Treat the range of representable values of floating-point types as [-inf, +inf] not as [-max, +max].

Summary:
Prior to r329065, we used [-max, max] as the range of representable
values because LLVM's `fptrunc` did not guarantee defined behavior when
truncating from a larger floating-point type to a smaller one. Now that
has been fixed, we can make clang follow normal IEEE 754 semantics in this
regard and take the larger range [-inf, +inf] as the range of representable
values.

In practice, this affects two parts of the frontend:
 * the constant evaluator no longer treats floating-point evaluations
   that result in +-inf as being undefined (because they no longer leave
   the range of representable values of the type)
 * UBSan no longer treats conversions to floating-point type that are
   outside the [-max, +max] range as being undefined

In passing, also remove the float-divide-by-zero sanitizer from
-fsanitize=undefined, on the basis that while it's undefined per C++
rules (and we disallow it in constant expressions for that reason), it
is defined by Clang / LLVM / IEEE 754.

Reviewers: rnk, BillyONeal

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63793

llvm-svn: 365272

1 files changed, 13 insertions, 9 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 5162eb16794..f01b42e7ff7 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -2212,10 +2212,8 @@ static bool HandleFloatToFloatCast(EvalInfo &Info, const Expr *E,
                                    APFloat &Result) {
   APFloat Value = Result;
   bool ignored;
-  if (Result.convert(Info.Ctx.getFloatTypeSemantics(DestType),
-                     APFloat::rmNearestTiesToEven, &ignored)
-      & APFloat::opOverflow)
-    return HandleOverflow(Info, E, Value, DestType);
+  Result.convert(Info.Ctx.getFloatTypeSemantics(DestType),
+                 APFloat::rmNearestTiesToEven, &ignored);
   return true;
 }
 
@@ -2236,10 +2234,8 @@ static bool HandleIntToFloatCast(EvalInfo &Info, const Expr *E,
                                  QualType SrcType, const APSInt &Value,
                                  QualType DestType, APFloat &Result) {
   Result = APFloat(Info.Ctx.getFloatTypeSemantics(DestType), 1);
-  if (Result.convertFromAPInt(Value, Value.isSigned(),
-                              APFloat::rmNearestTiesToEven)
-      & APFloat::opOverflow)
-    return HandleOverflow(Info, E, Value, DestType);
+  Result.convertFromAPInt(Value, Value.isSigned(),
+                          APFloat::rmNearestTiesToEven);
   return true;
 }
 
@@ -2457,11 +2453,19 @@ static bool handleFloatFloatBinOp(EvalInfo &Info, const Expr *E,
     LHS.subtract(RHS, APFloat::rmNearestTiesToEven);
     break;
   case BO_Div:
+    // [expr.mul]p4:
+    //   If the second operand of / or % is zero the behavior is undefined.
+    if (RHS.isZero())
+      Info.CCEDiag(E, diag::note_expr_divide_by_zero);
     LHS.divide(RHS, APFloat::rmNearestTiesToEven);
     break;
   }
 
-  if (LHS.isInfinity() || LHS.isNaN()) {
+  // [expr.pre]p4:
+  //   If during the evaluation of an expression, the result is not
+  //   mathematically defined [...], the behavior is undefined.
+  // FIXME: C++ rules require us to not conform to IEEE 754 here.
+  if (LHS.isNaN()) {
     Info.CCEDiag(E, diag::note_constexpr_float_arithmetic) << LHS.isNaN();
     return Info.noteUndefinedBehavior();
   }