[Bug c/66552] Missed optimization when shift amount is result of signed modulus

[Bug c/66552] Missed optimization when shift amount is result of signed modulus

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Target|x86/generic                 |x86, powerpc64

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
On powerpc64, with -O2, GCC emits the following 

f:
        srawi 9,4,5
        addze 9,9
        slwi 9,9,5
        subf 4,9,4
        srw 3,3,4
        rldicl 3,3,0,32
        blr

while Clang emits what looks like optimal code:

f:
        rlwinm 4, 4, 0, 27, 31
        srw 3, 3, 4
        blr

With the return expression rewritten as suggested (i.e., x >> (n & 31)), GCC
still emits an extra instruction compare to Clang.  I believe this aspect is
tracked in bug 66217.

        rldicl 4,4,0,59
        srw 3,3,4
        rldicl 3,3,0,32
        blr

[Bug rtl-optimization/66552] Missed optimization when shift amount is result of signed modulus

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-06-30
                 CC|                            |segher at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Please file a separate bug for that last problem (it seems to be a
combine problem; it has nothing to do with PR66217).  You can assign
it to me if you want.

Confirmed, btw.

[Bug rtl-optimization/66552] Missed optimization when shift amount is result of signed modulus

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #2)

I opened bug 66706.

[Bug rtl-optimization/66552] Missed optimization when shift amount is result of signed modulus

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

--- Comment #15 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jiu Fu Guo <guojiufu@gcc.gnu.org>:

https://gcc.gnu.org/g:222f312a773157f53396ba4cb287deb07f84cc34

commit r11-4033-g222f312a773157f53396ba4cb287deb07f84cc34
Author: guojiufu <guojiufu@linux.ibm.com>
Date:   Mon Oct 19 13:48:19 2020 +0800

    [PATCH] fold x << (n % C) to x << (n & C-1) if C meets power2

    This patch fixes PR66552 which is also as:
    https://gcc.gnu.org/pipermail/gcc-patches/2020-February/540930.html
    which requests to optimizes (x shift (n mod C)) to
    (x shift (n bit_and (C - 1))) when C is a constant and power of two.

    gcc/ChangeLog
    2020-10-19  Li Jia He  <helijia@gcc.gnu.org>

            PR tree-optimization/66552
            * match.pd (x << (n % C) -> x << (n & C-1)): New simplification.

    gcc/testsuite/ChangeLog
    2020-10-19  Li Jia He  <helijia@gcc.gnu.org>

            PR tree-optimization/66552
            * gcc.dg/pr66552.c: New testcase.

[Bug rtl-optimization/66552] Missed optimization when shift amount is result of signed modulus

[guojiufu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

Jiu Fu Guo <guojiufu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |guojiufu at gcc dot gnu.org
         Resolution|---                         |FIXED

--- Comment #16 from Jiu Fu Guo <guojiufu at gcc dot gnu.org> ---
Just confirmed the fix is ready in the trunk.