https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94092
--- Comment #11 from Mel Chen <bina2374 at gmail dot com> ---
(In reply to Mel Chen from comment #10)
> (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.
The test case gcc.dg/tree-ssa/ldist-33.c is failure after I added DSE.
/* { dg-do compile { target size32plus } } */
/* { dg-options "-O2 -ftree-loop-distribution -ftree-loop-distribute-patterns
-fdump-tree-ldist-details" } */
#define N (1024)
double a[N][N], b[N][N], c[N][N];
void
foo (void)
{
unsigned i, j, k;
for (i = 0; i < N; ++i)
for (j = 0; j < N; ++j)
{
c[i][j] = 0.0;
for (k = 0; k < N; ++k)
c[i][j] += a[i][k] * b[k][j];
}
}
/* { dg-final { scan-tree-dump "Loop nest . distributed: split to 1 loops and 1
library" "ldist" } } */
It is similar to the example I showed earlier. DSE eliminated 'c[i][j] = 0.0'
so no loop will be split. My question is how to handle this test case? Add
-fno-tree-dse into dg-options, modify this test case, delete this test case, or
others.
