Thread View: gnu.gcc.bug
2 messages
2 total messages
Started by James Clark
Wed, 08 Apr 1992 11:23
optimization bug
Author: James Clark
Date: Wed, 08 Apr 1992 11:23
Date: Wed, 08 Apr 1992 11:23
28 lines
1342 bytes
1342 bytes
gcc 2.1 misoptimizes the following program:
$ cat bug.c
main()
{
signed char c = -1;
printf("%d\n", c < 0);
exit(0);
}
$ gcc -v -O bug.c
Reading specs from /usr/local/lib/gcc-lib/sparc-sun-sunos4.1/2.1/specs
gcc version 2.1
/usr/local/lib/gcc-lib/sparc-sun-sunos4.1/2.1/cpp -lang-c -v -undef -D__GNUC__=2 -Dsparc -Dsun -Dunix -D__sparc__ -D__sun__ -D__unix__ -D__sparc -D__sun -D__unix -D__OPTIMIZE__ bug.c /usr/tmp/cca28545.i
GNU CPP version 2.1 (sparc)
/usr/local/lib/gcc-lib/sparc-sun-sunos4.1/2.1/cc1 /usr/tmp/cca28545.i -quiet -dumpbase bug.c -O -version -o /usr/tmp/cca28545.s
GNU C version 2.1 (sparc) compiled by GNU C version 2.0.
/usr/local/lib/gcc-lib/sparc-sun-sunos4.1/2.1/as -o bug.o /usr/tmp/cca28545.s
/usr/local/lib/gcc-lib/sparc-sun-sunos4.1/2.1/ld -e start -dc -dp /lib/crt0.o -L/usr/local/lib/gcc-lib/sparc-sun-sunos4.1/2.1/ -L/usr/local/lib/ bug.o -lgcc -lc -lgcc
$ ./a.out
0
$ gcc bug.c
$ ./a.out
1
$
James Clark
jjc@jclark.com
FIX FOR: optimization bug
Author: Chip Salzenberg
Date: Fri, 10 Apr 1992 13:31
Date: Fri, 10 Apr 1992 13:31
51 lines
2171 bytes
2171 bytes
A bug report from James Clark <jjc@jclark.com> reports that GCC 2.1
misoptimizes this program:
main()
{
signed char c = -1;
printf("%d\n", c < 0);
exit(0);
}
It prints zero instead of one.
I've found this bug. The optimizer is noting that "c" equals 0xFF,
but it is forgetting that 0xFF is a _byte_ value (QImode). So when
the optimizer SIGN_EXTENDs that constant, the constant looks like it
has already been extended. This bug results in a wrong value of 255
(which is, of course, not less than zero) instead of the correct -1.
The below patch makes GCC generate and retain the correct machine
modes for constants.
----------------------------------------------------------------------------
Thu Apr 9 17:22:19 1992 Chip Salzenberg (chip at tct.com)
* emit-rtl.c (gen_rtx): Generate CONST_INT with requested MODE.
(gen_lowpart_common): Ditto.
Index: emit-rtl.c
*************** gen_rtx (va_alist)
*** 257,260 ****
--- 257,261 ----
rt_val = rtx_alloc (code);
+ rt_val->mode = mode;
INTVAL (rt_val) = arg;
}
*************** gen_lowpart_common (mode, x)
*** 533,537 ****
: (GET_MODE_BITSIZE (mode) == HOST_BITS_PER_INT
&& GET_CODE (x) == CONST_INT) ? x
! : gen_rtx (CONST_INT, VOIDmode,
(GET_MODE_MASK (mode)
& (GET_CODE (x) == CONST_INT
--- 534,538 ----
: (GET_MODE_BITSIZE (mode) == HOST_BITS_PER_INT
&& GET_CODE (x) == CONST_INT) ? x
! : gen_rtx (CONST_INT, mode,
(GET_MODE_MASK (mode)
& (GET_CODE (x) == CONST_INT
--
Chip Salzenberg at Teltronics/TCT <chip@tct.com>, <73717.366@compuserve.com>
"Informix 4GL is not a 4G, and it's barely an L." -- John Tombs
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