On Mon, Oct 12, 2020 at 12:24 PM Uros Bizjak <ubiz...@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
