On Mon, 10 Feb 2025, Richard Biener wrote:
> The following addresses the fact that we keep an excessive amount of
> redundant DEBUG BEGIN_STMTs - in the testcase it sums up to 99.999%
> of all stmts, sucking up compile-time in IL walks. The patch amends
> the GIMPLE DCE code that elides redundant DEBUG BIND stmts, also
> pruning uninterrupted sequences of DEBUG BEGIN_STMTs, keeping only
> the last one.
>
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>
> For the testcase this brings down compile-time to 150% of -g0 levels
> (and only 215 out of originally 1981380 DEBUG BEGIN_STMTs kept).
>
> OK for trunk and possibly backports?
It regresses a few guality checks (and progresses one), I've looked
only into one, g++.dg/guality/pr67192.C, where we now see
FAIL: g++.dg/guality/pr67192.C -O[123sg] line 54 cnt == 15
because the breakpoint happens in the wrong place. But this shows
it "works" only by accident. The testcase is
__attribute__((noinline, noclone)) static void
f4 (void)
{
while (1) /* { dg-final { gdb-test 54 "cnt" "15" } } */
if (last ())
break;
else
do_it ();
do_it (); /* { dg-final { gdb-test 59 "cnt" "20" } } */
}
and we have two BEGIN_STMTs for line 54(!) originally:
[/space/rguenther/src/gcc/gcc/testsuite/g++.dg/guality/pr67192.C:54:3] #
DEBUG BEGIN_STMT
<D.2884>:
[/space/rguenther/src/gcc/gcc/testsuite/g++.dg/guality/pr67192.C:55:5] #
DEBUG BEGIN_STMT
...
[/space/rguenther/src/gcc/gcc/testsuite/g++.dg/guality/pr67192.C:54:3] #
DEBUG BEGIN_STMT
[/space/rguenther/src/gcc/gcc/testsuite/g++.dg/guality/pr67192.C:55:5]
goto <D.2884>;
and special code in make_blocks() moves the first BEGIN_STMT after
the label, altering when we reach a breakpoint on the line.
You can see that with the first BEGIN_STMT moved the patch will elide it,
and gdb will find the second location.
With removing only repeating BEGIN_STMT with exactly
the same location (unfortunately with uint64_t a bitmap no longer
works), we're "only" down to 996 BEGIN_STMTs for the testcase.
So I'm retesting the following.
Richard.
>From 38d49d3e2c0bf98e9e2a46e251ae0454b084cc8d Mon Sep 17 00:00:00 2001
From: Richard Biener <rguent...@suse.de>
Date: Mon, 10 Feb 2025 10:23:45 +0100
Subject: [PATCH] middle-end/118801 - excessive redundant DEBUG BEGIN_STMT
To: gcc-patches@gcc.gnu.org
The following addresses the fact that we keep an excessive amount of
redundant DEBUG BEGIN_STMTs - in the testcase it sums up to 99.999%
of all stmts, sucking up compile-time in IL walks. The patch amends
the GIMPLE DCE code that elides redundant DEBUG BIND stmts, also
pruning uninterrupted sequences of DEBUG BEGIN_STMTs, keeping only
the last of each set of DEBUG BEGIN_STMT with unique location.
PR middle-end/118801
* tree-ssa-dce.cc (eliminate_unnecessary_stmts): Prune
sequences of uninterrupted DEBUG BEGIN_STMTs, keeping only
the last of a set with unique location.
---
gcc/tree-ssa-dce.cc | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc
index be21a2d0b50..461283ba858 100644
--- a/gcc/tree-ssa-dce.cc
+++ b/gcc/tree-ssa-dce.cc
@@ -1508,6 +1508,7 @@ eliminate_unnecessary_stmts (bool aggressive)
/* Remove dead statements. */
auto_bitmap debug_seen;
+ hash_set<int_hash <location_t, 0>> locs_seen;
for (gsi = gsi_last_bb (bb); !gsi_end_p (gsi); gsi = psi)
{
stmt = gsi_stmt (gsi);
@@ -1670,6 +1671,15 @@ eliminate_unnecessary_stmts (bool aggressive)
remove_dead_stmt (&gsi, bb, to_remove_edges);
continue;
}
+ else if (gimple_debug_begin_stmt_p (stmt))
+ {
+ /* We are only keeping the last debug-begin in a series of
+ debug-begin stmts. */
+ if (locs_seen.add (gimple_location (stmt)))
+ remove_dead_stmt (&gsi, bb, to_remove_edges);
+ continue;
+ }
+ locs_seen.empty ();
bitmap_clear (debug_seen);
}
--
2.43.0
