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* [Bug c/29705] New: gcc optimizes necessary checks for isnan out of existence
@ 2006-11-03 22:27 james dot me at gmail dot com
2006-11-03 22:35 ` [Bug target/29705] " pinskia at gcc dot gnu dot org
0 siblings, 1 reply; 2+ messages in thread
From: james dot me at gmail dot com @ 2006-11-03 22:27 UTC (permalink / raw)
To: gcc-bugs
This bug applies at least to gcc version 4.0.3 and the current CVS 4.2.0
20061103. The assembly extracts included below were produced by gcc 4.2.0; the
gcc 4.0.3 extracts are analogous with respect to this problem but are different
in some respects. My particular host is an intel pentium4 xenon.
With certain math optimizations enabled, gcc assumes that a test for isnan will
always fail. This assumption is not correct, and results in the undesired
propagation of NANs into other computations and altered control flow.
This bug is exemplified by the following code which can be compiled without
including any header files. For brevity in the test case, instead of include
files I have used only the prototypes needed to silence gcc's warnings. The
code compiles the same, when <math.h> and <stdio.h> are included instead of the
prototypes, or when the prototypes are simply left out all together.
float logf(float);
int printf(const char *fmt, ...);
int isnan();
double test(int i0, int n, double a) {
double sum = 0.0;
int i;
for(i=i0; i<n; ++i) {
float x = logf((float)i);
sum += isnan(x) ? 0 : x;
}
return sum;
}
int main(void) {
printf("test(4, 6, 0) = %f\n", test(4,6,0));
printf("test(0, 2, 0) = %f\n", test(0,2,0));
printf("test(-2, 3, 0) = %f\n", test(-2,3,0));
return 0;
}
james@recoil:~/project/cf/util$ /home/james/local/gcc/bin/gcc -O2
-march=i686 -funsafe-math-optimizations -fno-math-errno test.c -o
test
james@recoil:~/project/cf/util$ ./test
test(4, 6, 0) = 2.995732
test(0, 2, 0) = -inf
test(-2, 3, 0) = nan <- incorrect, as written, test should preclude nan as a
result
james@recoil:~/project/cf/util$ /home/james/local/gcc/bin/gcc -O2
-march=i686 test.c -o testb -lm
james@recoil:~/project/cf/util$ ./testb
test(4, 6, 0) = 2.995732
test(0, 2, 0) = -inf
test(-2, 3, 0) = -inf <- correct
james@recoil:~/project/cf/util$ /home/james/local/gcc/bin/gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../gcc-4-2/configure --prefix=/home/james/local/gcc
Thread model: posix
gcc version 4.2.0 20061103 (prerelease)
The assembly produced for the test case that runs correctly has this as the
main loop:
.L5:
pushl %ebx
fildl (%esp)
addl $4, %esp
fstps (%esp)
fstpl -24(%ebp)
call logf
fucomi %st(0), %st ;; check for NAN
fldz
fcmovnu %st(1), %st ;; conditional based on check
fstp %st(1)
addl $1, %ebx
cmpl %esi, %ebx
fldl -24(%ebp)
faddp %st, %st(1)
jne .L5
The (over) optimized assembly has this for the loop:
.L5:
pushl %eax
addl $1, %eax
fildl (%esp)
addl $4, %esp
cmpl %edx, %eax
fldln2
fxch %st(1)
fyl2x ;; log is computed inline due to -funsafe-math-optimizations
faddp %st, %st(1) ;; result is added unconditionally this does
;; not belong in -funsafe-math-optimizations
;; (perhaps -funusable-math-optimizations)
jne .L5
Uros Bizjak reported (on the gcc mailing list) that
(1) gcc 4.3 does not exhibit this incorrect behavior
(2) this is a bug.
--
Summary: gcc optimizes necessary checks for isnan out of
existence
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: james dot me at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29705
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug target/29705] gcc optimizes necessary checks for isnan out of existence
2006-11-03 22:27 [Bug c/29705] New: gcc optimizes necessary checks for isnan out of existence james dot me at gmail dot com
@ 2006-11-03 22:35 ` pinskia at gcc dot gnu dot org
0 siblings, 0 replies; 2+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-11-03 22:35 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2006-11-03 22:35 -------
This is a dup of bug 19116 which was fixed in 4.3.0.
*** This bug has been marked as a duplicate of 19116 ***
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |DUPLICATE
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29705
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2006-11-03 22:27 [Bug c/29705] New: gcc optimizes necessary checks for isnan out of existence james dot me at gmail dot com
2006-11-03 22:35 ` [Bug target/29705] " pinskia at gcc dot gnu dot org
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