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sra: Do not leave work for DSE (that it can sometimes not perform)

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When looking again at the g++.dg/tree-ssa/pr109849.C testcase we
discovered that it generates terrible store-to-load forwarding stalls
because SRA was leaving behind aggregate loads but all the stores were
by scalar parts and DSE failed to remove the useless load.  SRA has
all the knowledge to remove the statement even now, so this small
patch makes it do so.

With this patch, the g++.dg/tree-ssa/pr109849.C micro-benchmark runs 9
times faster (on an AMD EPYC 75F3 machine).

gcc/ChangeLog:

2024-04-18  Martin Jambor  <mjambor@suse.cz>

	* tree-sra.cc (sra_modify_assign): Remove the original statement
	also when dealing with a store to a fully covered aggregate from a
	non-candidate.

gcc/testsuite/ChangeLog:

2024-04-23  Martin Jambor  <mjambor@suse.cz>

	* g++.dg/tree-ssa/pr109849.C: Also check that the aggeegate store
	to cur disappears.
	* gcc.dg/tree-ssa/ssa-dse-26.c: Instead of relying on DSE,
	check that the unwanted stores were removed at early SRA time.

(cherry picked from commit f6743695b4d2bd4da96e56a19157372f93b800bd)
This commit is contained in:
Martin Jambor 2024-05-09 16:39:44 +02:00
parent 137e7a86a4
commit 1a6c1c85b7
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GPG key ID: BF63C1BC3FA43540
3 changed files with 17 additions and 6 deletions
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3
gcc/testsuite/g++.dg/tree-ssa/pr109849.C
View file

@ -1,5 +1,5 @@
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-sra" } */
/* { dg-options "-O2 -fdump-tree-sra -fdump-tree-optimized" } */
#include <vector>
typedef unsigned int uint32_t;
@ -29,3 +29,4 @@ main()
}
/* { dg-final { scan-tree-dump "Created a replacement for stack offset" "sra"} } */
/* { dg-final { scan-tree-dump-not "cur = MEM" "optimized"} } */

6
gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
View file

@ -1,5 +1,5 @@
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-dse1-details -fno-short-enums -fno-tree-fre" } */
/* { dg-options "-O2 -fdump-tree-esra -fno-short-enums -fno-tree-fre" } */
/* { dg-skip-if "we want a BIT_FIELD_REF from fold_truth_andor" { ! lp64 } } */
/* { dg-skip-if "temporary variable names are not x and y" { mmix-knuth-mmixware } } */
@ -31,5 +31,5 @@ constraint_equal (struct constraint a, struct constraint b)
&& constraint_expr_equal (a.rhs, b.rhs);
}
/* { dg-final { scan-tree-dump-times "Deleted dead store: x = " 2 "dse1" } } */
/* { dg-final { scan-tree-dump-times "Deleted dead store: y = " 2 "dse1" } } */
/* { dg-final { scan-tree-dump-not "x = " "esra" } } */
/* { dg-final { scan-tree-dump-not "y = " "esra" } } */

10
gcc/tree-sra.cc
View file

@ -4854,8 +4854,18 @@ sra_modify_assign (gimple *stmt, gimple_stmt_iterator *gsi)
But use the RHS aggregate to load from to expose more
optimization opportunities. */
if (access_has_children_p (lacc))
{
generate_subtree_copies (lacc->first_child, rhs, lacc->offset,
0, 0, gsi, true, true, loc);
if (lacc->grp_covered)
{
unlink_stmt_vdef (stmt);
gsi_remove (& orig_gsi, true);
release_defs (stmt);
sra_stats.deleted++;
return SRA_AM_REMOVED;
}
}
}
return SRA_AM_NONE;