Support pure __builtin_bswap16 in switch cases

src/cil.ml

@@ -2956,7 +2956,7 @@ let initGccBuiltins () : unit =
     try
       assert (sizeInBits mod 8 = 0);
       let sizeInBytes = sizeInBits / 8 in
-      let sizedIntType = TInt (intKindForSize sizeInBytes false, []) in
+      let sizedIntType = TInt (intKindForSize sizeInBytes true, []) in
       let name = Printf.sprintf "__builtin_bswap%d" sizeInBits in
       H.add h name (sizedIntType, [ sizedIntType ], false)
     with Not_found ->

src/frontc/cabs2cil.ml

@@ -4752,6 +4752,24 @@ and doExp (asconst: bool)   (* This expression is used as a constant *)
                           prestype := intType
                   | _ -> ignore (warn "Invalid call to builtin_types_compatible_p");
                 end
+              else if fv.vname = "__builtin_bswap16" && asconst && isEmpty (!prechunk ()) then (* to support pure switch cases in Linux kernel *)
+                begin
+                  match !pargs with
+                  | [ arg ] -> begin
+                      piscall := false;
+                      prestype := TInt (IUShort, []); (* TODO: lookup via machdep? *)
+                      let arg = CastE (IntegerPromotion, intType, arg) in (* bitwise operators will promote uint16 to int *) (* TODO: consider machdep? *)
+                      pres :=
+                        CastE (Internal, !prestype, (* force promoted int back to uint16 *)
+                          BinOp (BOr,
+                            BinOp (Shiftlt, arg, integer 8, intType),
+                            BinOp (Shiftrt, arg, integer 8, intType),
+                          intType)
+                        );
+                    end
+                  | _ ->
+                    ignore (warn "Invalid call to builtin_bswap16");
+                end
             end
           | _ -> ()
         );

test/small1/builtin_bswap_const.c

@@ -0,0 +1,13 @@
+#include "testharness.h"
+
+int main (void)
+{
+  switch (1) {
+    case (__builtin_bswap16(256)):
+      break;
+    default:
+      E(1);
+  }
+
+  SUCCESS;
+}

test/testcil.pl

@@ -555,6 +555,7 @@ sub addToGroup {
 addTest("testrun/builtin4 ");
 addTest("test/builtin5 ");
 addTest("test/builtin6 ");
+addTest("testrun/builtin_bswap_const ");
 addTest("test/sync-1 _GNUCC=1");
 addTest("test/sync-2 _GNUCC=1");
 addTest("test/sync-3 _GNUCC=1");