Re: [patch 4/7] Immediate Values - i386 Optimization

Mathieu Desnoyers
2007-09-19T11:00:08Z

* Jeremy Fitzhardinge (jeremy@goop.org) wrote:
> Mathieu Desnoyers wrote:
>
> > +#define immediate_read(name) \
> > + ({ \
> > + __typeof__(name##__immediate) value; \
> > + switch (sizeof(value)) { \
> > + case 1: \
> > + asm ( ".section __immediate, \"a\", @progbits;\n\t" \
> > + ".long %1, (0f)+1, 1;\n\t" \
> > + ".previous;\n\t" \
> > + "0:\n\t" \
> > + "mov %2,%0;\n\t" \
>
> Given that you're relying on the exact instruction that this mov
> generates, it might be better to explicitly put the opcodes in with
> .byte. That way you're protected from the assembler deciding to
> generate some other form of the instruction (for whatever reason). I
> guess substituting in different registers would be a pain.
>

Good point. I thought it might come up, especially for 16 bits mov that
can be expressed under different forms, one of which has a prefix. I
would like to go for Peter's suggestion: putting the label _after_ the
instruction, since we know that we will be right after the immediate
value, but it has a drawback: we cannot insure correct alignment of the
immediate value in that case. But that would help not having to force
the register.

> Aside from that, is there any reason not to just put $0 in there rather
> than use %2?
>

Actually, no, since the initial value is written to the immediate value
references at early boot and at module load time. I originally thought
passing the referenced variable to it, but, as I recall, it brought
linker issues when the symbol was defined in another module. So yes,
just $0 is ok, I'll change that.

>
> > + ".long %1, (0f)+1, 4;\n\t" \
> > + ".previous;\n\t" \
> > + "1:\n\t" \
> > + ".org (1b)+(3-((1b)%%4)), 0x90;\n\t" \
> >
> Seems a little complex, but I couldn't come up with anything much better:
>
> .org . + 3 - (. & 3), 0x90
>
> You can use . rather than needing to define 1:, it doesn't need quite so
> many parens, and using &3 avoids the %% wart.
>

Yes, this one is tricky.. trying to align efficiently something on a 4
bytes address - 1 is not what gas is used to help doing.

> It's a pity that gas seems to generate plain 0x90 nops rather than
> long-nop forms here. I thought it could do that.
>

At least we will have at most 3 nops there.

Mathieu

> J