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* [Bug c/45034] New: "safe" conversion from unsigned to signed char gives broken code
@ 2010-07-22 21:13 mikpe at it dot uu dot se
2010-08-08 15:51 ` [Bug tree-optimization/45034] [4.3/4.4 Regression] " rguenth at gcc dot gnu dot org
0 siblings, 1 reply; 6+ messages in thread
From: mikpe at it dot uu dot se @ 2010-07-22 21:13 UTC (permalink / raw)
To: gcc-bugs
The following program (which I'll also attach later) gives unexpected results,
where signed char values are passed as non properly sign-extended ints:
> cat char-neg.c
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
static void fixnum_neg(signed char x, signed char *py, int *pv)
{
unsigned char ux, uy;
ux = (unsigned char)x;
uy = -ux;
*py = (uy <= 127) ? (signed char)uy : (-(signed char)(255 - uy) - 1);
*pv = (x == -128) ? 1 : 0;
}
void __attribute__((noinline)) foo(int x, int y, int v)
{
printf("test_neg: -(%d) => (%d, %d)\n", x, y, v);
if (y < -128 || y > 127)
abort();
}
int test_neg(void)
{
signed char x, y;
int v, err;
err = 0;
x = -128;
for (;;) {
fixnum_neg(x, &y, &v);
foo((int)x, (int)y, v);
if ((v && x != -128) || (!v && x == -128))
++err;
if (x == 127)
break;
++x;
}
return err;
}
int main(void)
{
if (CHAR_BIT != 8
|| SCHAR_MIN != -128 || SCHAR_MAX != 127
|| UCHAR_MAX != 255)
abort();
if (test_neg() != 0)
abort();
return 0;
}
> gcc -O2 -Wall -Wextra char-neg.c ; ./a.out
test_neg: -(-128) => (-128, 1)
test_neg: -(-127) => (-129, 0)
Abort
The abort shows that the `signed char' variable y is incorrectly extended to
int when passed to foo().
Passing -fwrapv eliminates the failure. Maybe I've been staring at this for
too long, but I can't see any signed overflow in this code.
The problem occurs with gcc 4.6/4.5/4.4/4.3/4.2, but not with 4.1 or older.
The program comes from some code which attempts to emulate machine-level
integer arithmetic and condition code settings. To validate the condition code
logic I used exhaustive testing on a smaller integer type (signed char), but
that broke as shown above. The assignment to *py in fixnum_neg() is one of
several attempts to cast from unsigned to signed char without (apparently)
triggering undefined behaviour due to signed overflow; other failed attempts
have included plain casts, assignment via a union, and memcpy() via a local
signed char temporary.
gcc was configured --with-gmp=... --with-mpfr=... --with-mpc=...
--disable-plugin --disable-lto --disable-nls --enable-threads=posix
--enable-checking=release --disable-libmudflap --enable-languages=c
--
Summary: "safe" conversion from unsigned to signed char gives
broken code
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: mikpe at it dot uu dot se
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45034
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45034] [4.3/4.4 Regression] "safe" conversion from unsigned to signed char gives broken code
2010-07-22 21:13 [Bug c/45034] New: " mikpe at it dot uu dot se
@ 2010-08-08 15:51 ` rguenth at gcc dot gnu dot org
0 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2010-08-08 15:51 UTC (permalink / raw)
To: gcc-bugs
------- Comment #19 from rguenth at gcc dot gnu dot org 2010-08-08 15:51 -------
Also fixed on the 4.5 branch.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
AssignedTo|rguenth at gcc dot gnu dot |unassigned at gcc dot gnu
|org |dot org
Status|ASSIGNED |NEW
Known to fail|4.3.2 4.2.0 |4.3.2 4.2.0 4.5.1
Known to work|4.1.2 4.6.0 |4.1.2 4.5.2 4.6.0
Summary|[4.3/4.4/4.5 Regression] |[4.3/4.4 Regression] "safe"
|"safe" conversion from |conversion from unsigned to
|unsigned to signed char |signed char gives broken
|gives broken code |code
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45034
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2011-08-01 16:42 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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[not found] <bug-45034-4@http.gcc.gnu.org/bugzilla/>
2011-06-27 10:37 ` [Bug tree-optimization/45034] [4.3/4.4 Regression] "safe" conversion from unsigned to signed char gives broken code rguenth at gcc dot gnu.org
2011-06-27 13:19 ` rguenth at gcc dot gnu.org
2011-08-01 15:32 ` [Bug tree-optimization/45034] [4.4 " sezeroz at gmail dot com
2011-08-01 16:41 ` ian at airs dot com
2011-08-01 16:42 ` ian at airs dot com
2010-07-22 21:13 [Bug c/45034] New: " mikpe at it dot uu dot se
2010-08-08 15:51 ` [Bug tree-optimization/45034] [4.3/4.4 Regression] " rguenth at gcc dot gnu dot org
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