[Bug tree-optimization/94092] Code size and performance degradations after -ftree-loop-distribute-patterns was enabled at -O[2s]+

[bina2374 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94092

--- Comment #10 from Mel Chen <bina2374 at gmail dot com> ---
(In reply to Richard Biener from comment #9)
> (In reply to Mel Chen from comment #8)
> > Sorry for using the bad example to describe the problem I am facing. Let me
> > clarify my question with a more precise example.
> > 
> > void array_mul(int N, int *C, short *A, short *B) {
> >   int i, j;
> >   for (i = 0; i < N; i++) {
> >     C[i] = 0; // Will be transformed to __builtin_memset
> >     for (j = 0; j < N; j++) {
> >       C[i] += (int)A[i * N + j] * (int)B[j];
> >     }
> >   }
> > }
> > 
> > If I compile the case with -O2 -fno-tree-loop-distribute-patterns, the store
> > operation 'C[i] = 0' can be eliminated by dead store elimination (dse3). But
> > without -fno-tree-loop-distribute-patterns, it will be transformed to memset
> > by loop distribution (ldist) because ldist executes before dse3. Finally the
> > memset will not be eliminated.
> > 
> > Another point is if there are other operations in the same level loop as the
> > store operation, is it really beneficial to do loop distribution and then
> > convert to builtin function?
> 
> Sure, it shows a cost modeling issue given that usually loop distribution
> merges partitions which touch the same memory stream (but IIRC maybe only
> for loads).  But more to the point we're missing to eliminate the dead store
> which should be appearant at least after PRE - LIM2 applied store motion
> but only PRE elides the resulting load of C[i].  Usually DCE and DSE come in
> pairs but after PRE we have DCE, CDDCE w/o accompaning DSE only with the
> next DSE only happening after loop distribution.
> 
> Which means we should eventually do
> 
> diff --git a/gcc/passes.def b/gcc/passes.def
> index e9ed3c7bc57..be3a9becde0 100644
> --- a/gcc/passes.def
> +++ b/gcc/passes.def
> @@ -254,6 +254,7 @@ along with GCC; see the file COPYING3.  If not see
>        NEXT_PASS (pass_sancov);
>        NEXT_PASS (pass_asan);
>        NEXT_PASS (pass_tsan);
> +      NEXT_PASS (pass_dse);
>        NEXT_PASS (pass_dce);
>        /* Pass group that runs when 1) enabled, 2) there are loops
>          in the function.  Make sure to run pass_fix_loops before

Yes, doing DSE before ldist is a simple and effective way.
This patch has been verified to be work on coremark. Not only improved
performance, but also code size.