Re: [Patch, i386] Limit unroll factor for certain loops on Corei7

[Xinliang David Li <davidxl at google dot com>]

The patch is good for google branches for now while waiting for upstream review.

David

On Sun, Dec 4, 2011 at 10:26 PM, Teresa Johnson <tejohnson@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 ?<tejohnson@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 <tejohnson@google.com> wrote:
>> On Fri, Dec 2, 2011 at 11:36 AM, Andi Kleen <andi@firstfloor.org> wrote:
>>> Teresa Johnson <tejohnson@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
>>>
>>> --
>>> ak@linux.intel.com -- Speaking for myself only
>>
>>
>>
>> --
>> Teresa Johnson?|?Software Engineer?|?tejohnson@google.com?|?408-460-2413
>
>
>
> --
> Teresa Johnson?|?Software Engineer?|?tejohnson@google.com?|?408-460-2413