[PATCH] D70413: [musttail] Don't forward incoming registers over call site registers

[Reid Kleckner via Phabricator via llvm-commits]

rnk marked an inline comment as done.
rnk added inline comments.


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Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:3901
+    // forwarding the original incoming value of AL. In these cases, the value
+    // passed at the call site wins.
     const auto &Forwards = X86Info->getForwardedMustTailRegParms();
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efriedma wrote:
> This doesn't seem right... either we need to forward the value, or we don't.
> 
> We shouldn't be forwarding AL for Windows callling conventions; that convention for specifying the number of xmm arguments only applies to the "Linux" varargs ABI.
So, I actually feel like musttail thunks really shouldn't assume a calling convention at all, they should simply preserve all non-CSR registers, including things like `RAX`, `R11`, `R10`, which are often scratch, and used for nest parameters, this CFG side parameter, and retpoline thunks. This makes it a much more general device for doing perfect forwarding. This change here is a step in that direction.


Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D70413/new/

https://reviews.llvm.org/D70413