[libc-commits] [libc] [llvm] [libc][CndVar] reimplmement conditional variable with FIFO ordering (PR #192748)

[Schrodinger ZHU Yifan via libc-commits]

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@@ -18,8 +19,8 @@ namespace LIBC_NAMESPACE_DECL {
 static_assert(sizeof(CndVar) == sizeof(cnd_t));
 
 LLVM_LIBC_FUNCTION(int, cnd_init, (cnd_t * cond)) {
-  CndVar *cndvar = reinterpret_cast<CndVar *>(cond);
-  return CndVar::init(cndvar) ? thrd_error : thrd_success;
+  new (cond) CndVar(false);
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SchrodingerZhu wrote:

@michaelrj-google  From my understanding, placement new is the correct way to tell the compiler: “from this point, start the lifetime of a `CndVar` object at this memory location (and effectively drop any previous assumptions about the object at that location)”.

As you can see here, we accept a pointer to public interface type `cnd_t` and start using it as our internal `CndVar*`, so placement new feels more appropriate. Consider if we LTO other operations into the same module, this is how the compiler can see that we actually initialized a `CndVar` object at this `cnd_t*` location and then access it via `CndVar*`.

Although `CndVar` and `cnd_t` are trivial objects anyway, I still prefer to make it more “correct” in the object model. That is why I prefer placement new when initializing `CndVar`.

https://github.com/llvm/llvm-project/pull/192748