Messages in this thread | | | Date | Mon, 2 Nov 2020 17:18:09 -0600 | From | Segher Boessenkool <> | Subject | Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces |
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On Mon, Nov 02, 2020 at 11:54:39PM +0100, Borislav Petkov wrote: > On Mon, Nov 02, 2020 at 02:01:13PM -0600, Segher Boessenkool wrote: > > It just asks the general_operand function, which (for registers) accepts > > the hard registers that are accessible. This does include the float and > > vector etc. registers, normally. > > > > But you usually have a pseudo-register there (which is always allowed > > here), unless you used some register asm variable. > > You mean like this: > > --- > int main(void) > { > register float foo asm ("xmm0") = 0.99f; > > asm volatile("movl %0, %%r8d\n\t" > "vmcall\n\t" > :: "g" (foo)); > > return 0; > } > --- > > That works ok AFAICT: > > --- > > 0000000000000000 <main>: > 0: 55 push %rbp > 1: 48 89 e5 mov %rsp,%rbp > 4: f3 0f 10 05 00 00 00 movss 0x0(%rip),%xmm0 # c <main+0xc> > b: 00 > c: 66 0f 7e c0 movd %xmm0,%eax > 10: 41 89 c0 mov %eax,%r8d > 13: 0f 01 c1 vmcall > 16: b8 00 00 00 00 mov $0x0,%eax > 1b: 5d pop %rbp > 1c: c3 retq > > ---
That is invalid actually: local register asm as input to an inline asm should use *that* register!
This is all correct until LRA ("reload"). Not that "movl %xmm0,$eax" works, but at least it screams its head off, as it should. And then LRA puts it in %eax, a different register than asked for.
> gcc smartypants shuffles it through a GPR before sticking it into %r8. > > It works too If I use a float immediate: > > --- > int main(void) > { > asm volatile("movl %0, %%r8d\n\t" > "vmcall\n\t" > :: "g" (0.99f)); > > return 0; > } > --- > > --- > 0000000000000000 <main>: > 0: 55 push %rbp > 1: 48 89 e5 mov %rsp,%rbp > 4: 41 b8 a4 70 7d 3f mov $0x3f7d70a4,%r8d > a: 0f 01 c1 vmcall > d: b8 00 00 00 00 mov $0x0,%eax > 12: 5d pop %rbp > 13: c3 retq > ---
(this one is correct code)
> or maybe I'm missing some freaky way to specify the input operand so > that I can make it take a float register. But even if I could, it would > error out when generating the asm, I presume, due to invalid insn or > whatnot.
Yes. But GCC doing what you should have said instead of doing what you said, is not good.
> > And pseudos usually are allocated a simple integer register during > > register allocation, in an asm that is. > > Interesting. > > > > Might even make people copying from bad examples > > > to go look at the docs first... > > > > Optimism is cool :-) > > In my experience so far, documenting stuff better might not always bring > the expected results but sometimes it does move people in the most > unexpected way and miracles happen. > > :-)))
Now if only we had time to document what we wrote! We *do* write docs to go with new code (maybe not enough always), but no one spends a lot of time on documenting the existing compiler, or with a larger view than just a single aspect of the compiler. Alas.
Segher
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