[PATCH 1/7] x86/Intel: restrict suffix derivation

H.J. Lu hjl.tools@gmail.com
Thu Aug 18 14:46:36 GMT 2022

On Wed, Aug 17, 2022 at 11:08 PM Jan Beulich <jbeulich@suse.com> wrote:
> On 17.08.2022 21:19, H.J. Lu wrote:
> > On Tue, Aug 16, 2022 at 12:30 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> While in some cases deriving an AT&T-style suffix from an Intel syntax
> >> memory operand size specifier is necessary, in many cases this is not
> >> only pointless, but has led to the introduction of various workarounds:
> >> Excessive use of IgnoreSize and NoRex64 as well as the ToDword and
> >> ToQword attributes. Suppress suffix derivation when we can clearly tell
> >> that the memory operand's size isn't going to be needed to infer the
> >> possible need for the low byte/word opcode bit or an operand size prefix
> >> (0x66 or REX.W).
> >>
> >> As a result ToDword and ToQword can be dropped entirely, plus a fair
> >> number of IgnoreSize and NoRex64 can also be got rid of. Note that
> >> IgnoreSize needs to remain on legacy encoded SIMD insns with GPR
> >> operand, to avoid emitting an operand size prefix in 16-bit mode. (Since
> >> 16-bit code using SIMD insns isn't well tested, clone an existing
> >> testcase just enough to cover a few insns which are potentially
> >> problematic but are being touched here.)
> >>
> >> As a side effect of folding the VCVT{,T}S{S,D,H}2SI templates,
> >> VCVT{,T}SH2SI will now allow L and Q suffixes, consistent with
> >> VCVT{,T}S{S,D}2SI. All of these remain inconsistent with their 2USI
> >> counterparts (which I think should also be corrected, but perhaps better
> >> in a separate change).
> >
> > I don't think allowing more unnecessary L and Q suffixes for AVX
> > instructions is desirable.   I prefer not to allow unnecessary L and
> > Q suffixes in folded entries.   We can add special entries to allow
> > the existing instructions with suffixes.
> I think we've been there before, and I continue to think that we should
> be consistent throughout the entire ISA in allowing suffixes when GPRs
> or their equivalent memory operands are involved. That's in the spirit
> of the original AT&T syntax intentions, after all. I have to admit that
> I find it particularly worrying that you suggest to introduce new
> templates, when the overall / long term goal is to reduce the set, to
> keep it manageable in spite of all the new additions that yer yet to
> come.
> As pointed out elsewhere, any inconsistencies here make it harder for
> people to write e.g. heavily macro-ized code. Similarly it can result
> in surprises when cloning existing code to deal with new extensions.

Will it work without unnecessary suffixes?


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