Now this idea I like, a lot, in terms of forcing users to understand
what they're dealing with. This would also give impetus to our Windows
folks to update configure for more sensible the default selection
behavior.
Thanks! Other thoughts?
David
On Tue, Nov 22, 2016 at 4:41 PM, Michael B. Smith
<mich...@smithcons.com <mailto:mich...@smithcons.com>> wrote:
Speaking only to my personal opinion – if you are compiling on
Windows, and the –mms-bitfields is not set and –no-ms-bitfields is
not set, then you should issue a warning to that effect.
*From:* Tinycc-devel [mailto:tinycc-devel-bounces+michael
<mailto:tinycc-devel-bounces%2Bmichael>=theessentialexchange....@nongnu.org
<mailto:theessentialexchange....@nongnu.org>] *On Behalf Of *David
Mertens
*Sent:* Tuesday, November 22, 2016 4:32 PM
*To:* tinycc-devel@nongnu.org <mailto:tinycc-devel@nongnu.org>
*Subject:* [Tinycc-devel] Patch: add option -mms-bitfields
Hello everyone,
Per previous discussion, here is a patch to implement a
command-line option, -mms-bitfields. This would alter tcc's
bitfield alignment algorithm in a backwards-incompatible way to be
a bit more consistent with gcc's algorithm. The old behavior is
traditionally attributed to MSVC, and so (consistent with gcc) the
command-line option to use the old behavior is -mms-bitfields. If
this patch gets applied, Windows users would be encouraged to add
-mms-bitfields to their Makefiles.
I would appreciate comments, bug reports, etc. If you care, please
give this a spin in the next 24 hours, or please tell me if you
need more time. Any tests illustrating discrepancies between gcc
and this algorithm would be especially appreciated!
Thanks!
David
diff --git a/libtcc.c b/libtcc.c
index f184502..b824c18 100644
--- a/libtcc.c
+++ b/libtcc.c
@@ -752,6 +752,7 @@ LIBTCCAPI TCCState *tcc_new(void)
s->alacarte_link = 1;
s->nocommon = 1;
s->warn_implicit_function_declaration = 1;
+ s->ms_bitfields = 0;
#ifdef CHAR_IS_UNSIGNED
s->char_is_unsigned = 1;
@@ -1508,6 +1509,7 @@ enum {
TCC_OPTION_Wl,
TCC_OPTION_W,
TCC_OPTION_O,
+ TCC_OPTION_mms_bitfields,
TCC_OPTION_m,
TCC_OPTION_f,
TCC_OPTION_isystem,
@@ -1571,6 +1573,7 @@ static const TCCOption tcc_options[] = {
{ "Wl,", TCC_OPTION_Wl, TCC_OPTION_HAS_ARG | TCC_OPTION_NOSEP },
{ "W", TCC_OPTION_W, TCC_OPTION_HAS_ARG | TCC_OPTION_NOSEP },
{ "O", TCC_OPTION_O, TCC_OPTION_HAS_ARG | TCC_OPTION_NOSEP },
+ { "mms-bitfields", TCC_OPTION_mms_bitfields, 0}, /* must go
before option 'm' */
{ "m", TCC_OPTION_m, TCC_OPTION_HAS_ARG },
{ "f", TCC_OPTION_f, TCC_OPTION_HAS_ARG | TCC_OPTION_NOSEP },
{ "isystem", TCC_OPTION_isystem, TCC_OPTION_HAS_ARG },
@@ -1854,6 +1857,9 @@ PUB_FUNC int tcc_parse_args(TCCState *s, int
argc, char **argv)
if (x > 0)
tcc_define_symbol(s, "__OPTIMIZE__", NULL);
break;
+ case TCC_OPTION_mms_bitfields:
+ s->ms_bitfields = 1;
+ break;
case TCC_OPTION_traditional:
case TCC_OPTION_pedantic:
case TCC_OPTION_pipe:
diff --git a/tcc-doc.texi b/tcc-doc.texi
index 5aab13c..9f136ed 100644
--- a/tcc-doc.texi
+++ b/tcc-doc.texi
@@ -176,6 +176,10 @@ In a script, it gives the following header:
#!/usr/local/bin/tcc -run -L/usr/X11R6/lib -lX11
@end example
+@item -mms-bitfields
+Use an algorithm for bitfield alignment consistent with MSVC.
Default is
+gcc's algorithm.
