[m-users.] users Digest, Vol 107, Issue 14

[Julien Fischer]

On Tue, 17 Oct 2023, Volker Wysk wrote:

> Am Dienstag, dem 17.10.2023 um 16:41 +1100 schrieb Julien Fischer:
>> On Mon, 16 Oct 2023, Volker Wysk wrote:
>>
>>> This really surprises me. But they don't state if they used the low-level-C
>>> (asm_fast) or the high-level-C backend. The former is faster.
>>
>> Let me fix that for you: the former is sometimes faster for some
>> programs on some systems for some versions of Mercury. For the Mercury
>> compiler, for example, which of the two is faster has flipped around
>> fairly regularly.
>
> Hmmm. That surprises me. From what I've learned here in this list, the low-
> level-C grade should be faster. It's a mixture of C and assembler, whereas
> the high-level-C is just C.

Right, but in asm_fast grades most of that assembler is just the labels
we use to avoid function calls. The code itself is predominently still
C. Most of the reamining uses of assember are aimed at preventing GCC
from "optimising" things in undesirable ways. A side effect of this is
arrangement is that it tends to inhibit optimizations in the C compiler,
whereas most C compiler optimsiations will work fine for high-level C
code.

> When installing the RODT verson of the compiler, it always says that you get
> a faster compiler, when doing both compilation passes.

Whether it recommends asm_fast.gc or hlc.gc depends on a number of
things, for example which C compiler you using. There's not a
significant difference between asm_fast.gc and hlc.gc (for the Mercury
compiler at least), so we tend to recommend asm_fast.gc if it can
be supported and hlc.gc otherwise.

Julien.