Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions

From: Linus Torvalds
Date: Wed Apr 01 2015 - 18:14:27 EST

On Wed, Apr 1, 2015 at 1:52 PM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> BTW, AMD64 docs do explicitly say that MOVs from segment registers
> to gpregs are zero-extending.

Yeah, I think anything even *remotely* recent enough to do 64-bit does

Even on the 32-bit side, anything that does register renaming is much
better off with zero-extension than with partial register writes.

And I found the "push" thing. It's actually documented:

"When pushing a segment selector onto the stack, the Pentium 4,
Intel Xeon, P6 family, and Intel486 processors
decrement the ESP register by the operand size and then write 2 bytes.
If the operand size is 32-bits, the upper
two bytes of the write are not modified"

but I can't find any similar documentation for the "mov
Sreg->register" thing. So now I'm starting to doubt my own memory.

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