On Tue, 16 Apr 2024 at 04:49, Alexandre Oliva <ol...@adacore.com> wrote:
>
>
> 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.
Aha, I was wondering why this was failing on ARM!
> 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.
>
> Regstrapped on x86_64-linux-gnu. Also tested with gcc-13 on arm-,
> aarch64-, x86- and x86_64-vxworks7r2. Ok to install?
Yes, thanks.
>
>
> for libstdc++-v3/ChangeLog
>
> * testsuite/29_atomics/atomic/compare_exchange_padding.cc:
> Disable SRA.
> ---
> .../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 2f18d426e7f7e..a6081968ca869 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;
>
> --
> Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/
> Free Software Activist GNU Toolchain Engineer
> More tolerance and less prejudice are key for inclusion and diversity
> Excluding neuro-others for not behaving ""normal"" is *not* inclusive
