[Bug c/47255] New: Missed optimization with loops, inline functions, and __attribute__((const))

2011-01-10 Thread in-gnu at baka dot org 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

   Summary: Missed optimization with loops, inline functions, and
__attribute__((const))
   Product: gcc
   Version: 4.5.2
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: in-...@baka.org


Created attachment 22942
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22942
Test program exhibiting missed optimization

Using gcc 4.5.2 on gentoo (or gcc 4.1.2 on RHEL 5.5), gcc fails to optimize a
case where a const function is called in a const way inside a loop, if the
const function is elgible for inlining.

The test case should print three lines if it succeeds.  In fact, the "World"
line is printed once per loop iteration.

How-To-Repeat:

Compile the attach test-case with -O3 or "-O -finline-small-functions
-finline-functions" -- more than three lines are printed.  Compile with -O2,
the correct number of lines (three) for full optimization are printed.

Release:
--
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-libgcj-multifile
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --enable-plugin
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20080704 (Red Hat 4.1.2-48)
--
Using built-in specs.
COLLECT_GCC=/usr/x86_64-pc-linux-gnu/gcc-bin/4.5.2/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/4.5.2/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with:
/n/startide/proj/startide/var-tmp/portage/sys-devel/gcc-4.5.2/work/gcc-4.5.2/configure
--prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.5.2
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.2/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.5.2
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.5.2/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.5.2/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.5.2/include/g++-v4
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable-altivec
--disable-fixed-point --without-ppl --without-cloog --disable-lto --enable-nls
--without-included-gettext --with-system-zlib --disable-werror
--enable-secureplt --enable-multilib --enable-libmudflap --disable-libssp
--enable-libgomp --enable-cld
--with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/4.5.2/python
--enable-checking=release --disable-libgcj --enable-languages=c,c++
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu --with-bugurl=http://bugs.gentoo.org/
--with-pkgversion='Gentoo 4.5.2 p1.0, pie-0.4.5'
Thread model: posix
gcc version 4.5.2 (Gentoo 4.5.2 p1.0, pie-0.4.5)
--

[Bug c/47255] Missed optimization with loops, inline functions, and __attribute__((const))

2011-01-11 Thread in-gnu at baka dot org 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

--- Comment #2 from Seth Robertson  2011-01-11 18:40:36 
UTC ---
Created attachment 22946
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22946
Revised test program exhibiting missed optimization

Well, my point is that even if I remove the obvious side-effect, gcc cannot
take advantage of the knowledge I am providing it.  If instead of a printf() I
make a system call which will return a (what I know/guarantee is, but is not
declared as, a) ((const)) value (like getgid(), getuid(), or geteuid()) and use
it in the expression, it is likewise not optimized.  I'm attaching a second
test program using these system calls.  Test with `strace ./x 2>&1 | grep get |
wc -l`.  When you compile w/`gcc -O2` you should get the number 12.  When you
compile w/`gcc -O3` or with "-finline-functions -finline-small-functions", you
get 21 (ie. it did more work than necessary).  Of course without any
optimization you get the maximum of 30.

I am perfectly willing to believe that inlining happens first and throws away
the information such that it will miss the opportunity to CSE.  I also believe
that if gcc could intuit that it was const (e.g. pure multiplication
operations) that it wouldn't duplicate the work. I'm just saying this is a
missed optimization.

There could be a dup bug somewhere, but none of the tickets with "pure" or
"const" and "__attributes__" subjects discuss it.  Nor "inline' and 'cse'.

[Bug tree-optimization/47255] Missed CSE optimization with inline functions, and __attribute__((const))

2011-02-07 Thread in-gnu at baka dot org 
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

Seth Robertson  changed:

   What|Removed |Added

  Attachment #22946|0   |1
is obsolete||

--- Comment #5 from Seth Robertson  2011-02-07 16:50:45 
UTC ---
Created attachment 23265
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23265
Correct version of revised test program exhibiting missed optimization

Hmm.  I somehow managed to not attach the correct second example, I must have
changed the program between the attach and submit steps or something.obsoleting
the incorrect test progam

But I as a developer have additional information not known to the compiler.  I
know that getgid and getuid are const FOR ME because I'm not going to be
running setuid and friends and those are not changible through external force
through standard APIs (unlike, say, current priority).  We have a way to
provide this sort of information to the compiler.  Why shouldn't the compiler
take advantage of the information I provide?