Re: [Tinycc-devel] Weird bitfield size handling, discrepancy with gcc
Hi all,
VC++ 32 and 64 both return:
t1 struct size: 2
t2 struct size: 8
t3 struct size: 8
Which IMHO is correct. I really don’t understand why gcc returns
t1 struct size: 2
t2 struct size: 4
t3 struct size: 4
To me,
struct t2 {
uint32_t op_type:1;
uint8_t op_flags;
};
op_type uses a plain uint32_t to store only one bit as there is another field
which is not a bit bitfield, C compile must allocate at least one extra byte in
this struct. The minimal size I expect is 5!
I changed you sample a little bit to see offset of op_flags:
#include
#include
#include
struct t1 {
uint8_t op_type:1;
uint8_t op_flags;
};
struct t2 {
uint32_t op_type:1;
uint8_t op_flags;
};
struct t3 {
unsigned op_type:1;
char op_flags;
};
int main() {
printf("t1 struct size: %ld\n", sizeof(struct t1));
printf("t2 struct size: %ld\n", sizeof(struct t2));
printf("t2 offset of op_flags: %ld\n", offsetof(struct t2, op_flags));
printf("t3 struct size: %ld\n", sizeof(struct t3));
return 0;
}
Visual C++ gives:
t1 struct size: 2
t2 struct size: 8
t2 offset of op_flags: 4
t3 struct size: 8
While gcc gives
c:\tmp>a.exe
t1 struct size: 2
t2 struct size: 4
t2 offset of op_flags: 1
t3 struct size: 4
Which IMHO is wrong. If later I change my mind with:
struct t2 {
uint32_t op_type:1;
uint32_t op_other_field: 12;
uint8_t op_flags;
};
I expect my struct will have the same size. I don't think we want tcc mimic
this behavior. Doing this will break Windows code that depends on this
behavior. The only solution is to add a compiler flag to ask for a specific
behavior.
From: Tinycc-devel [mailto:tinycc-devel-bounces+eligis=orange...@nongnu.org] On
Behalf Of Richard Brett
Sent: mardi 18 octobre 2016 06:08
To: tinycc-devel@nongnu.org
Subject: Re: [Tinycc-devel] Weird bitfield size handling, discrepancy with gcc
Hello
Is this really a problem for tcc? An old version of VC produces the same
sizes as tcc. The spec seems to say (not sure I'm reading this right, first
time I've read the spec)
"An implementation may allocate any addressable storage unit large enough to
hold a bitfield.snip. the order of bitfields within a unit is ( ...
high to low or . low to high) implementation defined. The alignment of
the addressable storage unit is undefined"
This seems to suggest that each implementation can do what it wants with
bitfields and that passing them between different compilers is probably
undefined.
Having said all that, I'm not overly worried if it gets changed, just seems
like risk for something that might not be broken. And, I am not an expert on C
compiler internals. I do pass structures a lot between tcc and other
compilers, but they are all carefully crafted with PACK directives/pragmas to
ensure exact memory layout, and I dont use bitfields as part of compiler -
hence why I looked at this.
Cheers
.Richard
David Mertens wrote:
Hello everyone,
I recently uncovered some segfaulting code when compiling code with macros that
manipulate certain Perl structs on 64-bit Linux. I boiled the problem down to a
discrepancy between how tcc and gcc determine the size needed by a series of
bit fields. The tcc-compiled function would get the Perl interpreter struct
produced by gcc-compiled code, then reach into the wrong memory slot for
something. A reduced example is provided below.
Question 1: Would anybody be opposed to changing tcc's behavior to match gcc's
behavior here? This could lead to binary incompatibility with object code
previously compiled with tcc, but that seems to me highly unlikely to be a real
problem for anyone.
Question 2: Does anybody know tccgen.c well enough to fix this? I can work on
it, but if anybody knows exactly where this goes wrong, it would save me a few
hours.
%<
#include
#include
struct t1 {
uint8_t op_type:1;
uint8_t op_flags;
};
struct t2 {
uint32_t op_type:1;
uint8_t op_flags;
};
struct t3 {
unsigned op_type:1;
char op_flags;
};
int main() {
printf("t1 struct size: %ld\n", sizeof(struct t1));
printf("t2 struct size: %ld\n", sizeof(struct t2));
printf("t3 struct size: %ld\n", sizeof(struct t3));
return 0;
}
>%
With tcc, this prints:
t1 struct size: 2
t2 struct size: 8
t3 struct size: 8
With gcc, this prints:
t1 struct size: 2
t2 struct size: 4
t3 struct size: 4
This suggests that with tcc, the number of bytes given to a series of bitfields
in a struct depends upon the integer type of the bitfield. In particular, plain
old "unsigned" is interpreted (in 64-bit context) to be "unsigned int", which
has 32 bits. This is incompatible with gcc's interpretation.
