Backport -Wdouble-promotion from Clang 3.8 (#6574)

This adds the Clang 3.8 `-Wdouble-promotion` warning group to DXC. This
is added as part of the mitigations for the HLSL 202x conforming
literals feature which (among other things) changes the conversion rank
of unsuffixed literal values.

Resolves #6571

Author: llvm-beanz

tools/clang/include/clang/Basic/DiagnosticGroups.td

@@ -44,6 +44,7 @@ def BoolConversion : DiagGroup<"bool-conversion", [PointerBoolConversion,
 def IntConversion : DiagGroup<"int-conversion">;
 def EnumConversion : DiagGroup<"enum-conversion">;
 def FloatConversion : DiagGroup<"float-conversion">;
+def DoublePromotion : DiagGroup<"double-promotion">; // HLSL Change - backport.
 def EnumTooLarge : DiagGroup<"enum-too-large">;
 def UnsupportedNan : DiagGroup<"unsupported-nan">;
 def NonLiteralNullConversion : DiagGroup<"non-literal-null-conversion">;

tools/clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2639,6 +2639,13 @@ def warn_impcast_null_pointer_to_integer : Warning<
 def warn_impcast_floating_point_to_bool : Warning<
     "implicit conversion turns floating-point number into bool: %0 to %1">,
     InGroup<ImplicitConversionFloatingPointToBool>;
+// HLSL Change - Begin
+// This backports the Clang change to emit warnings for floating point
+// promotions.
+def warn_impcast_double_promotion : Warning<
+  "implicit conversion increases floating-point precision: %0 to %1">,
+  InGroup<DoublePromotion>, DefaultIgnore;
+// HLSL Change - End
 
 def warn_impcast_pointer_to_bool : Warning<
     "address of%select{| function| array}0 '%1' will always evaluate to "

tools/clang/lib/Sema/SemaChecking.cpp

@@ -7135,12 +7135,15 @@ void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
     if (TargetBT && TargetBT->isFloatingPoint()) {
       // ...then warn if we're dropping FP rank.
 
-      // HLSL Change - unless source is literal float
+      // HLSL Change Begin - Warn on both promotions and conversions.
+      // Don't warn on Literal float.
       if (SourceBT->getKind() == BuiltinType::LitFloat)
         return;
+      int Order = S.getASTContext().getFloatingTypeOrder(QualType(SourceBT, 0),
+                                                         QualType(TargetBT, 0));
 
       // Builtin FP kinds are ordered by increasing FP rank.
-      if (SourceBT->getKind() > TargetBT->getKind()) {
+      if (Order > 0) {
         // Don't warn about float constants that are precisely
         // representable in the target type.
         Expr::EvalResult result;
@@ -7156,7 +7159,10 @@ void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
           return;
 
         DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_float_precision);
+      } else if (Order < 0) {
+        DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_double_promotion);
       }
+      // HLSL Change End
       return;
     }