Re: [PATCH 10/10] x86: correct VFPCLASSP{S,D} operand size handling

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Fri, Aug 9, 2019 at 8:58 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 09.08.2019 16:57,  H.J. Lu  wrote:
> > On Fri, Aug 9, 2019 at 12:55 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 08.08.2019 18:22,  H.J. Lu  wrote:
> >>> On Thu, Aug 8, 2019 at 12:47 AM Jan Beulich <JBeulich@suse.com> wrote:
> >>>>
> >>>> On 07.08.2019 20:00,  H.J. Lu  wrote:
> >>>>> On Wed, Aug 7, 2019 at 1:29 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>
> >>>>>> On 06.08.2019 23:11,  H.J. Lu  wrote:
> >>>>>>> On Tue, Aug 6, 2019 at 7:29 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>>>>>>>
> >>>>>>>> With AVX512VL disabled (e.g. when writing code for the Knights family
> >>>>>>>> of processors) these insns aren't ambiguous when used with a memory
> >>>>>>>> source, and hence should be accepted without suffix or operand size
> >>>>>>>> specifier. When AVX512VL is enabled, to be consistent with this as
> >>>>>>>> well as other ambiguous operand size handling it seems better to just
> >>>>>>>> wanrn about the ambiguity in AT&T mode, and still default to 512-bit
> >>>>>>>> operands (on the assumption that the code may have been written without
> >>>>>>>> AVX512VL in mind yet).
> >>>>>>>>
> >>>>>>>> gas/
> >>>>>>>> 2019-08-XX  Jan Beulich  <jbeulich@suse.com>
> >>>>>>>>
> >>>>>>>>             * config/tc-i386.c (avx512): New (at file scope), moved from
> >>>>>>>>             (check_VecOperands): ... here.
> >>>>>>>>             (process_suffix): Add [XYZ]MMword operand size handling.
> >>>>>>>>             * testsuite/gas/i386/noavx512-2.s, testsuite/gas/i386/noreg16.s,
> >>>>>>>>             testsuite/gas/i386/noreg32.s, testsuite/gas/i386/noreg64.s: Add
> >>>>>>>>             VFPCLASS tests.
> >>>>>>>>             * testsuite/gas/i386/noavx512-2.l, testsuite/gas/i386/noreg16.d,
> >>>>>>>>             testsuite/gas/i386/noreg16.l, testsuite/gas/i386/noreg32.d,
> >>>>>>>>             testsuite/gas/i386/noreg32.l, testsuite/gas/i386/noreg64.d,
> >>>>>>>>             testsuite/gas/i386/noreg64.l: Adjust expectations.
> >>>>>>>>
> >>>>>>>> opcodes/
> >>>>>>>> 2019-08-XX  Jan Beulich  <jbeulich@suse.com>
> >>>>>>>>
> >>>>>>>>             * i386-opc.tbl (vfpclasspd, vfpclassps): Add Unspecified.
> >>>>>>>>             * i386-tbl.h: Re-generate.
> >>>>>>>>
> >>>>>>>
> >>>>>>> We should keep the suffix even if AVX512VL isn't enabled so that
> >>>>>>> we don't need to check if AVX512VL isn't enabled to interpret the
> >>>>>>> instruction.
> >>>>>>
> >>>>>> But that's wrong (and fixing this is the whole point of this patch).
> >>>>>> As you've said elsewhere, unambiguous (SIMD in particular) insns
> >>>>>> should not require any use of suffixes.
> >>>>>>
> >>>>>
> >>>>> When I look at such instruction, I should be able to tell what it is without
> >>>>> checking if AVX512VL is enabled.
> >>>>
> >>>> This entirely depends on the context: When all you think about is
> >>>> Knights hardware (or AVX512F/AVX512DQ in more general terms), then
> >>>> seeing (and in particular being _forced_ to use) the suffix is
> >>>> confusing (wrong). No-one says that in this case the suffix won't
> >>>> be allowed anymore. The problem here seems to be your use of "I",
> >>>> when instead you should also be considering how other people may
> >>>> view things.
> >>>>
> >>>> As a general statement: The assembler should accept (without any
> >>>> diagnostic) anything that's unambiguously mappable to a valid
> >>>> encoding.
> >>>>
> >>>
> >>> When I debug assembly codes:
> >>>
> >>> vfpclasspd $0, (%eax), %k0
> >>>
> >>> it is hard to tell the memory operand size.
> >>
> >> You're kidding? The presented patch makes no change whatsoever to
> >> the disassembler (as can also be seen from the testsuite extensions
> >> it makes). Doing so would actually be quite hard I think without it
> >> even knowing of the distinction between Knights family and other
> >> processors, and without having any CPU capabilities attribute
> >> attached to insns.
> >>
> >
> > I can compile assembly codes with -g and use gdb to step
> > through the assembly code:
> >
> > Breakpoint 2, __strstr_sse2_unaligned ()
> >      at ../sysdeps/x86_64/multiarch/strstr-sse2-unaligned.S:22
> > 22 movzbl (%rsi), %eax
> > (gdb) next
> > 23 testb %al, %al
> > (gdb)
> >
> > When I see
> >
> > 22   vfpclasspd $0, (%eax), %k0
> >
> > I can't tell what the memory size is.
>
> Excuse me, but when you go through source code it is assumed
> that you know what your source code means and does. No-one

The assembly source code can come from anywhere.

> requires you to omit the suffix. But equally no-one should be
> required to specify a suffix just to meet your taste.

These instructions can take different memory sizes.  One should
be able to tell what the memory size is by looking at it. Is that too
much to ask?

> Let me be frank here: If you continue to refuse to allow this
> change in, I'll have to make it work correctly for Intel syntax
> mode only (which requires more code for no gain), just to avoid
> the need to have you ack the change. I don't think though that
> this would be a good course of action.
>
> Jan



-- 
H.J.