[gcc r11-6502] add alignment to enable store merging in strict-alignment targets

[Alexandre Oliva]

https://gcc.gnu.org/g:758abf1ae3139a5e3d556fd2cc5636c813629547

commit r11-6502-g758abf1ae3139a5e3d556fd2cc5636c813629547
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Wed Jan 6 08:05:40 2021 -0300

    add alignment to enable store merging in strict-alignment targets
    
    In g++.dg/opt/store-merging-2.C, the natural alignment of types T and
    S is a single byte, so we shouldn't expect store merging on
    strict-alignment platforms.  Indeed, without something like the
    adjust-alignment pass to bump up the alignment of the automatic
    variable, as in GCC 10, the optimization does not occur.
    
    This patch adjusts the test so that the required alignment is
    expressly stated, and so we don't rely on its accidentally being there
    to get the desired optimization.
    
    
    for  gcc/testsuite/ChangeLog
    
            * g++.dg/opt/store-merging-2.C: Add the required alignment.

Diff:
---
 gcc/testsuite/g++.dg/opt/store-merging-2.C | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/gcc/testsuite/g++.dg/opt/store-merging-2.C b/gcc/testsuite/g++.dg/opt/store-merging-2.C
index 3c17033764b..b1ad7018a17 100644
--- a/gcc/testsuite/g++.dg/opt/store-merging-2.C
+++ b/gcc/testsuite/g++.dg/opt/store-merging-2.C
@@ -4,7 +4,9 @@
 // { dg-options "-O2 -flifetime-dse=2 -fdump-tree-store-merging-details" }
 // { dg-final { scan-tree-dump "New sequence of 2 stores to replace old one of 3 stores" "store-merging" } }
 
-struct T { char a[128]; };
+/* The alignment is necessary for store-merging to take place on
+   strict-alignment targets.  */
+struct __attribute__ ((__aligned__ (4))) T { char a[128]; };
 struct S { S () : a () { a.a[12] = 0; a.a[13] = 1; a.a[14] = 0; a.a[15] = 6; } T a; };
 void foo (S &);
 void bar (void) { S s; foo (s); }