+
@item -mfloat-abi (ARM only)
Select the float ABI. Possible values: @code{softfp} and @code{hard}
diff --git a/tcc.c b/tcc.c
index 28f3ae9..fbfbffa 100644
--- a/tcc.c
+++ b/tcc.c
@@ -97,6 +97,7 @@ static void help(void)
" -o outfile set output filename\n"
" -run run compiled source\n"
" -fflag set or reset (with 'no-' prefix) 'flag'
(see man page)\n"
+ " -mms-bitfields use bitfield alignment consistent
with MSVC\n"
" -Wwarning set or reset (with 'no-' prefix)
'warning' (see man page)\n"
" -w disable all warnings\n"
" -v show version\n"
diff --git a/tcc.h b/tcc.h
index 180838d..cbdebd3 100644
--- a/tcc.h
+++ b/tcc.h
@@ -605,6 +605,7 @@ struct TCCState {
int old_struct_init_code; /* use old algorithm to init
array in struct when there is no '{' used.
Liuux 2.4.26 can't find initrd when compiled
with a new algorithm */
int dollars_in_identifiers; /* allows '$' char in
indentifiers */
+ int ms_bitfields; /* if true, emulate MS algorithm for
aligning bitfields */
/* warning switches */
int warn_write_strings;
diff --git a/tccgen.c b/tccgen.c
index ba1757e..d064024 100644
--- a/tccgen.c
+++ b/tccgen.c
@@ -3154,7 +3154,7 @@ static void parse_attribute(AttributeDef *ad)
/* enum/struct/union declaration. u is either VT_ENUM or VT_STRUCT */
static void struct_decl(CType *type, AttributeDef *ad, int u)
{
- int a, v, size, align, maxalign, c, offset, flexible;
+ int a, v, size, align, maxalign, c, offset, flexible,
extra_bytes;
int bit_size, bit_pos, bsize, bt, lbit_pos, prevbt;
Sym *s, *ss, *ass, **ps;
AttributeDef ad1;
@@ -3235,6 +3235,7 @@ static void struct_decl(CType *type,
AttributeDef *ad, int u)
while (tok != '}') {
parse_btype(&btype, &ad1);
while (1) {
+ extra_bytes = 0;
if (flexible)
tcc_error("flexible array member '%s' not at the
end of struct",
get_tok_str(v, NULL));
@@ -3310,9 +3311,9 @@ static void struct_decl(CType *type,
AttributeDef *ad, int u)
/* zero size: means to pad */
bit_pos = 0;
} else {
- /* we do not have enough room ?
- did the type change?
- is it a union? */
+ /* if type change, union, or will overrun
+ * allignment slot, start at a newly
+ * alligned slot */
if ((bit_pos + bit_size) > bsize ||
bt != prevbt || a == TOK_UNION)
bit_pos = 0;
@@ -3322,15 +3323,30 @@ static void struct_decl(CType *type,
AttributeDef *ad, int u)
(bit_pos << VT_STRUCT_SHIFT) |
(bit_size << (VT_STRUCT_SHIFT + 6));
bit_pos += bit_size;
+ /* without ms-bitfields, allocate the
+ * minimum number of bytes necessary,
+ * adding single bytes as needed */
+ if (!tcc_state->ms_bitfields) {
+ if (lbit_pos == 0)
+ /* minimum bytes for new
bitfield */
+ size = (bit_size + 7) / 8;
+ else {
+ /* enough spare bits already
allocated? */
+ bit_size = (lbit_pos - 1) % 8
+ 1 + bit_size;
+ if (bit_size > 8) /* doesn't
fit */
+ extra_bytes = (bit_size -
1) / 8;
+ }
+ }
}
prevbt = bt;
} else {
bit_pos = 0;
}
if (v != 0 || (type1.t & VT_BTYPE) ==
VT_STRUCT) {
- /* add new memory data only if starting
- bit field */
- if (lbit_pos == 0) {
+ /* add new memory data only if starting bit
+ field or adding bytes to existing bit
field */
+ if (extra_bytes) c += extra_bytes;
+ else if (lbit_pos == 0) {
if (a == TOK_STRUCT) {
c = (c + align - 1) & -align;
offset = c;
diff --git a/tests/tcctest.c b/tests/tcctest.c
index 5851fb4..b0fe8f3 100644
--- a/tests/tcctest.c
+++ b/tests/tcctest.c
@@ -1634,6 +1634,15 @@ void bitfield_test(void)
st2.f3 = a;
st2.f2++;
printf("%lld %lld %lld\n", st2.f1, st2.f2, st2.f3);
+ struct sbf3 {
+ int f1 : 7;
+ int f2 : 1;
+ char f3;
+ int f4 : 8;
+ int f5 : 1;
+ int f6 : 16;
+ } st3;
+ printf("sizeof(st3) = %d\n", sizeof(st3));
}
#ifdef __x86_64__
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