On Thu, 22 Sep 2022, Richard Biener wrote:
> On Thu, 22 Sep 2022, juzhe.zh...@rivai.ai wrote:
>
> > From: Ju-Zhe Zhong <juzhe.zh...@rivai.ai>
> >
> > This patch fix issue: PR 99407
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99407
> >
> > The enhancement implementation is simple:
> > 1.Search gimple statement in program reverse order.
> > 2.Queue the store statement which may be possible kill the def
> > of previous store statement.
> > 3.Perform dse_def_ref_analysis to remove stores will not kill
> > any def.
> > For example:
> > a[i_18] = _5;
> > ...
> > foo (&a);
> > a[i_18] = _7;
> >
> > a[i_18] = _7 is queued at the begining and will be removed
> > in dse_def_ref_analysis.
> > 4.Remove the store if the def is confirmed to be killed.
>
> But we already do the very same thing in dse_classify_store, I fail
> to see why we need to have an alternate implementation? It also
> seems to be quadratic in the size of a basic-block?
>
> The issue with dse_classify_store is that it relies on
> ref_maybe_used_by_stmt_p but that doesn't handle
>
> a[i] = ..;
> .. = a[i+1];
>
> but when seeing a[_1] vs. a[_2] (two variable offsets), it gives
> up, asserting may-aliasing. We do have infrastructure to catch
> such cases with data reference analysis. If we want to catch
> these cases we should use that instead. Given we have a
> DSE/DCE pass pair right before loop optimizations we could even
> move those inside of the loop pipeline and perform this more
> expensive checks conditional on loop/scev availability.
Oh, and when doing non-loop aware analysis we don't need SCEV. The
following optimizes the testcase but as said I don't think we want
to perform this for each of the DSE passes since it can be somewhat
expensive, at least without doing more caching (we could keep a
stmt -> data-ref hash-map and compute data-refs at most once for each
statement, that would make it more acceptable).
Richard.
>From 515b213e9d06c2bd36160e66728f57e48095bb84 Mon Sep 17 00:00:00 2001
From: Richard Biener <rguent...@suse.de>
Date: Thu, 22 Sep 2022 09:40:40 +0200
Subject: [PATCH] tree-optimization/99407 - DSE with data-ref analysis
To: gcc-patches@gcc.gnu.org
* tree-ssa-dse.c (dse_classify_store): Use data-ref analysis
to disambiguate more uses.
---
gcc/tree-ssa-dse.cc | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
index 34cfd1a8802..340a54f4105 100644
--- a/gcc/tree-ssa-dse.cc
+++ b/gcc/tree-ssa-dse.cc
@@ -45,6 +45,8 @@ along with GCC; see the file COPYING3. If not see
#include "ipa-modref.h"
#include "target.h"
#include "tree-ssa-loop-niter.h"
+#include "cfgloop.h"
+#include "tree-data-ref.h"
/* This file implements dead store elimination.
@@ -1019,6 +1021,25 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
/* If the statement is a use the store is not dead. */
else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
{
+ if (is_gimple_assign (use_stmt))
+ {
+ data_reference_p dra, drb;
+ dra = create_data_ref (NULL, NULL, ref->ref, stmt,
+ false, false);
+ drb = create_data_ref (NULL, NULL,
+ gimple_assign_rhs1 (use_stmt),
+ use_stmt, false, false);
+ bool alias_p = dr_may_alias_p (dra, drb, NULL);
+ free_data_ref (dra);
+ free_data_ref (drb);
+ if (!alias_p)
+ {
+ if (gimple_vdef (use_stmt))
+ defs.safe_push (use_stmt);
+ continue;
+ }
+ }
+
/* Handle common cases where we can easily build an ao_ref
structure for USE_STMT and in doing so we find that the
references hit non-live bytes and thus can be ignored.
--
2.35.3
