Your message dated Sun, 30 May 2010 19:47:14 +0200
with message-id <4c02a4a2.9070...@debian.org>
and subject line tagging gcc-4.1 bug reports as wontfix, and closing them
has caused the Debian Bug report #418795,
regarding gcc-4.1: profiling (-pg) FPU bound application using
-funsafe-math-optimizations yields incorrect timing
to be marked as done.
This means that you claim that the problem has been dealt with.
If this is not the case it is now your responsibility to reopen the
Bug report if necessary, and/or fix the problem forthwith.
(NB: If you are a system administrator and have no idea what this
message is talking about, this may indicate a serious mail system
misconfiguration somewhere. Please contact ow...@bugs.debian.org
immediately.)
--
418795: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=418795
Debian Bug Tracking System
Contact ow...@bugs.debian.org with problems
--- Begin Message ---
Package: gcc-4.1
Version: 4.1.1-21
Severity: normal
I am profiling an application that performs a lot of log2(.) operations. I
wanted to use
-funsafe-math-optimizations to have log2 executed by the FPU directly, instead
of using
a function call to the c library. Strangely enough, although wall-clock time
and CPU time
go down, the number of seconds that gprof reports now roughly quadruples,
exceeding the
total CPU time by a factor of two.
The attached c++ file reproduces the problem. I get
** run without -funsafe-math-optimizations **
g++ -pg test.cpp && time ./a.out && gprof ./a.out
8.01531e+09
real 0m42.432s
user 0m42.143s
sys 0m0.148s
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls Ts/call Ts/call name
100.00 13.11 13.11 main
0.00 13.11 0.00 1 0.00 0.00 global constructors keyed
to main
0.00 13.11 0.00 1 0.00 0.00
__static_initialization_and_destruction_0(int, int)
** run with -funsafe-math-optimizations **
g++ -funsafe-math-optimizations -pg test.cpp && time ./a.out && gprof ./a.out
8.01531e+09
real 0m30.165s
user 0m29.898s
sys 0m0.060s
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls Ts/call Ts/call name
100.00 74.87 74.87 main
0.00 74.87 0.00 1 0.00 0.00 global constructors keyed
to main
0.00 74.87 0.00 1 0.00 0.00
__static_initialization_and_destruction_0(int, int)
Notice that when enabling -funsafe-math-optimizations
- wall clock time goes down from 43 to 31 sec.
- gprof time goes up from 14 to 75 sec.
- it is strange that
* without -funsafe-math-optimizations the gprof time (14 sec) is much lower
than wall clock time (43 sec)
* with -funsafe-math-optimizations the gprof time (75 sec) is much higher
than wall clock time (31 sec)
the first (*) can be explained by samples that occur in the log2 system call.
the second (*) can not be correct. I suspect that the instrumentation code that
gcc inserts into the
compiled program expects the FPU to be in a specific IEEE/ansi state, which it
is not when the
-funsafe-math-optimizations flag is used.
With kind regards,
Wouter Koolen-Wijkstra
-- System Information:
Debian Release: lenny/sid
APT prefers testing
APT policy: (990, 'testing'), (500, 'unstable'), (500, 'stable')
Architecture: i386 (i686)
Kernel: Linux 2.6.18v10 (SMP w/2 CPU cores; PREEMPT)
Locale: LANG=en_US, LC_CTYPE=en_US (charmap=ISO-8859-1)
Shell: /bin/sh linked to /bin/bash
Versions of packages gcc-4.1 depends on:
ii binutils 2.17-3 The GNU assembler, linker and bina
ii cpp-4.1 4.1.1-21 The GNU C preprocessor
ii gcc-4.1-base 4.1.1-21 The GNU Compiler Collection (base
ii libc6 2.3.6.ds1-13 GNU C Library: Shared libraries
ii libgcc1 1:4.1.1-21 GCC support library
ii libssp0 4.1.1-21 GCC stack smashing protection libr
Versions of packages gcc-4.1 recommends:
ii libc6-dev 2.3.6.ds1-13 GNU C Library: Development Librari
pn libmudflap0-dev <none> (no description available)
-- no debconf information
#include <cmath>
#include <iostream>
using namespace std;
int main(int argc, const char ** argv) {
// do something that takes noticeable time (~30 sec)
// and that does many floating point operations
double d = 0;
for (int i = 1; i < 300000000; i++) {
d += log2(i);
}
cout << d << endl;
return 0;
}
--- End Message ---
--- Begin Message ---
tag 471767 + wontfix
tag 425527 + wontfix
tag 435907 + wontfix
tag 424918 + wontfix
tag 426302 + wontfix
tag 418795 + wontfix
tag 426302 + moreinfo
tag 418795 + moreinfo
tag 394325 + wontfix
tag 394325 + moreinfo
tag 355621 + wontfix
tag 355621 + moreinfo
thanks
tagging issues reported against gcc-4.1 and closing them. Many of these reports
had unanswered requests for missing information for a long time. If any of
these issues are seen with the current default gcc (gcc-4.4), please add the
missing information, reassign the report and reopen. If the issue is
reproducible with 4.4, please check with gcc-4.5 (experimental) and current
trunk (gcc-snapshot) too.
--- End Message ---
