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Re: MS/CW-style inline assembly for GCC
"Richard Earnshaw" <Richard.Earnshaw@arm.com> writes:
> On Fri, 2004-05-07 at 22:38, Paul Koning wrote:
>
> > I think an asm statement with any semantics OTHER than "make this
> > block of instructions appear in the binary exactly as written" is
> > badly broken. (Yes, that's why the MIPS assembler is bogus --
> > although there at least you can tell it ".set noreorder".)
> >
>
> I used to think that way myself, but these days I'm not so sure. Modern
> processors perform abysmally if code is not correctly scheduled, and
> there's no way that a programmer can tune their code for every variant
> of a CPU on the market. Allowing the tools to reschedule the contents
> of ASM blocks is potentially a major win.
>
> It's true that there are cases where the ordering is subtly important,
> but as you say, there are ways to avoid scheduling in those cases.
>
> It's always dangerous to say "never"!
Why wouldn't you write the code in C in these cases? It seems to me
that if you're willing to have the compiler schedule the code, you
might as well let it select the instructions and do register
allocation as well.
--
- Geoffrey Keating <geoffk@geoffk.org>