The relevant code is, I think, in tccgen.c's struct_decl. However, I can't
quite tease apart where the declaration comes in, and how it effect struct size
calculations.
David
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write
[Tinycc-devel] core dump on FreeBSD with last commit "configure: --triplet= option, Makefile: cleanup"
Hi Grischka, This one tccpp_new/delete and other cleanups commit 0be098929a062d706057d7beb78666daa52bac49 still works on FreeBSD with of course C library not found as previously reported But next commit configure: --triplet= option, Makefile: cleanupmob commit 02919cd27506e25dacdbe72dad1ae2718eb75991 Get worse as it now core dumps (I notice that Triplet is empty, is it wanted?): $ ./configure Binary directory /usr/local/bin TinyCC directory/usr/local/lib/tcc Library directory /usr/local/lib Include directory /usr/local/include Manual directory/usr/local/share/man Info directory /usr/local/share/info Doc directory /usr/local/share/doc Target root prefix - Source path /home/jullien/tinycc C compiler gcc Target OS FreeBSD CPU x86-64 Big Endian no Profiling no Cross compilers no Use libgcc no Triplet - Creating config.mak and config.h ... gmake test $ gmake test gmake -C tests gmake[1]: Entering directory '/usr/home/jullien/tinycc/tests' hello-exe ../tcc -B.. -I../include -I.. -I.. ../examples/ex1.c -o hello || (../tcc -vv; exit 1) && ./hello Segmentation fault (core dumped) tcc version 0.9.26 (x86-64 FreeBSD) install: /usr/local/lib/tcc include: /usr/local/lib/tcc/include /usr/local/include /usr/include libraries: /usr/lib /lib /usr/local/lib crt: /usr/lib elfinterp: /libexec/ld-elf.so.1 ___ Tinycc-devel mailing list Tinycc-devel@nongnu.org https://lists.nongnu.org/mailman/listinfo/tinycc-devel
Re: [Tinycc-devel] Weird bitfield size handling, discrepancy with gcc
Hello
Is this really a problem for tcc? An old version of VC produces the
same sizes as tcc. The spec seems to say (not sure I'm reading this
right, first time I've read the spec)
"An implementation may allocate any addressable storage unit large
enough to hold a bitfield.snip. the order of bitfields within a
unit is ( ... high to low or . low to high) implementation
defined. The alignment of the addressable storage unit is undefined"
This seems to suggest that each implementation can do what it wants with
bitfields and that passing them between different compilers is probably
undefined.
Having said all that, I'm not overly worried if it gets changed, just
seems like risk for something that might not be broken. And, I am not
an expert on C compiler internals. I do pass structures a lot between
tcc and other compilers, but they are all carefully crafted with PACK
directives/pragmas to ensure exact memory layout, and I dont use
bitfields as part of compiler - hence why I looked at this.
Cheers
.Richard
David Mertens wrote:
Hello everyone,
I recently uncovered some segfaulting code when compiling code with
macros that manipulate certain Perl structs on 64-bit Linux. I boiled
the problem down to a discrepancy between how tcc and gcc determine
the size needed by a series of bit fields. The tcc-compiled function
would get the Perl interpreter struct produced by gcc-compiled code,
then reach into the wrong memory slot for something. A reduced example
is provided below.
Question 1: Would anybody be opposed to changing tcc's behavior to
match gcc's behavior here? This could lead to binary incompatibility
with object code previously compiled with tcc, but that seems to me
highly unlikely to be a real problem for anyone.
Question 2: Does anybody know tccgen.c well enough to fix this? I can
work on it, but if anybody knows exactly where this goes wrong, it
would save me a few hours.
%<
#include
#include
struct t1 {
uint8_t op_type:1;
uint8_t op_flags;
};
struct t2 {
uint32_t op_type:1;
uint8_t op_flags;
};
struct t3 {
unsigned op_type:1;
char op_flags;
};
int main() {
printf("t1 struct size: %ld\n", sizeof(struct t1));
printf("t2 struct size: %ld\n", sizeof(struct t2));
printf("t3 struct size: %ld\n", sizeof(struct t3));
return 0;
}
>%
With tcc, this prints:
t1 struct size: 2
t2 struct size: 8
t3 struct size: 8
With gcc, this prints:
t1 struct size: 2
t2 struct size: 4
t3 struct size: 4
This suggests that with tcc, the number of bytes given to a series of
bitfields in a struct depends upon the integer type of the bitfield.
