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

H. Peter Anvin
2007-09-19T16:16:35Z

Mathieu Desnoyers wrote:
>
> Ok, let's have a good look at what we want:
>
> 1 - get a pointer to the beginning of the immediate value within the
> instruction.
> 2 - make sure that the immediate value, within the instruction, is
> written to atomically wrt all CPUs, even on older architectures
> where non aligned writes are not atomic.
>

I think you'll find that even on modern architectures cross-cacheline
writes aren't atomic.

Anyway, disregard the bit I mentioned about one byte longer; I've been
staring myself too blind at instruction variants lately (lots of
arguments about how to generate various instructions using NASM syntax).

>
> * 4 bytes
> B8 + rd MOV r32, imm32 (1 byte opcode)
> C7 /0 MOV r/m32, imm32 (2 bytes opcode)
> (the 2 bytes opcode can be a problem)
>

If gas generates the C7 opcodes by default, then that's a bug, nothing less.

-hpa