Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 12 Oct 2020 11:56:45 -0700 | Subject | Re: [GIT PULL] x86/asm updates for v5.10 |
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On Mon, Oct 12, 2020 at 11:41 AM Uros Bizjak <ubizjak@gmail.com> wrote: > > GCC does not distinguish between %ah and %al and it is not possible to pass "%ah" to the assembly. To access the high part of the %ax register, %h modifier has to be used in the assembly template.
Do you know whether that's true for clang too, for example?
Also note that even if the _asm_ might get "%al", maybe the compiler decided to use "%ah" for something else?
I have memories of gcc using the high registers at some point, but it might have been some special case code - and it might also be very historical.
[ Goes off and checks ]
In fact, I can still find gcc generating high register code, although it's quite possible that yes, it's only peephole bit extract instruction kind of use..
I also find that clang generates code that uses the high byte registers, although again, that's not from any knowledge of clang internals, and just by looking at my kernel image disassembly.
So yes, it _may_ all be just peepholes, but it's not obvious that this is all safe.
Linus
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