Emit a warning for floating-point size changes after implicit conversions

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1387,3 +1387,6 @@ def BranchProtection : DiagGroup<"branch-protection">;
 // HLSL diagnostic groups
 // Warnings for HLSL Clang extensions
 def HLSLExtension : DiagGroup<"hlsl-extensions">;
+
+// Warnings about double precision arithmetic in device code
+def DoubleInDevice : DiagGroup<"double-in-device">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -125,6 +125,9 @@ def warn_float_compare_literal : Warning<
 def warn_double_const_requires_fp64 : Warning<
   "double precision constant requires %select{cl_khr_fp64|cl_khr_fp64 and __opencl_c_fp64}0, "
   "casting to single precision">;
+def warn_on_double_precision_use : Warning<
+  "double precision arithmetic used in device code. reduced FP64 performance expected on GPU's.">,
+  InGroup<DoubleInDevice>;
 def err_half_const_requires_fp16 : Error<
   "half precision constant requires cl_khr_fp16">;
 

clang/lib/Sema/SemaExpr.cpp

@@ -4019,10 +4019,15 @@ ExprResult Sema::ActOnNumericConstant(const Token &Tok, Scope *UDLScope) {
         }
       } else if (getLangOpts().OpenCL && !getOpenCLOptions().isAvailableOption(
                                              "cl_khr_fp64", getLangOpts())) {
+
         // Impose single-precision float type when cl_khr_fp64 is not enabled.
         Diag(Tok.getLocation(), diag::warn_double_const_requires_fp64)
             << (getLangOpts().getOpenCLCompatibleVersion() >= 300);
         Res = ImpCastExprToType(Res, Context.FloatTy, CK_FloatingCast).get();
+      } else if (getLangOpts().SYCLIsDevice) {
+        /* Emit a warning if double precision arithmetic is used in device
+         * kernel code */
+        Diag(Tok.getLocation(), diag::warn_on_double_precision_use);
       }
     }
   } else if (!Literal.isIntegerLiteral()) {