In particular, plain old "unsigned" is interpreted (in 64-bit context)
to be "unsigned int", which has 32 bits. This is incompatible with
gcc's interpretation.
The relevant code is, I think, in tccgen.c's struct_decl. However, I
can't quite tease apart where the declaration comes in, and how it
effect struct size calculations.
David
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." -- Brian Kernighan
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[Tinycc-devel] Weird bitfield size handling, discrepancy with gcc
Hello everyone,
I recently uncovered some segfaulting code when compiling code with macros
that manipulate certain Perl structs on 64-bit Linux. I boiled the problem
down to a discrepancy between how tcc and gcc determine the size needed by
a series of bit fields. The tcc-compiled function would get the Perl
interpreter struct produced by gcc-compiled code, then reach into the wrong
memory slot for something. A reduced example is provided below.
Question 1: Would anybody be opposed to changing tcc's behavior to match
gcc's behavior here? This could lead to binary incompatibility with object
code previously compiled with tcc, but that seems to me highly unlikely to
be a real problem for anyone.
Question 2: Does anybody know tccgen.c well enough to fix this? I can work
on it, but if anybody knows exactly where this goes wrong, it would save me
a few hours.
%<
#include
#include
struct t1 {
uint8_t op_type:1;
uint8_t op_flags;
};
struct t2 {
uint32_t op_type:1;
uint8_t op_flags;
};
struct t3 {
unsigned op_type:1;
char op_flags;
};
int main() {
printf("t1 struct size: %ld\n", sizeof(struct t1));
printf("t2 struct size: %ld\n", sizeof(struct t2));
printf("t3 struct size: %ld\n", sizeof(struct t3));
return 0;
}
>%
With tcc, this prints:
t1 struct size: 2
t2 struct size: 8
t3 struct size: 8
With gcc, this prints:
t1 struct size: 2
t2 struct size: 4
t3 struct size: 4
This suggests that with tcc, the number of bytes given to a series of
bitfields in a struct depends upon the integer type of the bitfield. In
particular, plain old "unsigned" is interpreted (in 64-bit context) to be
"unsigned int", which has 32 bits. This is incompatible with gcc's
interpretation.
The relevant code is, I think, in tccgen.c's struct_decl. However, I can't
quite tease apart where the declaration comes in, and how it effect struct
size calculations.
David
--
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." -- Brian Kernighan
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Re: [Tinycc-devel] [FreeBSD support] __aligned(n) + __pure issues
Hi, $ find /usr/lib -name crt1.o /usr/lib/crt1.o -Original Message- From: grischka [mailto:gris...@gmx.de] Sent: lundi 17 octobre 2016 18:32 To: Christian JULLIEN; tinycc-devel@nongnu.org Subject: Re: [Tinycc-devel] [FreeBSD support] __aligned(n) + __pure issues Christian JULLIEN wrote: > tcc: error: file 'crt1.o' not found Can you look where your file is? $ find /usr/lib -name crt1.o -- gr ___ Tinycc-devel mailing list Tinycc-devel@nongnu.org https://lists.nongnu.org/mailman/listinfo/tinycc-devel
Re: [Tinycc-devel] [FreeBSD support] __aligned(n) + __pure issues
Christian JULLIEN wrote: tcc: error: file 'crt1.o' not found Can you look where your file is? $ find /usr/lib -name crt1.o -- gr ___ Tinycc-devel mailing list Tinycc-devel@nongnu.org https://lists.nongnu.org/mailman/listinfo/tinycc-devel
Re: [Tinycc-devel] Use TCC with GSL under Windows
Dear Michael,
Thank you.
I'm using the latest release, but not the latest version as of the
source code. Are you suggesting me to compile tcc itself from source codes?
Warm regards,
Xianwen
On 14.10.2016 17:56, Michael Matz wrote:
> Hi,
>
> On Tue, 11 Oct 2016, Chen, Xianwen wrote:
>
>> In file included from Bessel_Function.c:1:
>> In file included from C:/cygwin64/usr/include//stdio.h:60:
>> In file included from C:/cygwin64/usr/include//sys/reent.h:15:
>> C:/cygwin64/usr/include//sys/_types.h:168: error: ',' expected (got "__wch")
> From a bit googling that's probably this typedef:
>
> typedef struct
> {
> int __count;
> union
> {
> wint_t __wch;
> unsigned char __wchb[4];
> } __value;/* Value so far. */
> } _mbstate_t;
>
> If so, that means the type wint_t isn't defined, which should have been
> defined by when __need_wint_t itself was defined before
> including it. That in turn means you're using an old version of TCC,
> because commit 0961a384 from April 2014 fixed this.
>
> That is, update your TCC.
>
>
> Ciao,
> Michael.
>
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