On Wed, Mar 29, 2017 at 11:05 AM, Richard Biener <rguent...@suse.de> wrote:
>
> After quite some pondering over this and other related bugs I propose
> the following for GCC 7 which tames down PRE a bit (back to levels
> of GCC 6). Technically it's the wrong place to fix this, we do
> have measures in place during elimination but they are not in effect
> at -O2. For GCC 8 I'd like to be more aggressive there but that
> would require to enable predictive commoning at -O2 (with some
> limits to its unrolling) to not lose optimization opportunities.
>
> The other option is to ignore this issue and postpone the solution
> to GCC 8.
>
> Bootstrapped / tested on x86_64-unknown-linux-gnu.
>
> Any preference?
+1 enabling predcom at O2 for this kind of transformation, remember I
once suggested that.
Thanks,
bin
>
> Thanks,
> Richard.
>
> 2017-03-29 Richard Biener <rguent...@suse.de>
>
> PR tree-optimization/77498
> * tree-ssa-pre.c (phi_translate_1): Do not allow simplifications
> to non-constants over backedges.
>
> * gfortran.dg/pr77498.f: New testcase.
>
> Index: gcc/tree-ssa-pre.c
> ===================================================================
> *** gcc/tree-ssa-pre.c (revision 246026)
> --- gcc/tree-ssa-pre.c (working copy)
> *************** phi_translate_1 (pre_expr expr, bitmap_s
> *** 1468,1477 ****
> leader for it. */
> if (constant->kind != CONSTANT)
> {
> ! unsigned value_id = get_expr_value_id (constant);
> ! constant = find_leader_in_sets (value_id, set1, set2);
> ! if (constant)
> ! return constant;
> }
> else
> return constant;
> --- 1468,1487 ----
> leader for it. */
> if (constant->kind != CONSTANT)
> {
> ! /* Do not allow simplifications to non-constants over
> ! backedges as this will likely result in a loop PHI node
> ! to be inserted and increased register pressure.
> ! See PR77498 - this avoids doing predcoms work in
> ! a less efficient way. */
> ! if (find_edge (pred, phiblock)->flags & EDGE_DFS_BACK)
> ! ;
> ! else
> ! {
> ! unsigned value_id = get_expr_value_id (constant);
> ! constant = find_leader_in_sets (value_id, set1, set2);
> ! if (constant)
> ! return constant;
> ! }
> }
> else
> return constant;
> Index: gcc/testsuite/gfortran.dg/pr77498.f
> ===================================================================
> --- gcc/testsuite/gfortran.dg/pr77498.f (nonexistent)
> +++ gcc/testsuite/gfortran.dg/pr77498.f (working copy)
> @@ -0,0 +1,36 @@
> +! { dg-do compile }
> +! { dg-options "-O2 -ffast-math -fdump-tree-pre" }
> +
> + subroutine foo(U,V,R,N,A)
> + integer N
> + real*8 U(N,N,N),V(N,N,N),R(N,N,N),A(0:3)
> + integer I3, I2, I1
> +C
> + do I3=2,N-1
> + do I2=2,N-1
> + do I1=2,N-1
> + R(I1,I2,I3)=V(I1,I2,I3)
> + * -A(0)*( U(I1, I2, I3 ) )
> + * -A(1)*( U(I1-1,I2, I3 ) + U(I1+1,I2, I3 )
> + * + U(I1, I2-1,I3 ) + U(I1, I2+1,I3 )
> + * + U(I1, I2, I3-1) + U(I1, I2, I3+1) )
> + * -A(2)*( U(I1-1,I2-1,I3 ) + U(I1+1,I2-1,I3 )
> + * + U(I1-1,I2+1,I3 ) + U(I1+1,I2+1,I3 )
> + * + U(I1, I2-1,I3-1) + U(I1, I2+1,I3-1)
> + * + U(I1, I2-1,I3+1) + U(I1, I2+1,I3+1)
> + * + U(I1-1,I2, I3-1) + U(I1-1,I2, I3+1)
> + * + U(I1+1,I2, I3-1) + U(I1+1,I2, I3+1) )
> + * -A(3)*( U(I1-1,I2-1,I3-1) + U(I1+1,I2-1,I3-1)
> + * + U(I1-1,I2+1,I3-1) + U(I1+1,I2+1,I3-1)
> + * + U(I1-1,I2-1,I3+1) + U(I1+1,I2-1,I3+1)
> + * + U(I1-1,I2+1,I3+1) + U(I1+1,I2+1,I3+1) )
> + enddo
> + enddo
> + enddo
> + return
> + end
> +
> +! PRE shouldn't do predictive commonings job here (and in a bad way)
> +! ??? It still does but not as bad as it could. Less prephitmps
> +! would be better, pcom does it with 6.
> +! { dg-final { scan-tree-dump-times "# prephitmp" 9 "pre" } }
