#include <stdio.h>
#include <math.h>
int main()
{
double a= 10.0;
double b= 1e+308;
printf("%d %d %d\n", isinf(a*b), __builtin_isinf(a*b), __isinf(a*b));
return 0;
}
mtay...@drizzle-dev:~$ gcc -o test test.c
mtay...@drizzle-dev:~$ ./test
0 0 1
mtay...@drizzle-dev:~$ gcc -o test test.c -std=c99
mtay...@drizzle-dev:~$ ./test
1 0 1
mtay...@drizzle-dev:~$ gcc -o test test.c -mfpmath=sse -march=pentium4
mtay...@drizzle-dev:~$ ./test
1 1 1
mtay...@drizzle-dev:~$ g++ -o test test.c
mtay...@drizzle-dev:~$ ./test
1 0 1
Originally I found the simple isinf() case to be different on x86 than x86-64,
ppc32 and sparc (32 and 64).
After more research, I found that x86-64 uses the sse instructions to do it
(and using sse is the only way for __builtin_isinf() to produce correct
results). For the g++ built version, it calls __isinf() instead of inlining
(and as can be seen, the __isinf() version is always correct).
Specifically, it's because the optimised 387 code is doing the math in double
extended precision inside the FPU. 10.0*1e308 fits in 80bits but not in 64bit.
Any code that forces it to be stored and loaded gets the correct result too.
e.g.
mtay...@drizzle-dev:~$ cat test-simple.c
#include <stdio.h>
#include <math.h>
int main()
{
double a= 10.0;
double b= 1e+308;
volatile double c= a*b;
printf("%d\n", isinf(c));
return 0;
}
mtay...@drizzle-dev:~$ gcc -o test-simple test-simple.c
mtay...@drizzle-dev:~$ ./test-simple
1
With this code you can easily see the load and store:
8048407: dc 0d 18 85 04 08 fmull 0x8048518
804840d: dd 5d f0 fstpl -0x10(%ebp)
8048410: dd 45 f0 fldl -0x10(%ebp)
8048413: d9 e5 fxam
While if you remove volatile, the load and store doesn't happen (at least on
-O3, on -O0 it hasn't been optimised away):
8048407: dc 0d 18 85 04 08 fmull 0x8048518
804840d: c7 44 24 04 10 85 04 movl $0x8048510,0x4(%esp)
8048414: 08
8048415: c7 04 24 01 00 00 00 movl $0x1,(%esp)
804841c: d9 e5 fxam
This is also a regression from 4.2.4 as it just calls isinf() and doesn't
expand the 387 code inline. My guess is the 387 optimisation was added in 4.3.
Recommended fix: store and load in the 387 version so to operate on same
precision as elsewhere.
--
Summary: 387 optimised __builtin_isinf() gives incorrect result
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: regression
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: stewart at flamingspork dot com
GCC build triplet: gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu12)
GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39228
