Pulling this one back as I have a better solution, patch coming shortly.
Thanks,
Teresa
On Fri, Mar 16, 2012 at 3:33 PM, Teresa Johnson <tejohn...@google.com> wrote:
>
> Ping - now that stage 1 is open, could someone review?
>
> Thanks,
> Teresa
>
> On Sun, Dec 4, 2011 at 10:26 PM, Teresa Johnson <tejohn...@google.com> wrote:
> > Latest patch which improves the efficiency as described below is
> > included here. Boostrapped and checked again with
> > x86_64-unknown-linux-gnu. Could someone review?
> >
> > Thanks,
> > Teresa
> >
> > 2011-12-04 Teresa Johnson <tejohn...@google.com>
> >
> > * loop-unroll.c (decide_unroll_constant_iterations): Call loop
> > unroll target hook.
> > * config/i386/i386.c (ix86_loop_unroll_adjust): New function.
> > (TARGET_LOOP_UNROLL_ADJUST): Define hook for x86.
> >
> > ===================================================================
> > --- loop-unroll.c (revision 181902)
> > +++ loop-unroll.c (working copy)
> > @@ -547,6 +547,9 @@ decide_unroll_constant_iterations (struc
> > if (nunroll > (unsigned) PARAM_VALUE (PARAM_MAX_UNROLL_TIMES))
> > nunroll = PARAM_VALUE (PARAM_MAX_UNROLL_TIMES);
> >
> > + if (targetm.loop_unroll_adjust)
> > + nunroll = targetm.loop_unroll_adjust (nunroll, loop);
> > +
> > /* Skip big loops. */
> > if (nunroll <= 1)
> > {
> > Index: config/i386/i386.c
> > ===================================================================
> > --- config/i386/i386.c (revision 181902)
> > +++ config/i386/i386.c (working copy)
> > @@ -60,6 +60,7 @@ along with GCC; see the file COPYING3.
> > #include "fibheap.h"
> > #include "opts.h"
> > #include "diagnostic.h"
> > +#include "cfgloop.h"
> >
> > enum upper_128bits_state
> > {
> > @@ -38370,6 +38371,82 @@ ix86_autovectorize_vector_sizes (void)
> > return (TARGET_AVX && !TARGET_PREFER_AVX128) ? 32 | 16 : 0;
> > }
> >
> > +/* If LOOP contains a possible LCP stalling instruction on corei7,
> > + calculate new number of times to unroll instead of NUNROLL so that
> > + the unrolled loop will still likely fit into the loop stream detector.
> > */
> > +static unsigned
> > +ix86_loop_unroll_adjust (unsigned nunroll, struct loop *loop)
> > +{
> > + basic_block *body, bb;
> > + unsigned i;
> > + rtx insn;
> > + bool found = false;
> > + unsigned newunroll;
> > +
> > + if (ix86_tune != PROCESSOR_COREI7_64 &&
> > + ix86_tune != PROCESSOR_COREI7_32)
> > + return nunroll;
> > +
> > + /* Look for instructions that store a constant into HImode (16-bit)
> > + memory. These require a length-changing prefix and on corei7 are
> > + prone to LCP stalls. These stalls can be avoided if the loop
> > + is streamed from the loop stream detector. */
> > + body = get_loop_body (loop);
> > + for (i = 0; i < loop->num_nodes; i++)
> > + {
> > + bb = body[i];
> > +
> > + FOR_BB_INSNS (bb, insn)
> > + {
> > + rtx set_expr, dest;
> > + set_expr = single_set (insn);
> > + if (!set_expr)
> > + continue;
> > +
> > + dest = SET_DEST (set_expr);
> > +
> > + /* Don't reduce unroll factor in loops with floating point
> > + computation, which tend to benefit more heavily from
> > + larger unroll factors and are less likely to bottleneck
> > + at the decoder. */
> > + if (FLOAT_MODE_P (GET_MODE (dest)))
> > + {
> > + free (body);
> > + return nunroll;
> > + }
> > +
> > + if (!found
> > + && GET_MODE (dest) == HImode
> > + && CONST_INT_P (SET_SRC (set_expr))
> > + && MEM_P (dest))
> > + {
> > + found = true;
> > + /* Keep walking loop body to look for FP computations above.
