Enhanced implementation of -Wfloat-equal to check for comparisons against

floating-point literals that are represented exactly by the APFloat in
FloatingLiteral. For such literals, we do not emit a warning since such checks are
often performed in real code to see if a variable has changed from its original
value. This heuristic clearly can lead to false negatives, but the hope is it will
significantly reduce false positives to help make the compiler flag more useful.

llvm-svn: 44424

clang/Sema/SemaChecking.cpp

@@ -715,6 +715,24 @@ void Sema::CheckFloatComparison(SourceLocation loc, Expr* lex, Expr *rex) {
       if (DRL->getDecl() == DRR->getDecl())
         EmitWarning = false;
   
+  
+  // Special case: check for comparisons against literals that can be exactly
+  //  represented by APFloat.  In such cases, do not emit a warning.  This
+  //  is a heuristic: often comparison against such literals are used to
+  //  detect if a value in a variable has not changed.  This clearly can
+  //  lead to false negatives.
+  if (EmitWarning) {
+    if (FloatingLiteral* FLL = dyn_cast<FloatingLiteral>(LeftExprSansParen)) {
+      if (FLL->isExact())
+        EmitWarning = false;
+    }
+    else
+      if (FloatingLiteral* FLR = dyn_cast<FloatingLiteral>(RightExprSansParen)){
+        if (FLR->isExact())
+          EmitWarning = false;
+    }
+  }
+  
   // Check for comparisons with builtin types.
   if (EmitWarning)           
     if (CallExpr* CL = dyn_cast<CallExpr>(LeftExprSansParen))