[gcc r14-10017] [libstdc++] [testsuite] disable SRA for compare_exchange_padding

[Alexandre Oliva <aoliva@gcc.gnu.org>]

https://gcc.gnu.org/g:dcf0bd14cda706be8d0c18963812beefca51df39

commit r14-10017-gdcf0bd14cda706be8d0c18963812beefca51df39
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Apr 18 08:00:59 2024 -0300

    [libstdc++] [testsuite] disable SRA for compare_exchange_padding
    
    On arm-vx7r2, the uses of as.load() as initializer get SRAed, so the
    padding bits in the tests are not what we might expect from full-word
    struct copies.
    
    I tried adding a function to perform bitwise copying, but even taking
    the as.load() argument by const&, we'd still construct a temporary
    with SRAed field-wise copying.  Unable to find another way to ensure
    we wouldn't get a temporary, I went for disabling SRA.
    
    
    for  libstdc++-v3/ChangeLog
    
            * testsuite/29_atomics/atomic/compare_exchange_padding.cc:
            Disable SRA.

Diff:
---
 libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
index 2f18d426e7f..a6081968ca8 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
@@ -1,6 +1,7 @@
 // { dg-do run { target c++20 } }
 // { dg-require-atomic-cmpxchg-word "" }
 // { dg-add-options libatomic }
+// { dg-additional-options "-fno-tree-sra" }
 
 #include <atomic>
 #include <cstring>
@@ -26,10 +27,10 @@ main ()
   s.s = 42;
 
   std::atomic<S> as{ s };
-  auto ts = as.load();
+  auto ts = as.load(); // SRA might prevent copying of padding bits here.
   VERIFY( !compare_struct(s, ts) ); // padding cleared on construction
   as.exchange(s);
-  auto es = as.load();
+  auto es = as.load(); // SRA might prevent copying of padding bits here.
   VERIFY( compare_struct(ts, es) ); // padding cleared on exchange
 
   S n;