1 files changed, 49 insertions, 0 deletions
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index f9f82cdeef4..2b0d8c903b4 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -11400,6 +11400,55 @@ CheckImplicitConversion(Sema &S, Expr *E, QualType T, SourceLocation CC,
     }
   }
 
+  // If we are casting an integer type to a floating point type, we might
+  // lose accuracy if the floating point type has a narrower significand
+  // than the integer type. Issue warnings for that accuracy loss. 
+  if (SourceBT && TargetBT && SourceBT->isIntegerType() &&
+      TargetBT->isFloatingType()) {
+    // Determine the number of precision bits in the source integer type.
+    IntRange SourceRange = GetExprRange(S.Context, E, S.isConstantEvaluated());
+    unsigned int SourcePrecision = SourceRange.Width;
+
+    // Determine the number of precision bits in the
+    // target floating point type.
+    unsigned int TargetPrecision = llvm::APFloatBase::semanticsPrecision(
+      S.Context.getFloatTypeSemantics(QualType(TargetBT, 0)));
+
+    if (SourcePrecision > 0 && TargetPrecision > 0 &&
+        SourcePrecision > TargetPrecision) {
+
+      llvm::APSInt SourceInt;
+      if (E->isIntegerConstantExpr(SourceInt, S.Context)) {
+        // If the source integer is a constant, convert it to the target
+        // floating point type. Issue a warning if the value changes
+        // during the whole conversion.
+        llvm::APFloat TargetFloatValue(
+          S.Context.getFloatTypeSemantics(QualType(TargetBT, 0)));
+        llvm::APFloat::opStatus ConversionStatus =
+          TargetFloatValue.convertFromAPInt(SourceInt,
+          SourceBT->isSignedInteger(), llvm::APFloat::rmNearestTiesToEven);
+
+        if (ConversionStatus != llvm::APFloat::opOK) {
+          std::string PrettySourceValue = SourceInt.toString(10);
+          SmallString<32> PrettyTargetValue;
+          TargetFloatValue.toString(PrettyTargetValue,
+            TargetPrecision);
+          
+          S.DiagRuntimeBehavior(
+            E->getExprLoc(), E,
+            S.PDiag(diag::warn_impcast_integer_float_precision_constant)
+              << PrettySourceValue << PrettyTargetValue
+              << E->getType() << T
+              << E->getSourceRange() << clang::SourceRange(CC));
+        }
+      } else {
+        // Otherwise, the implicit conversion may lose precision.
+        DiagnoseImpCast(S, E, T, CC,
+          diag::warn_impcast_integer_float_precision);
+      }
+    }
+  }
+
   DiagnoseNullConversion(S, E, T, CC);
 
   S.DiscardMisalignedMemberAddress(Target, E);