Inconsistent treatment of "lone" zero-width bit fields

(i.e. not preceded by another bit field).


git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@1516 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e

cparser/Bitfields.ml

@@ -88,19 +88,25 @@ let rec transf_members env id count = function
         m :: transf_members env id count ms
       else begin
         let (nbits, bitfields, ml') = pack_bitfields env id ml in
-        let carrier = sprintf "__bf%d" count in
-        let carrier_typ = TInt(unsigned_ikind_for_carrier nbits, []) in
-        List.iter
-          (fun (name, pos, sz, signed, signed2) ->
-            if name <> "" then
-              Hashtbl.add bitfield_table
-                (id, name)
-                {bf_carrier = carrier; bf_carrier_typ = carrier_typ;
-                 bf_pos = pos; bf_size = sz;
-                 bf_signed = signed; bf_signed_res = signed2})
-          bitfields;
-        { fld_name = carrier; fld_typ = carrier_typ; fld_bitfield = None}
-        :: transf_members env id (count + 1) ml'
+        if nbits = 0 then
+          (* Lone zero-size bitfield: just ignore *)
+          transf_members env id count ml'
+        else begin
+          (* Create integer field of sufficient size for this bitfield group *)
+          let carrier = sprintf "__bf%d" count in
+          let carrier_typ = TInt(unsigned_ikind_for_carrier nbits, []) in
+          List.iter
+            (fun (name, pos, sz, signed, signed2) ->
+              if name <> "" then
+                Hashtbl.add bitfield_table
+                  (id, name)
+                  {bf_carrier = carrier; bf_carrier_typ = carrier_typ;
+                   bf_pos = pos; bf_size = sz;
+                   bf_signed = signed; bf_signed_res = signed2})
+            bitfields;
+          { fld_name = carrier; fld_typ = carrier_typ; fld_bitfield = None}
+          :: transf_members env id (count + 1) ml'
+        end
       end
 
 let transf_composite env su id ml =

cparser/Cutil.ml

@@ -216,12 +216,14 @@ let pack_bitfields ml =
             pack (nbits + n) ms (* add to current word *)
   in
   let (nbits, ml') = pack 0 ml in
-  let sz =
-    if nbits <= 8 then 1 else
-    if nbits <= 16 then 2 else
-    if nbits <= 32 then 4 else
-    if nbits <= 64 then 8 else assert false in
-  (sz, ml')
+  let (sz, al) =
+    (* A lone bitfield of width 0 consumes no space and aligns to 1 *) 
+    if nbits = 0 then (0, 1) else
+    if nbits <= 8 then (1, 1) else
+    if nbits <= 16 then (2, 2) else
+    if nbits <= 32 then (4, 4) else
+    if nbits <= 64 then (8, 8) else assert false in
+  (sz, al, ml')
 
 (* Natural alignment, in bytes *)
 
@@ -264,8 +266,8 @@ let alignof_struct_union env members =
         | None -> None
         | Some a -> align_rec (max a al) rem
       end else begin
-        let (sz, ml') = pack_bitfields ml in
-        align_rec (max sz al) ml'
+        let (s, a, ml') = pack_bitfields ml in
+        align_rec (max a al) ml'
       end
   in align_rec 1 members
 
@@ -355,8 +357,8 @@ let sizeof_struct env members =
         | Some a, Some s -> sizeof_rec (align ofs a + s) rem
         | _, _ -> None
       end else begin
-        let (sz, ml') = pack_bitfields ml in
-        sizeof_rec (align ofs sz + sz) ml'
+        let (s, a, ml') = pack_bitfields ml in
+        sizeof_rec (align ofs a + s) ml'
       end
   in sizeof_rec 0 members
 

test/regression/Makefile

@@ -9,7 +9,7 @@ LIBS=$(LIBMATH)
 # Can run and have reference output in Results
 
 TESTS=bitfields1 bitfields2 bitfields3 bitfields4 \
-  bitfields5 bitfields6 \
+  bitfields5 bitfields6 bitfields7 \
   expr1 initializers volatile2 \
   funct3 expr5 struct7 struct8 casts1 casts2
 

test/regression/Results/bitfields7

+g_22.f2 = 1

test/regression/bitfields7.c

+#include <stdio.h>
+
+struct S0 {
+  signed f0 : 12;
+  unsigned f1 : 28;
+};
+
+struct S5 {
+  struct S0  f1;
+  unsigned : 0;
+  signed f2 : 26;
+};
+
+struct S5 g_22 = {{0,0},1};
+
+int main(int argc, char* argv[])
+{
+   printf("g_22.f2 = %d\n", g_22.f2);
+   return 0;
+}