Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 12 Oct 2020 12:33:49 -0700 | Subject | Re: [GIT PULL] x86/asm updates for v5.10 |
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On Mon, Oct 12, 2020 at 12:24 PM Uros Bizjak <ubizjak@gmail.com> wrote: > > I don't think it is even possible to write to a part of a register in the asm. An example:
But this example is the *reverse* of what I worry about.
I worry about the asm writing not to a "part" of a register, but to *more* than we told the compiler we'd write to.
If we told the compiler we're only writing to %al, then I could see the compiler using %ah for something, and scheduling that "somethihng" to after the inline asm that said it was only modifying the low bits.
Now, I do believe you're right that gcc (and probably clang) simply doesn't track %ah liveness and clobbering separately from %al.
But it still stinks as a concept when this isn't actually documented anywhere I can tell.
See my worry?
Linus
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