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[Bug middle-end/41072] New: Alias stacked type cast interpretation regression
- From: "sergei_lus at yahoo dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 14 Aug 2009 18:38:41 -0000
- Subject: [Bug middle-end/41072] New: Alias stacked type cast interpretation regression
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
The following code:
#include <stdio.h>
float floatsisf(int x) {
float ans = 0.5F;
long lexp = 24;
unsigned long frac = x;
unsigned long norm = 0x800000;
unsigned long xint;
for (; frac < norm; frac <<= 1)
--lexp;
xint = (unsigned long)frac;
(((unsigned short *)(char *)&(ans)))[1] = (((unsigned short *)(char
*)&(ans)))[1] & 0xff80 | ((xint) >> 16 ) & 0x007f;
(((unsigned short *)(char *)&(ans)))[0] = (xint) & 0xffff;
fprintf(stderr,"scale got (%f)\n",ans);
return ans;
}
main(){
int i;
for(i=1;i<10;i++) floatsisf(i);
}
uses an obscure way to communicate with alias analyzer. The way I understand
the original intent, once you cast &ans to (char *), alias analyzer should
treat it as "char" == "alias everything" case. With GCC 3.3.3 and 3.4.6 that
was the case. Starting with GCC 4.4.0 it does not - the leftmost (unsigned
short *) cast seems to permit speculation about the two references, resulting
in a wrong code generated.
More details:
>/xxx/x86_gcc_4.4/bin/bin/gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --enable-threads=posix
--prefix=/prj/dsp/qdsp6_aus/users/slarin/x86_gcc_4.4/bin
--enable-languages=c,c++ --disable-checking
Thread model: posix
gcc version 4.4.0 (GCC)
>/xxx/x86_gcc_4.4/bin/bin/gcc -O1 test.c
>./a.out
scale got (0.500000)
scale got (0.500000)
scale got (0.750000)
scale got (0.500000)
scale got (0.625000)
scale got (0.750000)
scale got (0.875000)
scale got (0.500000)
scale got (0.562500)
>/xxx/x86_gcc_4.4/bin/bin/gcc -O1 -fstrict-aliasing test.c
slarin@l1:~/test_gcc/qcc/bad> ./a.out
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
scale got (0.500000)
Same exercise with:
> gcc -v
Reading specs from /usr/lib64/gcc-lib/x86_64-suse-linux/3.3.3/specs
Configured with: ../configure --enable-threads=posix --prefix=/usr
--with-local-prefix=/usr/local --infodir=/usr/share/info
--mandir=/usr/share/man --enable-languages=c,c++,f77,objc,java,ada
--disable-checking --libdir=/usr/lib64 --enable-libgcj
--with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib64
--with-system-zlib --enable-shared --enable-__cxa_atexit x86_64-suse-linux
Thread model: posix
gcc version 3.3.3 (SuSE Linux)
... yields correct results either way.
--
Summary: Alias stacked type cast interpretation regression
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: sergei_lus at yahoo dot com
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41072