Messages in this thread | | | Date | Wed, 07 May 2014 10:40:07 +0200 | From | Paolo Bonzini <> | Subject | Re: [RFC PATCH 3/4] KVM: emulate: avoid per-byte copying in instruction fetches |
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Il 07/05/2014 06:36, Bandan Das ha scritto: >> > + _x = *(_type __aligned(1) *) &_fc->data[ctxt->_eip - _fc->start]; \ > For my own understanding, how does the __aligned help here ?
Except for 16-byte SSE accesses, x86 doesn't distinguish aligned and unaligned accesses. You can read 4 bytes at 0x2345 and the processor will do the right thing. Still it's better to tell the compiler what we're doing.
> Wouldn't > that result in unaligned accesses that will actually impact performance ?
These accesses *can* and will in fact be unaligned. For example, say you have "mov ax,0x1234" which is 0xb8 0x34 0x12. When you read it into the fetch cache, you will have data[0] == 0xb8, data[1] == 0x34, data[2] == 0x12. Fetching the 16-bit immediate from data[1] will then be an unaligned access.
Paolo
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