Hi,
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).
Bootstrapped and tested on x86_64. OK for master?
Given that the patch is simple but can sometimes have large benefit,
could it possibly be backported to gcc-14 branch even if it is not a
regression (at least not in the last decade) in a few weeks?
Thanks,
Martin
gcc/ChangeLog:
2024-04-18 Martin Jambor <mjam...@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 <mjam...@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.
---
gcc/testsuite/g++.dg/tree-ssa/pr109849.C | 3 ++-
gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c | 6 +++---
gcc/tree-sra.cc | 14 ++++++++++++--
3 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr109849.C
b/gcc/testsuite/g++.dg/tree-ssa/pr109849.C
index cd348c0f590..d06dbb10482 100644
--- a/gcc/testsuite/g++.dg/tree-ssa/pr109849.C
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr109849.C
@@ -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"} } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
index 43152de5616..1d01392c595 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dse-26.c
@@ -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" } } */
diff --git a/gcc/tree-sra.cc b/gcc/tree-sra.cc
index 32fa28911f2..8040b0c5645 100644
--- a/gcc/tree-sra.cc
+++ b/gcc/tree-sra.cc
@@ -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);
+ {
+ 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;
--
2.44.0
