On Wed, Sep 14, 2011 at 8:24 AM, Richard Sandiford <rdsandif...@googlemail.com> wrote: > At the moment, fwprop will propagate constants and registers > even if no further rtl simplifications are possible: > > if (REG_P (new_rtx) || CONSTANT_P (new_rtx)) > flags |= PR_CAN_APPEAR; > > What do you think about extending this to subregs? The reason for > asking is that on NEON, vector loads like vld4 are represented as a load > of a single monolithic register followed by subreg extractions of each > vector: > > (set (reg:OI FULL) (...)) > (set (reg:V2SI V0) (subreg:V2SI (reg:OI FULL) 0)) > (set (reg:V2SI V1) (subreg:V2SI (reg:OI FULL) 16)) > (set (reg:V2SI V2) (subreg:V2SI (reg:OI FULL) 32)) > (set (reg:V2SI V3) (subreg:V2SI (reg:OI FULL) 48)) > > Nothing ever propagates these subregs, so the separate moves > survive until IRA. This has three problems: > > - We generally want the registers allocated to V0...V3 to be the same > as FULL, so that the four subreg moves become nops. And this often > happens in simple examples. But if register pressure is relatively > high, these moves can sometimes cause IRA to spill in cases where > it doesn't if the subregs are used instead of each Vi. > > - Perhaps related, register pressure becomes harder to estimate. > > - These moves can interfere with pre-reload scheduling. > > In combination with the MODES_TIEABLE_P patch that I posted here: > > http://gcc.gnu.org/ml/gcc-patches/2011-09/msg00626.html > > this patch significantly improves the code generated for several libav > loops. Unfortunately, I don't have a setup that can do meaningful > x86_64 performance measurements, but a diff of the before and after > output for libav showed many cases where the patch removed moves. > > What do you think? Alternatives include propagating in lower-subreg, > or maybe only in the second fwprop pass. > > Richard > > > gcc/ > * fwprop.c (propagate_rtx): Also set PR_CAN_APPEAR for subregs. > > Index: gcc/fwprop.c > =================================================================== > --- gcc/fwprop.c 2011-08-26 09:58:28.829540497 +0100 > +++ gcc/fwprop.c 2011-08-26 10:14:03.767707504 +0100 > @@ -664,7 +664,7 @@ propagate_rtx (rtx x, enum machine_mode > return NULL_RTX; > > flags = 0; > - if (REG_P (new_rtx) || CONSTANT_P (new_rtx)) > + if (REG_P (new_rtx) || CONSTANT_P (new_rtx) || GET_CODE (new_rtx) == > SUBREG) > flags |= PR_CAN_APPEAR; > if (!for_each_rtx (&new_rtx, varying_mem_p, NULL)) > flags |= PR_HANDLE_MEM; >
A SUBREG may not be REG nor CONSTANT. Don't you need to check REG_P/CONSTANT_P on SUBREG? -- H.J.
