Messages in this thread | | | Date | Tue, 3 Nov 2020 17:44:35 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v5 04/17] x86/acrn: Introduce hypercall interfaces |
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On Mon, Nov 02, 2020 at 05:18:09PM -0600, Segher Boessenkool wrote: > 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.
Screams how?
It builds fine without a single peep with -Wall here.
Btw, that's a MOVD - not a MOVL. MOVD can do xmm -> gpr moves. And singlestepping it with gdb does, well, something, which is clearly wrong but nothing complains:
=> 0x555555555131 <main+12>: movd %xmm0,%eax
and %xmm0 has:
(gdb) p $xmm0 $2 = {v4_float = {0.99000001, 0, 0, 0}, v2_double = {5.2627153433055495e-315, 0}, v16_int8 = {-92, 112, 125, 63, ^^^^^^^^^^
so that is correct.
0 <repeats 12 times>}, v8_int16 = {28836, 16253, 0, 0, 0, 0, 0, 0}, v4_int32 = {1065185444, 0, 0, 0}, v2_int64 = { 1065185444, 0}, uint128 = 1065185444}
Then:
movd %xmm0,%eax rax 0x3f7d70a4
and that's the hex representation of the 0.99 float.
and that same value goes into %r8d:
mov %eax,%r8d r8 0x3f7d70a4
:-)
> Yes. But GCC doing what you should have said instead of doing what you > said, is not good.
Oh well, should I open a low prio bug, would that help?
I probably should test with the latest gcc first, though...
> 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.
The problem every big project has.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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