[llvm] [Transforms] Introduce BuildBuiltins.h atomic helpers (PR #134455)

[Michael Kruse via llvm-commits]

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@@ -462,11 +462,91 @@ TLI_DEFINE_ENUM_INTERNAL(atomic_load)
 TLI_DEFINE_STRING_INTERNAL("__atomic_load")
 TLI_DEFINE_SIG_INTERNAL(Void, SizeT, Ptr, Ptr, Int)
 
+/// int8_t __atomic_load_1(void *ptr, int memorder);
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Meinersbur wrote:

This is the GCC-defined ABI, see https://gcc.gnu.org/wiki/Atomic/GCCMM/LIbrary#GCC_intrinsics for reference. 

`__atomic_load_N` returns the load value, possible due to having a known size that fits into a register (or pre-allocated stack slot, depending on calling convention). `__atomic_load` reads a dynamic number of bytes and hence cannot be pre-allocated/passed by register.

https://github.com/llvm/llvm-project/pull/134455