Messages in this thread | | | Date | Wed, 19 Sep 2007 10:30:32 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [patch 4/7] Immediate Values - i386 Optimization |
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H. Peter Anvin wrote: > 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. >
Cross-cache-line, sure. But what about just not sizeof aligned? If its enough to avoid cross-cache-line, then that's simpler.
Which is something I was going to comment on: Mathieu, you try to align the constant itself, but you don't prevent the instruction overall from crossing a cache line. Given how delicate all this stuff is, it seems like a good idea to do that.
>> * 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. >
Well, in this case, it might be preferred if it brings the constant into alignment without explicit padding :)
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