add alignment to enable store merging in strict-alignment targets

add alignment to enable store merging in strict-alignment targets

[Alexandre Oliva]

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.

Regstrapped on x86_64-linux-gnu, also tested on x-arm-wrs-vxworks7r2.
Ok to install?


for  gcc/testsuite/ChangeLog

	* g++.dg/opt/store-merging-2.C: Add the required alignment.
---
 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 3c17033764b75..b1ad7018a1789 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); }

-- 
Alexandre Oliva, happy hacker  https://FSFLA.org/blogs/lxo/
   Free Software Activist         GNU Toolchain Engineer
        Vim, Vi, Voltei pro Emacs -- GNUlius Caesar

Re: add alignment to enable store merging in strict-alignment targets

[Jeff Law]

On 1/5/21 12:46 AM, Alexandre Oliva wrote:
> 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.
>
> Regstrapped on x86_64-linux-gnu, also tested on x-arm-wrs-vxworks7r2.
> Ok to install?
>
>
> for  gcc/testsuite/ChangeLog
>
> 	* g++.dg/opt/store-merging-2.C: Add the required alignment.
OK
jeff