Expr *LHS, Expr *RHS,
+                                      BinaryOperatorKind Opcode) {
+  Expr *LeftExprSansParen = LHS->IgnoreParenImpCasts();
+  Expr *RightExprSansParen = RHS->IgnoreParenImpCasts();
+
+  FPOptions FPO = LHS->getFPFeaturesInEffect(getLangOpts());
+  bool NoHonorNaNs = FPO.getNoHonorNaNs();
+  bool NoHonorInfs = FPO.getNoHonorInfs();
+  llvm::APFloat Value(0.0);
+  bool IsConstant;
+  IsConstant = !LHS->isValueDependent() &&
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andykaylor wrote:

You could use a lambda here to avoid code duplication for LHS and RHS checking.

https://github.com/llvm/llvm-project/pull/76873