[Bug target/56110] New: Sub-optimal code: unnecessary CMP after AND

[Bug target/56110] New: Sub-optimal code: unnecessary CMP after AND

[tilman@code-monkey.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56110

             Bug #: 56110
           Summary: Sub-optimal code: unnecessary CMP after AND
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tilman@code-monkey.de
            Target: arm


Compiling this code

unsigned f1 (unsigned x, unsigned m)
{
    if (m & 0x008080)
        x >>= 8;

    return x;
}

with gcc 4.7.2 gives this code for ARMv5:

$ armv5tel-softfloat-linux-gnueabi-gcc -O2 -S -o- f1.c
[...]
    ldr    r3, .L6
    and    r3, r1, r3       @ XXX
    cmp    r3, #0           @ XXX
    movne    r0, r0, lsr #8
    bx    lr
[...]

AFAICS we could get rid of the CMP if we used ANDS instead of AND:

    ldr    r3, .L6
    ands    r3, r1, r3
    movne    r0, r0, lsr #8
    bx    lr

[Bug target/56110] Sub-optimal code: unnecessary CMP after AND

[tilman@code-monkey.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56110

--- Comment #1 from Tilman Sauerbeck <tilman@code-monkey.de> 2013-02-16 16:49:34 UTC ---
Changing the literal in the test function so that it fits in 8 bits makes gcc
go with the TST instruction instead of AND+CMP:

unsigned f2 (unsigned x, unsigned m)
{
    if (m & 0x80)
        x >>= 8;

    return x;
}

=>
    tst    r1, #128
    movne    r0, r0, lsr #8
    bx    lr

So I guess I shouldn't ask why gcc generates AND+CMP instead of ANDS, but why
it chooses not to use TST.

[Bug target/56110] Sub-optimal code: unnecessary CMP after AND

[tilman@code-monkey.de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56110

--- Comment #2 from Tilman Sauerbeck <tilman@code-monkey.de> 2013-03-16 09:01:17 UTC ---
Created attachment 29677
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29677
WIP patch

I'm not really sure if this patch is good or bad. The discussion on the ML died
off eventually (maybe because I asked one too many silly questions).

Anyway, without guidance I won't be able to finish this, but I want to post my
WIP here at least.

[Bug target/56110] Sub-optimal code: unnecessary CMP after AND

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56110

Ramana Radhakrishnan <ramana at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-08-05
                 CC|                            |ramana at gcc dot gnu.org
     Ever confirmed|0                           |1