[Sema] Implement -Wdouble-promotion for clang.

GCC has a warning called -Wdouble-promotion, which warns you when
an implicit conversion increases the width of a floating point type.

This is useful when writing code for architectures that can perform
hardware FP ops on floats, but must fall back to software emulation for
larger types (i.e. double, long double).

This fixes PR15109 <https://llvm.org/bugs/show_bug.cgi?id=15109>.

Thanks to Carl Norum for the patch!

llvm-svn: 251588

4 files changed, 46 insertions, 0 deletions

diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 1f77181c6b6..042244125b8 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -46,6 +46,7 @@ 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">;
 def EnumTooLarge : DiagGroup<"enum-too-large">;
 def UnsupportedNan : DiagGroup<"unsupported-nan">;
 def NonLiteralNullConversion : DiagGroup<"non-literal-null-conversion">;
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index b902efb1ca9..d2945a0bd41 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2590,6 +2590,9 @@ def warn_impcast_complex_scalar : Warning<
 def warn_impcast_float_precision : Warning<
   "implicit conversion loses floating-point precision: %0 to %1">,
   InGroup<Conversion>, DefaultIgnore;
+def warn_impcast_double_promotion : Warning<
+  "implicit conversion increases floating-point precision: %0 to %1">,
+  InGroup<DoublePromotion>, DefaultIgnore;
 def warn_impcast_float_integer : Warning<
   "implicit conversion turns floating-point number into integer: %0 to %1">,
   InGroup<FloatConversion>, DefaultIgnore;
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index a8d882a635e..8d9a1b23bb4 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -7217,6 +7217,14 @@ void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
           return;
 
         DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_float_precision);
+
+      }
+      // ... or possibly if we're increasing rank, too
+      else if (TargetBT->getKind() > SourceBT->getKind()) {
+        if (S.SourceMgr.isInSystemMacro(CC))
+          return;
+
+        DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_double_promotion);
       }
       return;
     }
diff --git a/clang/test/Sema/warn-double-promotion.c b/clang/test/Sema/warn-double-promotion.c
new file mode 100644
index 00000000000..b6fd0c5ec62
--- /dev/null
+++ b/clang/test/Sema/warn-double-promotion.c
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s -Wdouble-promotion
+
+float ReturnFloatFromDouble(double d) {
+  return d;
+}
+
+float ReturnFloatFromLongDouble(long double ld) {
+  return ld;
+}
+
+double ReturnDoubleFromLongDouble(long double ld) {
+  return ld;
+}
+
+double ReturnDoubleFromFloat(float f) {
+  return f;  //expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
+}
+
+long double ReturnLongDoubleFromFloat(float f) {
+  return f;  //expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
+}
+
+long double ReturnLongDoubleFromDouble(double d) {
+  return d;  //expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+}
+
+void Convert(float f, double d, long double ld) {
+  d = f;  //expected-warning{{implicit conversion increases floating-point precision: 'float' to 'double'}}
+  ld = f; //expected-warning{{implicit conversion increases floating-point precision: 'float' to 'long double'}}
+  ld = d; //expected-warning{{implicit conversion increases floating-point precision: 'double' to 'long double'}}
+  f = d;
+  f = ld;
+  d = ld;
+}