[PATCH] D77689: [X86] Codegen for preallocated

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


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Comment at: llvm/test/CodeGen/X86/musttail-indirect.ll:58
 
+; FIXME: This generates a lot of code even at -O2, any better way to do this? Same with all the preallocated versions of functions below.
+; CHECK-LABEL: g_thunk_2:
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rnk wrote:
> This is interesting, we forgot about this when writing the LangRef.
> 
> If you look at the previous inalloca test, it forwards its inalloca parameter directly to the musttail callee. It doesn't allocate new memory. That's the real use case for this `musttail` thing, and we probably need to bend the verifier rules to allow direct forwarding of preallocated arguments to a musttail call site.
> 
> I think for now, committing this as is with a FIXME and coming back to it later would be fine. I assume that the verifier rejects it if you remove the setup and arg calls.
I don't think it makes sense to commit this test as-is.  The verifier should reject the combination of musttail and a preallocated bundle.  The llvm.call.preallocated.setup would have to allocate memory on top of the argument, and there's no reasonable way to prove that's safe.

And yes, we probably need to bend the rules for the "preallocated" attribute to allow forwarding arguments in musttail calls.


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https://reviews.llvm.org/D77689