[llvm-dev] function returning small-ish struct

Tim Northover via llvm-dev llvm-dev at lists.llvm.org
Fri Apr 29 19:33:53 PDT 2016

On 29 April 2016 at 19:04, Lawrence, Peter via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Has anyone else done this in an llvm backend, if so are the relevant source
> files in the public domain, and can you point me to them.

This is pretty common. Both ARM targets certainly do it. For example
in lib/Target/AArch64/AArch64CallingConvention.td, RetCC_AArch64_AAPCS
returns i64 arguments in X0-X7 even though real C integers arguments
could only ever use X0 & X1 (and that's for __int128). The rest come
into play for larger structs.

> Also if so, is your solution entirely in the backend, or do you modify clang
> as well (to not use “hidden pointer argument”).

You definitely don't want Clang using a hidden pointer argument for
this; it would be really tricky to undo that in the backend. Normally
the Clang-side involves picking a "close enough" scalar type so that
the usual backend rules kick in properly. (things like iN, [N x iM]
are useful here).



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