------- Comment #6 from rakdver at atrey dot karlin dot mff dot cuni dot cz
2007-04-20 16:09 -------
Subject: Re: Missed invariant out of the loop with conditionals and shifts
> void quantum_cnot(int control, int target, unsigned long *state, int size)
> {
> int i;
>
> for(i=0; i<size; i++)
> {
> if (state[i] & ((unsigned long) 1 << control))
> state[i] ^= ((unsigned long) 1 << target);
> }
> }
>
> (and more similar loops in libquantum). It would be nice if rtl
> loop-invariant
> motion could detect this form:
>
> (insn 23 22 24 4 (parallel [
> (set (reg:DI 67)
> (lshiftrt:DI (reg:DI 62 [ D.1992 ])
> (subreg:QI (reg/v:SI 63 [ control ]) 0)))
> (clobber (reg:CC 17 flags))
> ]) 470 {*lshrdi3_1_rex64} (nil)
> (nil))
>
> (insn 24 23 25 4 (parallel [
> (set (reg:SI 68)
> (and:SI (subreg:SI (reg:DI 67) 0)
> (const_int 1 [0x1])))
> (clobber (reg:CC 17 flags))
> ]) 301 {*andsi_1} (nil)
> (nil))
>
> and move the invariant (1 << control).
how exactly do you imagine this transformation should work? The insns
you show essentially are
tmp = x >> control;
z = tmp & 1;
I do not see how to transform just this pattern profitably (without also
seeing that z is only used in condition).
I could hack something in, however handling just this single pattern
specially seems a bit weird to me.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29789
