[llvm-dev] Current preferred approach for handling 'byval' struct arguments

[Reid Kleckner via llvm-dev]

On Sat, Mar 18, 2017 at 8:37 AM, Alex Bradbury <asb at asbradbury.org> wrote:

> In the RISC-V calling convention, large scalars (larger than 2 GPRs)
> are passed indirect, just like large aggregates. e.g. an i128 or a
> long double on a 32-bit platform. It's tempting to let the frontend
> emit i128 and fp128 arguments/return values, however making the
> argument indirect is somewhat easier in the frontend.


This is a great example for why the responsibilities are currently weirdly
split between the frontend and backend. The more ABI lowering you do in the
frontend, the more information is available for the mid-level optimizer to
hack on. If the backend was responsible for creating a temporary i128 value
in memory and taking its address, the mid-level would never have an
opportunity to optimize those memory loads and stores, or realize that the
callee never modifies its argument, making the copy is unnecessary.

Of course, there are many drawbacks to the current split of
responsibilities, so it's definitely a tradeoff.
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