On Tue, Oct 2, 2012 at 6:40 PM, Kumar, Venkataramanan
<venkataramanan.ku...@amd.com> wrote:
> Hi Richi,
>
> (Snip)
>> + (!cst_and_fits_in_hwi (step))
>> + {
>> + if( loop->inner != NULL)
>> + {
>> + if (dump_file && (dump_flags & TDF_DETAILS))
>> + {
>> + fprintf (dump_file, "Reference %p:\n", (void *) ref);
>> + fprintf (dump_file, "(base " );
>> + print_generic_expr (dump_file, base, TDF_SLIM);
>> + fprintf (dump_file, ", step ");
>> + print_generic_expr (dump_file, step, TDF_TREE);
>> + fprintf (dump_file, ")\n");
>
> No need to repeat this - all references are dumped when we gather them.
> (Snip)
>
> The dumping happens at "record_ref" which is called after these statements to
> record these references.
>
> When the step is invariant we return from the function without recording the
> references.
>
> so I thought of dumping the references here.
>
> Is there a cleaner way to dump the references at one place?
Yes, call dump_mem_ref then, instead of repeating parts of its body.
Richard.
> Regards,
> Venkat.
>
>
>
> -----Original Message-----
> From: Richard Guenther [mailto:rguent...@suse.de]
> Sent: Tuesday, October 02, 2012 5:42 PM
> To: Kumar, Venkataramanan
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [Patch] Fix PR53397
>
> On Mon, 1 Oct 2012, venkataramanan.ku...@amd.com wrote:
>
>> Hi,
>>
>> The below patch fixes the FFT/Scimark regression caused by useless
>> prefetch generation.
>>
>> This fix tries to make prefetch less aggressive by prefetching arrays
>> in the inner loop, when the step is invariant in the entire loop nest.
>>
>> GCC currently tries to prefetch invariant steps when they are in the
>> inner loop. But does not check if the step is variant in outer loops.
>>
>> In the scimark FFT case, the trip count of the inner loop varies by a
>> non constant step, which is invariant in the inner loop.
>> But the step variable is varying in outer loop. This makes inner loop
>> trip count small (at run time varies sometimes as small as 1
>> iteration)
>>
>> Prefetching ahead x iteration when the inner loop trip count is
>> smaller than x leads to useless prefetches.
>>
>> Flag used: -O3 -march=amdfam10
>>
>> Before
>> ** **
>> ** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
>> ** for details. (Results can be submitted to p...@nist.gov) **
>> ** **
>> Using 2.00 seconds min time per kenel.
>> Composite Score: 550.50
>> FFT Mflops: 38.66 (N=1024)
>> SOR Mflops: 617.61 (100 x 100)
>> MonteCarlo: Mflops: 173.74
>> Sparse matmult Mflops: 675.63 (N=1000, nz=5000)
>> LU Mflops: 1246.88 (M=100, N=100)
>>
>>
>> After
>> ** **
>> ** SciMark2 Numeric Benchmark, see http://math.nist.gov/scimark **
>> ** for details. (Results can be submitted to p...@nist.gov) **
>> ** **
>> Using 2.00 seconds min time per kenel.
>> Composite Score: 639.20
>> FFT Mflops: 479.19 (N=1024)
>> SOR Mflops: 617.61 (100 x 100)
>> MonteCarlo: Mflops: 173.18
>> Sparse matmult Mflops: 679.13 (N=1000, nz=5000)
>> LU Mflops: 1246.88 (M=100, N=100)
>>
>> GCC regression "make check -k" passes with x86_64-unknown-linux-gnu
>> New tests that PASS:
>>
>> gcc.dg/pr53397-1.c scan-assembler prefetcht0 gcc.dg/pr53397-1.c
>> scan-tree-dump aprefetch "Issued prefetch"
>> gcc.dg/pr53397-1.c (test for excess errors) gcc.dg/pr53397-2.c
>> scan-tree-dump aprefetch "loop variant step"
>> gcc.dg/pr53397-2.c scan-tree-dump aprefetch "Not prefetching"
>> gcc.dg/pr53397-2.c (test for excess errors)
>>
>>
>> Checked CPU2006 and polyhedron on latest AMD processor, no regressions noted.
>>
>> Ok to commit in trunk?
>>
>> regards,
>> Venkat
>>
>> gcc/ChangeLog
>> +2012-10-01 Venkataramanan Kumar <venkataramanan.ku...@amd.com>
>> +
>> + * tree-ssa-loop-prefetch.c (gather_memory_references_ref):$
>> + Perform non constant step prefetching in inner loop, only $
>> + when it is invariant in the entire loop nest. $
>> + * testsuite/gcc.dg/pr53397-1.c: New test case $
>> + Checks we are prefecthing for loop invariant steps$
>> + * testsuite/gcc.dg/pr53397-2.c: New test case$
>> + Checks we are not prefecthing for loop variant steps
>> +
>>
>>
>> Index: gcc/testsuite/gcc.dg/pr53397-1.c
>> ===================================================================
>> --- gcc/testsuite/gcc.dg/pr53397-1.c (revision 0)
>> +++ gcc/testsuite/gcc.dg/pr53397-1.c (revision 0)
>> @@ -0,0 +1,28 @@
>> +/* Prefetching when the step is loop invariant. */
>> +
>> +/* { dg-do compile } */
>> +/* { dg-options "-O3 -fprefetch-loop-arrays
>> +-fdump-tree-aprefetch-details --param min-insn-to-prefetch-ratio=3
>> +--param simultaneous-prefetches=10 -fdump-tree-aprefetch-details" }
>> +*/
>> +
>> +
>> +double data[16384];
>> +void prefetch_when_non_constant_step_is_invariant(int step, int n) {
>> + int a;
>> + int b;
>> + for (a = 1; a < step; a++) {
>> + for (b = 0; b < n; b += 2 * step) {
>> +
>> + int i = 2*(b + a);
>> + int j = 2*(b + a + step);
>> +
>> +
>> + data[j] = data[i];
>> + data[j+1] = data[i+1];
>> + }
>> + }
>> +}
>> +
>> +/* { dg-final { scan-tree-dump "Issued prefetch" "aprefetch" } } */
>> +/* { dg-final { scan-assembler "prefetcht0" } } */
>
> This (and the case below) needs to be adjusted to only run on the appropriate
> hardware. See for example gcc.dg/tree-ssa/prefetch-8.c for how to do this.
