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SubjectRe: [PATCH] x86-64: use 32-bit XOR to zero registers
>>> 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.

Jan


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