> > */
> > + }
> > + }
> > + }
> > + free (body);
> > +
> > + if (!found)
> > + return nunroll;
> > +
> > + if (dump_file)
> > + {
> > + fprintf (dump_file,
> > + ";; Loop contains HImode store of const (possible LCP
> > stalls),\n");
> > + fprintf (dump_file,
> > + " reduce unroll factor to fit into Loop Stream
> > Detector\n");
> > + }
> > +
> > + /* On corei7 the loop stream detector can hold 28 uops, so
> > + don't allow unrolling to exceed that many instructions. */
> > + newunroll = 28 / loop->av_ninsns;
> > + if (newunroll < nunroll)
> > + return newunroll;
> > +
> > + return nunroll;
> > +}
> > +
> > /* Initialize the GCC target structure. */
> > #undef TARGET_RETURN_IN_MEMORY
> > #define TARGET_RETURN_IN_MEMORY ix86_return_in_memory
> > @@ -38685,6 +38762,9 @@ ix86_autovectorize_vector_sizes (void)
> > #define TARGET_INIT_LIBFUNCS darwin_rename_builtins
> > #endif
> >
> > +#undef TARGET_LOOP_UNROLL_ADJUST
> > +#define TARGET_LOOP_UNROLL_ADJUST ix86_loop_unroll_adjust
> > +
> > struct gcc_target targetm = TARGET_INITIALIZER;
> >
> >
> > #include "gt-i386.h"
> >
> >
> > On Fri, Dec 2, 2011 at 12:11 PM, Teresa Johnson <tejohn...@google.com>
> > wrote:
> >> On Fri, Dec 2, 2011 at 11:36 AM, Andi Kleen <a...@firstfloor.org> wrote:
> >>> Teresa Johnson <tejohn...@google.com> writes:
> >>>
> >>> Interesting optimization. I would be concerned a little bit
> >>> about compile time, does it make a measurable difference?
> >>
> >> I haven't measured compile time explicitly, but I don't it should,
> >> especially after I address your efficiency suggestion (see below),
> >> since it will just have one pass over the instructions in innermost
> >> loops.
> >>
> >>>
> >>>> The attached patch detects loops containing instructions that tend to
> >>>> incur high LCP (loop changing prefix) stalls on Core i7, and limits
> >>>> their unroll factor to try to keep the unrolled loop body small enough
> >>>> to fit in the Corei7's loop stream detector which can hide LCP stalls
> >>>> in loops.
> >>>
> >>> One more optimization would be to optimize padding for this case,
> >>> the LSD only works if the loop is not spread over too many 32 byte
> >>> chunks. So if you detect the loop is LSD worthy always pad to 32 bytes
> >>> at the beginning.
> >>
> >> Thanks for the suggestion, I will look at doing that in follow-on tuning.
> >>
> >>>
> >>>> To do this I leveraged the existing TARGET_LOOP_UNROLL_ADJUST target
> >>>> hook, which was previously only defined for s390. I added one
> >>>> additional call to this target hook, when unrolling for constant trip
> >>>> count loops. Previously it was only called for runtime computed trip
> >>>> counts. Andreas, can you comment on the effect for s390 of this
> >>>> additional call of the target hook, since I can't measure that?
> >>>
> >>> On Sandy-Bridge there's also the decoded icache which is much larger,
> >>> but also has some restrictions. It would be nice if this optimization
> >>> was general enough to handle this case too.
> >>>
> >>> In general I notice that the tree loop unroller is too aggressive
> >>> recently:
> >>> a lot of loops that probably shouldn't be unrolled (like containing
> >>> function calls etc.) are unrolled at -O3. So probably a better cost
> >>> model for unrolling would make sense anyways.
> >>
> >> These are both good suggestions, and I will look into Sandy Bridge
> >> heuristics in follow-on work, since we will need to tune for that
> >> soon.
> >>
> >>>
> >>>> + /* Don't reduce unroll factor in loops with floating point
> >>>> + computation, which tend to benefit more heavily from
> >>>> + larger unroll factors and are less likely to bottleneck
> >>>> + at the decoder. */
> >>>> + has_FP = loop_has_FP_comp(loop);
> >>>
> >>> You could cache the loop body and pass it in here.
> >>
> >> That is a great idea, and in fact, I think I will do away with this
> >> separate function completely for this patch. I can more efficiently
> >> look for the FP computation while I am looking for the half word
> >> stores. I'll do that and send a follow up with the new patch.
> >>
> >>>
> >>> Patch looks reasonable to me, but I cannot approve.
> >>
> >> Thanks!
> >> Teresa
> >>
> >>>
> >>> -Andi
> >>>
> >>> --
> >>> a...@linux.intel.com -- Speaking for myself only
> >>
> >>
> >>
> >> --
> >> Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
> >
> >
> >
> > --
> > Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
>
>
>
> --
> Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
--
Teresa Johnson | Software Engineer | tejohn...@google.com | 408-460-2413
