Messages in this thread | | | Date | Thu, 26 Jul 2018 13:45:37 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86-64: use 32-bit XOR to zero registers |
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* Pavel Machek <pavel@ucw.cz> wrote:
> 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@infradead.org> 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.
Agreed - mind sending a patch that adds it?
> But Ingo (?) told me everyone knows about this quirk...
I was wrong.
Thanks,
Ingo
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