Treat constant contexts as being in the default rounding mode.

This addresses a regression where pretty much all C++ compilations using
-frounding-math now fail, due to rounding being performed in constexpr
function definitions in the standard library.

This follows the "manifestly constant evaluated" approach described in
https://reviews.llvm.org/D87528#2270676 -- evaluations that are required
to succeed at compile time are permitted even in regions with dynamic
rounding modes, as are (unfortunately) the evaluation of the
initializers of local variables of const integral types.

Differential Revision: https://reviews.llvm.org/D89360

clang/lib/AST/ExprConstant.cpp

@@ -2528,6 +2528,11 @@ static llvm::RoundingMode getActiveRoundingMode(EvalInfo &Info, const Expr *E,
 /// Check if the given evaluation result is allowed for constant evaluation.
 static bool checkFloatingPointResult(EvalInfo &Info, const Expr *E,
                                      APFloat::opStatus St) {
+  // In a constant context, assume that any dynamic rounding mode or FP
+  // exception state matches the default floating-point environment.
+  if (Info.InConstantContext)
+    return true;
+
   FPOptions FPO = E->getFPFeaturesInEffect(Info.Ctx.getLangOpts());
   if ((St & APFloat::opInexact) &&
       FPO.getRoundingMode() == llvm::RoundingMode::Dynamic) {

clang/test/Sema/rounding-math.c

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -triple x86_64-linux -verify=norounding %s
+// RUN: %clang_cc1 -triple x86_64-linux -std=c17 -verify=rounding-std %s -frounding-math
+// RUN: %clang_cc1 -triple x86_64-linux -std=gnu17 -verify=rounding-gnu %s -frounding-math
+
+#define fold(x) (__builtin_constant_p(x) ? (x) : (x))
+
+double a = 1.0 / 3.0;
+
+#define f(n) ((n) * (1.0 / 3.0))
+_Static_assert(fold((int)f(3)) == 1, "");
+
+typedef int T[fold((int)f(3))];
+typedef int T[1];
+
+enum Enum { enum_a = (int)f(3) };
+
+struct Bitfield {
+  unsigned int n : 1;
+  unsigned int m : fold((int)f(3));
+};
+
+void bitfield(struct Bitfield *b) {
+  b->n = (int)(6 * (1.0 / 3.0)); // norounding-warning {{changes value from 2 to 0}}
+}
+
+void vlas() {
+  // Under -frounding-math, this is a VLA.
+  // FIXME: Due to PR44406, in GNU mode we constant-fold the initializer resulting in a non-VLA.
+  typedef int vla[(int)(-3 * (1.0 / 3.0))]; // norounding-error {{negative size}} rounding-gnu-error {{negative size}}
+  struct X { vla v; }; // rounding-std-error {{fields must have a constant size}}
+}

clang/test/SemaCXX/rounding-math.cpp

@@ -0,0 +1,41 @@
+// RUN: %clang_cc1 -triple x86_64-linux -verify=norounding %s
+// RUN: %clang_cc1 -triple x86_64-linux -verify=rounding %s -frounding-math
+// rounding-no-diagnostics
+
+#define fold(x) (__builtin_constant_p(x) ? (x) : (x))
+
+constexpr double a = 1.0 / 3.0;
+
+constexpr int f(int n) { return int(n * (1.0 / 3.0)); }
+
+using T = int[f(3)];
+using T = int[1];
+
+enum Enum { enum_a = f(3) };
+
+struct Bitfield {
+  unsigned int n : 1;
+  unsigned int m : f(3);
+};
+
+void f(Bitfield &b) {
+  b.n = int(6 * (1.0 / 3.0)); // norounding-warning {{changes value from 2 to 0}}
+}
+
+const int k = 3 * (1.0 / 3.0);
+static_assert(k == 1, "");
+
+void g() {
+  // FIXME: Constant-evaluating this initializer is surprising, and violates
+  // the recommended practice in C++ [expr.const]p12:
+  //
+  //   Implementations should provide consistent results of floating-point
+  //   evaluations, irrespective of whether the evaluation is performed during
+  //   translation or during program execution.
+  const int k = 3 * (1.0 / 3.0);
+  static_assert(k == 1, "");
+}
+
+int *h() {
+  return new int[int(-3 * (1.0 / 3.0))]; // norounding-error {{too large}}
+}