Re: [PATCH] vect, tree-optimization/105219: Disable epilogue vectorization when peeling for alignment

[Richard Biener <rguenther@suse.de>]

On Tue, 26 Apr 2022, Richard Sandiford wrote:

> "Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com> writes:
> > Hi,
> >
> > This patch disables epilogue vectorization when we are peeling for 
> > alignment in the prologue and we can't guarantee the main vectorized 
> > loop is entered.  This is to prevent executing vectorized code with an 
> > unaligned access if the target has indicated it wants to peel for 
> > alignment. We take this conservative approach as we currently do not 
> > distinguish between peeling for alignment for correctness or for 
> > performance.
> >
> > A better codegen would be to make it skip to the scalar epilogue in case 
> > the main loop isn't entered when alignment peeling is required. However, 
> > that would require a more aggressive change to the codebase which we 
> > chose to avoid at this point of development.  We can revisit this option 
> > during stage 1 if we choose to.
> >
> > Bootstrapped on aarch64-none-linux and regression tested on 
> > aarch64-none-elf.
> >
> > gcc/ChangeLog:
> >
> >      PR tree-optimization/105219
> >      * tree-vect-loop.cc (vect_epilogue_when_peeling_for_alignment): New 
> > function.
> >      (vect_analyze_loop): Use vect_epilogue_when_peeling_for_alignment 
> > to determine
> >      whether to vectorize epilogue.
> >      * testsuite/gcc.target/aarch64/pr105219.c: New.
> >      * testsuite/gcc.target/aarch64/pr105219-2.c: New.
> >      * testsuite/gcc.target/aarch64/pr105219-3.c: New.
> >
> > diff --git a/gcc/testsuite/gcc.target/aarch64/pr105219-2.c b/gcc/testsuite/gcc.target/aarch64/pr105219-2.c
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..c97d1dc100181b77af0766e08407e1e352f604fe
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/aarch64/pr105219-2.c
> > @@ -0,0 +1,29 @@
> > +/* { dg-do run } */
> > +/* { dg-options "-O3 -march=armv8.2-a -mtune=thunderx -fno-vect-cost-model" } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-march=*" } { "-march=armv8.2-a" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-mtune=*" } { "-mtune=thunderx" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-mcpu=*" } } */
> 
> I think this should be in gcc.dg/vect, with the options forced
> for { target aarch64 }.
> 
> Are the skips necessary?  It looks like the test should work correctly
> for all options/targets.
> 
> > +/* PR 105219.  */
> > +int data[128];
> > +
> > +void __attribute((noipa))
> > +foo (int *data, int n)
> > +{
> > +  for (int i = 0; i < n; ++i)
> > +    data[i] = i;
> > +}
> > +
> > +int main()
> > +{
> > +  for (int start = 0; start < 16; ++start)
> > +    for (int n = 1; n < 3*16; ++n)
> > +      {
> > +        __builtin_memset (data, 0, sizeof (data));
> > +        foo (&data[start], n);
> > +        for (int j = 0; j < n; ++j)
> > +          if (data[start + j] != j)
> > +            __builtin_abort ();
> > +      }
> > +  return 0;
> > +}
> > +
> > diff --git a/gcc/testsuite/gcc.target/aarch64/pr105219-3.c b/gcc/testsuite/gcc.target/aarch64/pr105219-3.c
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..444352fc051b787369f6f1be6236d1ff0fc2d392
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/aarch64/pr105219-3.c
> > @@ -0,0 +1,15 @@
> > +/* { dg-do compile } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-march=*" } { "-march=armv8.2-a" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-mtune=*" } { "-mtune=thunderx" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-mcpu=*" } } */
> > +/* { dg-options "-O3 -march=armv8.2-a -mtune=thunderx -fno-vect-cost-model -fdump-tree-vect-all" } */
> > +/* PR 105219.  */
> > +int data[128];
> > +
> > +void foo (void)
> > +{
> > +  for (int i = 0; i < 9; ++i)
> > +    data[i + 1] = i;
> > +}
> > +
> > +/* { dg-final { scan-tree-dump "EPILOGUE VECTORIZED" "vect" } } */
> > diff --git a/gcc/testsuite/gcc.target/aarch64/pr105219.c b/gcc/testsuite/gcc.target/aarch64/pr105219.c
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..bbdefb549f6a4e803852f69d20ce1ef9152a526c
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/aarch64/pr105219.c
