Messages in this thread | | | Date | Tue, 18 Sep 2007 15:12:35 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [patch 4/7] Immediate Values - i386 Optimization |
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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
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