Re: [PATCH] Wasm support patch 2 (static syscalls)

[Nicholas Wilson <nicholas.wilson@realvnc.com>]

That's some sophisticated link-time optimisation you're expecting there! DCE on a single source file wouldn't do it.

When you have callers like __setxid, it's not until the final link that that kind of elimination could be run; just by examining setxid.c there's no way the compiler could eliminate anything. And, it would rely on several stages of inlining taking place, which you really can't guarantee. (Not to mention that the Clang LLD linker doesn't do LTO.) The traditional unix linking model isn't really geared up for this level of link-time inlining and optimising.

The current Emscripten fork of Musl implements the "static syscalls" a little differently. They have retained the numeric constants for the syscalls, but the calls are redirected to functions with names like "__syscall_42" instead of "__syscall_getpid". At the end of the day, there isn't a big difference between my approach here and the current Emscripten one - I've just reduced the amount of Musl code that needs to be touched to support the static syscalls, and I can't see how to reduce the changes further...

Nick

________________________________________
From: Szabolcs Nagy <nsz@port70.net>
Sent: 28 November 2017 14:05:31
To: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] Wasm support patch 2 (static syscalls)

* Nicholas Wilson <nicholas.wilson@realvnc.com> [2017-11-28 13:23:15 +0000]:
> With a jump table like that, you can't get static linkage. The compiler will link in *every* syscall.

use a better compiler with dce then.