Messages in this thread | | | Date | Fri, 14 Oct 2022 19:41:29 -0700 | From | "H. Peter Anvin" <> | Subject | RE: [PATCH v3 3/6] x86/gsseg: make asm_load_gs_index() take an u16 |
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On October 14, 2022 5:28:25 AM PDT, David Laight <David.Laight@ACULAB.COM> wrote: >From: Xin Li >> Sent: 13 October 2022 21:02 >> >> From: "H. Peter Anvin (Intel)" <hpa@zytor.com> >> >> Let gcc know that only the low 16 bits of load_gs_index() argument >> actually matter. It might allow it to create slightly better >> code. However, do not propagate this into the prototypes of functions >> that end up being paravirtualized, to avoid unnecessary changes. > >Using u16 will almost always make the code worse. >At some point the value has to be masked and/or extended >to ensure an out of range value doesn't appear in >a register. > > David > >- >Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK >Registration No: 1397386 (Wales) > >
Is that a general statement or are you actually invoking it in this case? This is about it being a narrowing input, *removing* such constraints.
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