Re: [PATCH] x86-64: use 32-bit XOR to zero registers

[Pavel Machek]

On Tue 2018-06-26 08:38:22, Henrique de Moraes Holschuh wrote:
> On Tue, 26 Jun 2018, Jan Beulich wrote:
> > >>> On 25.06.18 at 18:33, <rdunlap@xxxxxxxxxxxxx> wrote:
> > > On 06/25/2018 03:25 AM, Jan Beulich wrote:
> > >> Some Intel CPUs don't recognize 64-bit XORs as zeroing idioms - use
> > >> 32-bit ones instead.
> > > 
> > > Hmph.  Is that considered a bug (errata)?
> > 
> > No.
> > 
> > > URL/references?
> > 
> > Intel's Optimization Reference Manual says so (in rev 040 this is in section
> > 16.2.2.5 "Zeroing Idioms" as a subsection of the Goldmont/Silvermont
> > descriptions).
> > 
> > > Are these changes really only zeroing the lower 32 bits of the register?
> > > and that's all that the code cares about?
> > 
> > No - like all operations targeting a 32-bit register, the result is zero
> > extended to the entire 64-bit destination register.
> 
> Missing information that would have been helpful in the commit message:
> 
> When the processor can recognize something as a zeroing idiom, it
> optimizes that operation on the front-end.  Only 32-bit XOR r,r is
> documented as a zeroing idiom according to the Intel optimization
> manual.  While a few Intel processors recognize the 64-bit version of
> XOR r,r as a zeroing idiom, many won't.
> 
> Note that the 32-bit operation extends to the high part of the 64-bit
> register, so it will zero the entire 64-bit register.  The 32-bit
> instruction is also one byte shorter.

Actually, I believe that should be comment in code. But Ingo (?) told
me everyone knows about this quirk...
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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