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Change the struct to ensure there is a padding byte between the members, which was previously not the case for byte-aligned targets such as cris-elf. Add a new helper function to verify that the padding bits in a struct S and in the std::atomic<S> are zero, without needing to disable SRA. Then rewrite the test in terms of that helper, so we check directly that the atomic object has the correct value and has no padding bits. Previously we loaded a value (with -fno-tree-sra to hopefully preserve padding) and then just using memcmp on that value to see if it was bitwise identical to another value, but didn't actually check the members were correct or that the padding was clear. Also add a loop around the weak compare-exchange, to tolerate spurious failures as described in https://gcc.gnu.org/pipermail/gcc-patches/2026-March/710289.html libstdc++-v3/ChangeLog: * testsuite/29_atomics/atomic/compare_exchange_padding.cc: Rewrite to be robust against spurious failures of weak compare exchange, and to check padding bits more directly.
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