>
>> +/* { dg-final { cleanup-tree-dump "aprefetch" } } */
>> Index: gcc/testsuite/gcc.dg/pr53397-2.c
>> ===================================================================
>> --- gcc/testsuite/gcc.dg/pr53397-2.c (revision 0)
>> +++ gcc/testsuite/gcc.dg/pr53397-2.c (revision 0)
>> @@ -0,0 +1,29 @@
>> +/* Not prefetching when the step is loop variant. */
>> +
>> +/* { dg-do compile } */
>> +/* { dg-options "-O3 -fprefetch-loop-arrays
>> +-fdump-tree-aprefetch-details --param min-insn-to-prefetch-ratio=3
>> +--param simultaneous-prefetches=10 -fdump-tree-aprefetch-details" }
>> +*/
>> +
>> +
>> +double data[16384];
>> +void donot_prefetch_when_non_constant_step_is_variant(int step, int
>> +n) {
>> + int a;
>> + int b;
>> + for (a = 1; a < step; a++,step*=2) {
>> + for (b = 0; b < n; b += 2 * step) {
>> +
>> + int i = 2*(b + a);
>> + int j = 2*(b + a + step);
>> +
>> +
>> + data[j] = data[i];
>> + data[j+1] = data[i+1];
>> + }
>> + }
>> +}
>> +
>> +/* { dg-final { scan-tree-dump "Not prefetching" "aprefetch" } } */
>> +/* { dg-final { scan-tree-dump "loop variant step" "aprefetch" } } */
>> +
>> +/* { dg-final { cleanup-tree-dump "aprefetch" } } */
>> +
>> Index: gcc/tree-ssa-loop-prefetch.c
>> ===================================================================
>> --- gcc/tree-ssa-loop-prefetch.c (revision 191642)
>> +++ gcc/tree-ssa-loop-prefetch.c (working copy)
>> @@ -523,6 +523,7 @@
>> tree base, step;
>> HOST_WIDE_INT delta;
>> struct mem_ref_group *agrp;
>> + loop_p ploop;
>>
>> if (get_base_address (ref) == NULL)
>> return false;
>> @@ -537,10 +538,50 @@
>> if (may_be_nonaddressable_p (base))
>> return false;
>>
>> - /* Limit non-constant step prefetching only to the innermost loops.
>> */
>> - if (!cst_and_fits_in_hwi (step) && loop->inner != NULL)
>> - return false;
>> + /* Limit non-constant step prefetching only to the innermost loops and
>> + only when the step is invariant in the entire loop nest. */ if
>> + (!cst_and_fits_in_hwi (step))
>> + {
>> + if( loop->inner != NULL)
>> + {
>> + if (dump_file && (dump_flags & TDF_DETAILS))
>> + {
>> + fprintf (dump_file, "Reference %p:\n", (void *) ref);
>> + fprintf (dump_file, "(base " );
>> + print_generic_expr (dump_file, base, TDF_SLIM);
>> + fprintf (dump_file, ", step ");
>> + print_generic_expr (dump_file, step, TDF_TREE);
>> + fprintf (dump_file, ")\n");
>
> No need to repeat this - all references are dumped when we gather them.
>
>> + fprintf (dump_file, "Ignoring %p, non-constant step
>> prefetching\
>> + is limited to inner most loops \n",(void *) ref);
>> + }
>> + return false;
>> + }
>> + else
>> + {
>> + ploop = loop;
>>
>> + while (loop_outer (ploop))
>> + {
>> + if (!expr_invariant_in_loop_p (ploop , step))
>> + {
>> + if (dump_file && (dump_flags & TDF_DETAILS))
>> + {
>> + fprintf (dump_file, "Reference %p:\n", (void *) ref);
>> + fprintf (dump_file, "(base " );
>> + print_generic_expr (dump_file, base, TDF_SLIM);
>> + fprintf (dump_file, ", step ");
>> + print_generic_expr (dump_file, step, TDF_TREE);
>> + fprintf (dump_file, ")\n");
>
> Likewise.
>
>> + fprintf (dump_file, "Not prefetching, ignoring %p due
>> to loop variant step\n",(void *) ref);
>> + }
>> + return false;
>> + }
>> + ploop = loop_outer (ploop);
>> + }
>
> Instead of the loop checking expr_invariant_in_loop_p for each sub-loop you
> can query expr_invariant_in_loop_p once, for the outermost loop.
>
> Please add a new function to cfgloop.h to access this outermost loop:
>
> /* Returns the outermost loop of the loop nest that contains LOOP. */
>
> static inline struct loop *
> loop_outermost (const struct loop *loop) {
> unsigned n = VEC_length (loop_p, loop->superloops);
>
> if (n <= 1)
> return loop;
>
> return VEC_index (loop_p, loop->superloops, 1); }
>
> Thanks,
> Richard.
>
>