> > @@ -0,0 +1,28 @@
> > +/* { dg-do run { target aarch64_sve128_hw } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-march=*" } { "-march=armv8.2-a+sve" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-mtune=*" } { "-mtune=thunderx" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-mcpu=*" } } */
> > +/* { dg-skip-if "incompatible options" { *-*-* } { "-msve-vector-bits=*" } { "-msve-vector-bits=128" } } */
> > +/* { dg-options "-O3 -march=armv8.2-a+sve -msve-vector-bits=128 -mtune=thunderx" } */
> 
> Same here.
> 
> > +/* PR 105219.  */
> > +int a;
> > +char b[60];
> > +short c[18];
> > +short d[4][19];
> > +long long f;
> > +void e(int g, int h, short k[][19]) {
> > +  for (signed i = 0; i < 3; i += 2)
> > +    for (signed j = 1; j < h + 14; j++) {
> > +      b[i * 14 + j] = 1;
> > +      c[i + j] = k[2][j];
> > +      a = g ? k[i][j] : 0;
> > +    }
> > +}
> > +int main() {
> > +  e(9, 1, d);
> > +  for (long l = 0; l < 6; ++l)
> > +    for (long m = 0; m < 4; ++m)
> > +      f ^= b[l + m * 4];
> > +  if (f)
> > +    __builtin_abort ();
> > +}
> > diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> > index d7bc34636bd52b2f67cdecd3dc16fcff684dba07..a23e6181dec8126bcb691ea9474095bf65483863 100644
> > --- a/gcc/tree-vect-loop.cc
> > +++ b/gcc/tree-vect-loop.cc
> > @@ -2942,6 +2942,38 @@ vect_analyze_loop_1 (class loop *loop, vec_info_shared *shared,
> >    return opt_loop_vec_info::success (loop_vinfo);
> >  }
> >  
> > +/* Function vect_epilogue_when_peeling_for_alignment
> > +
> > +   PR 105219: If we are peeling for alignment in the prologue then we do not
> > +   vectorize the epilogue unless we are certain we will enter the main
> > +   vectorized loop.  This is to prevent entering the vectorized epilogue in
> > +   case there aren't enough iterations to enter the main loop.
> > +*/
> > +
> > +static bool
> > +vect_epilogue_when_peeling_for_alignment (loop_vec_info loop_vinfo)
> > +{
> > +  if (vect_use_loop_mask_for_alignment_p (loop_vinfo))
> > +    return true;
> > +
> > +  int prologue_peeling = LOOP_VINFO_PEELING_FOR_ALIGNMENT (loop_vinfo);
> > +  if (prologue_peeling > 0 && LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
> > +    {
> > +      poly_uint64 niters_for_main
> > +	= upper_bound (LOOP_VINFO_VECT_FACTOR (loop_vinfo),
> > +		       LOOP_VINFO_COST_MODEL_THRESHOLD (loop_vinfo));
> > +      niters_for_main
> > +	= upper_bound (niters_for_main,
> > +		       LOOP_VINFO_VERSIONING_THRESHOLD (loop_vinfo));
> > +      niters_for_main += prologue_peeling;
> > +      if (maybe_le (LOOP_VINFO_INT_NITERS (loop_vinfo), niters_for_main))
> > +	return false;
> > +    }
> > +  else if (prologue_peeling < 0)
> 
> I was surprised that this tests < 0 rather than != 0.  If that's the
> right behaviour, could you add a comment saying why?  I would have
> expected:
> 
>   prologue_peeling > 0 && !LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> 
> to be handled more conservatively than:
> 
>   prologue_peeling > 0 && LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> 
> LGTM otherwise, but Richard B should have the final say.

It works for me.  Note it strictly prefers peeling for alignment over
epilogue vectorization (which is probably OK in most cases), but the
number of targets affected is quite small.

For GCC 13 we should see to make skipping to the scalar epilogue
possible.

Thanks,
Richard.

> Thanks,
> Richard
> 
> > +    return false;
> > +  return true;
> > +}
> > +
> >  /* Function vect_analyze_loop.
> >  
> >     Apply a set of analyses on LOOP, and create a loop_vec_info struct
> > @@ -3151,7 +3183,8 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
> >  		}
> >  	    }
> >  	  /* For now only allow one epilogue loop.  */
> > -	  if (first_loop_vinfo->epilogue_vinfos.is_empty ())
> > +	  if (first_loop_vinfo->epilogue_vinfos.is_empty ()
> > +	      && vect_epilogue_when_peeling_for_alignment (first_loop_vinfo))
> >  	    {
> >  	      first_loop_vinfo->epilogue_vinfos.safe_push (loop_vinfo);
> >  	      poly_uint64 th = LOOP_VINFO_VERSIONING_THRESHOLD (loop_vinfo);
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)