On Monday, 11 June 2018 at 18:34:58 UTC, Johannes Pfau wrote:
I understand that you actually need to reimplement memcpy, as
in your microcontroller usecase you don't want to have any C
runtime. So you'll basically have to rewrite the C runtime
parts D depends on.
However, I think for memcpy and similar functions you're
probably better off keeping the C interface. This directly
provides the benefit of compiler intrinsics/optimizations. And
marking memcpy as nothrow/pure/ system/nogc is simple* either
way. For the D implementation, the compiler will verify this
for you, for the C implementation, you have to mark the
function depending on the C implementation. But that's mostly
trivial.
My plans go beyond microcontrollers. Mostly, I'd like to be able
to use more features of D without having to link in a pre-built
runtime. This is especially convenient for cross-compiling
scenarios. By replacing the runtime hooks with templates, and
the software building blocks with D implementations, we'd no
longer need to obtain a C toolchain and a pre-compiled druntime
library just to get a build for our target. We'd just need a
cross-compiler like LDC or GDC. After adding druntime to the
import path, we'd have everything we need for our target. Try,
today, to create an LDC cross-compiler for a Windows Host and an
ARM target like the Raspberry Pi. You'll quickly realize what a
hassle it all is.
Another issue with memcpy being generated by compilers is noone
knows what it's actually doing without building and looking at
the assembly. And the assembly will change based what's being
built. Have you tried to look at the GCC implementation. I
have, and I never want to again. If the implementation were
templated in D, we'd be able to see exactly what code would be
generated when by simply viewing the D runtime source code. It'd
be easier to understand, predict, port, and enhance.
There isn't one single game-changing benefit this endeavor would
bring, and I guess that's why it's difficult to comprehend, but
collectively, all the little things it would enable would make it
quite compelling.
Mike
