[llvm-dev] LLVM and System V ABI Inconsistency Question

[Craig Topper via llvm-dev]

Hi Vanessa,

I think the two users of the final classification,
X86_64ABIInfo::classifyReturnType and X86_64ABIInfo::classifyArgumentType,
don't look at Hi if Lo == Memory. They both return from their respective
"switch (Lo)". So I guess the code took a shortcut and didn't
bother updating Hi.

~Craig


On Tue, Sep 14, 2021 at 5:15 PM Sochat, Vanessa via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi LLVM Community,
>
>
>
> I’m hoping for some insight to a question I stumbled on – I’ve been trying
> various social networks and contacts to no avail! I was recently looking at
> the System V ABI Document for X86 and comparing to the LLVM source code,
> and I had a few questions about register allocation. Basically, when we are
> doing the post merge step in classifying, say, a struct, the first rule
> says that we check for either lo *or* hi to be in memory, and then set
> the whole thing to memory. But in the implementation, it only checks hi
> (and then changes lo). I thought if it’s not an oversight, it might be the
> case that lo can never be set to memory, or something like that? I’m hoping
> that you have some insight because I’m not sure if this is an oversight on
> my part! I shared some screenshots here:
>
>
>
> https://twitter.com/vsoch/status/1435687187979268099
>
>
>
> And the link to the code in question is here:
>
>
>
>
> https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/TargetInfo.cpp#L2726
>
> Thank you so much for your help! 😊
>
>
>
> Best,
>
>
>
> Vanessa
>
>
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> llvm-dev at lists.llvm.org
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