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


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


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