------- 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