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

Mathieu Desnoyers
2007-09-19T11:01:09Z

* H. Peter Anvin (hpa@zytor.com) wrote:
> Jeremy Fitzhardinge 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.
>
> Allowing different registers should be doable, but if so, one would have
> to put 0: at the *end* of the instruction and use (0f)-4 instead, since
> the non-%eax forms are one byte longer.
>
> This also seems "safer", since an imm32 is always the last thing in the
> instruction.
>

The idea is very interesting, but then, if I can't be sure of the size
of my instruction, how can I align the immediate value properly ?

> -hpa