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Re: ARM conditional instruction optimisation bug (feature?)


On 7/30/09, Zoltán Kócsi <zoltan@bendor.com.au> wrote:
> On the ARM every instruction can be executed conditionally. GCC very
> cleverly uses this feature:
>
> int bar ( int x, int a, int b )
> {
>   if ( x )
>
>      return a;
>    else
>      return b;
> }
>
> compiles to:
>
> bar:
>        cmp     r0, #0          // test x
>        movne   r0, r1          // retval = 'a' if !0 ('ne')
>        moveq   r0, r2          // retval = 'b' if 0 ('eq')
>        bx      lr
>
> However, the following function:
>
> extern unsigned array[ 128 ];
>
> int     foo( int x )
> {
>   int     y;
>
>   y = array[ x & 127 ];
>
>   if ( x & 128 )
>
>      y = 123456789 & ( y >> 2 );
>   else
>      y = 123456789 & y;
>
>   return y;
> }
>
> compiled with gcc 4.4.0, using -Os generates this:
>
> foo:
>
>        ldr     r3, .L8
>        tst     r0, #128
>        and     r0, r0, #127
>        ldr     r3, [r3, r0, asl #2]
>        ldrne   r0, .L8+4            ***
>        ldreq   r0, .L8+4            ***
>        movne   r3, r3, asr #2
>        andne   r0, r3, r0           ***
>        andeq   r0, r3, r0           ***
>        bx      lr
> .L8:
>        .word   array
>        .word   123456789
>
> The lines marked with the *** -s do the same, one executing if the
> condition is one way, the other if the condition is the opposite.
> That is, together they perform one unconditional instruction, except
> that they use two instuctions (and clocks) instead of one.
>
> Compiling with -O2 makes things even worse, because an other issue hits:
> gcc sometimes changes a "load constant" to a "generate the constant on
> the fly" even when the latter is both slower and larger, other times it
> chooses to load a constant even when it can easily (and more cheaply)
> generate it from already available values. In this particular case it
> decides to build the constant from pieces and combines that with
> the generate an unconditional instruction using two complementary
> conditional instructions method, resulting in this:
>
> foo:
>        ldr     r3, .L8
>        tst     r0, #128
>        and     r0, r0, #127
>        ldr     r0, [r3, r0, asl #2]
>        movne   r0, r0, asr #2
>        bicne   r0, r0, #-134217728
>        biceq   r0, r0, #-134217728
>        bicne   r0, r0, #10747904
>        biceq   r0, r0, #10747904
>        bicne   r0, r0, #12992
>        biceq   r0, r0, #12992
>        bicne   r0, r0, #42
>        biceq   r0, r0, #42
>        bx      lr
> .L8:
>        .word   array
>
> Should I report a bug?

This looks like my bug PR21803 (gcc.gnu.org/PR21803). Can you check if
the ce3 pass creates this code? (Compile with -fdump-rtl-all and look
at the .ce3 dump and one dump before to see if the .ce3 pass created
your funny sequence.)

If your problem is indeed caused by the ce3 pass, you should add your
problem to PR21803, change the "Component" field to "middle-end", and
adjust the bug summary to make it clear that this is not ia64
specific.

Ciao!
Steven


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