[PATCH] D74094: Reapply: [IRGen] Emit lifetime intrinsics around temporary aggregate argument allocas

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

I can't easily tell you because the "standalone example" involves a million templates that I don't know anything about, and nobody seems to have done the analysis to identify the specific source of the miscompile.

What I can tell you is that there is an enormous semantic difference between passing a pr-value argument to a parameter that's passed by reference and to a parameter that's passed by value.  In the former case, the reference is bound to a temporary with full-expression lifetime, and it is totally reasonable to return its address.  In the latter case, the parameter object will often have a lifetime bound by the current function, and you cannot return its address without it being UB.

Looking closer, my previous guess that this was accidentally triggering for reference parameters seems to be wrong, because that case is filtered out earlier in the function.  I'm not sure what the problem is exactly, then, unless the code indeed has UB.


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https://reviews.llvm.org/D74094