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

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

[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: unassigned@gcc.gnu.org
        ReportedBy: in-gnu@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))

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-11 12:38:09 UTC ---
There is a duplicate somewhere for this bug.  Inlining happens before any
CSE is done which is what you see.  If you remove the side-effect the
multiplications will be CSEd instead after inlining.

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

[in-gnu at baka dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

--- Comment #2 from Seth Robertson <in-gnu at baka dot org> 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))

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.01.12 11:10:22
          Component|c                           |tree-optimization
     Ever Confirmed|0                           |1

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-12 11:10:22 UTC ---
Sure, it is a missed optimization.  It's even "easy" to fix when you accept
a general compile-time slowdown (just schedule some CSE passes before
inlining).
When you want to avoid the compile-time slowdown then it's not so easy
(it's a usual trade-off with the case where there is no CSE opportunity but
earlier inlining would result in better code).

You can force GCC to not inline the function with using
__attribute__((const,noinline)) (but it of course will then be not
inlined at all).

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

[bonzini at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

Paolo Bonzini <bonzini at gnu dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bonzini at gnu dot org

--- Comment #4 from Paolo Bonzini <bonzini at gnu dot org> 2011-02-07 10:57:54 UTC ---
I think this is invalid.  const attributes are a hint to GCC regarding parts of
the program that it cannot see, but IMHO the const/pure/nothrow on a function
that is static and a leaf should have no effect on code generation (since GCC
can infer just as much).

So, in the first example GCC is "fixing" a wrong usage of const on part of the
program.

In the second example attached, there is no use of syscalls and GCC properly
optimizes out square2 and square3.  If syscalls were added, the bug would be
about missed attributes on the syscalls.  BTW, getgid, getuid etc. are pure but
not const.

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

[in-gnu at baka dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

Seth Robertson <in-gnu at baka dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #22946|0                           |1
        is obsolete|                            |

--- Comment #5 from Seth Robertson <in-gnu at baka dot org> 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?

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

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
   Last reconfirmed|2011-01-12 11:10:22         |2021-11-27

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

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47255

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |khim at google dot com

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 114305 has been marked as a duplicate of this bug. ***