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

H. Peter Anvin
2007-09-18T22:17:28Z

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.

-hpa