RE: [PATCH 4/8] Support APX NDD

["Cui, Lili" <lili.cui@intel.com>]

> Subject: Re: [PATCH 4/8] Support APX NDD
> 
> On 25.10.2023 10:10, Cui, Lili wrote:
> >> On 22.10.2023 16:05, Cui, Lili wrote:
> >>>>> --- /dev/null
> >>>>> +++ b/gas/testsuite/gas/i386/x86-64-apx-ndd.s
> >>>>> @@ -0,0 +1,156 @@
> >>>>> +# Check 64bit APX NDD instructions with evex prefix encoding
> >>>>> +
> >>>>> +	.allow_index_reg
> >>>>> +	.text
> >>>>> +_start:
> >>>>> +cmovge 0x90909090(%eax),%edx,%r8d cmovle
> >>>>> +0x90909090(%eax),%edx,%r8d cmovg  0x90909090(%eax),%edx,%r8d
> >>>>> +imul   0x90909(%eax),%edx,%r8d
> >>>>> +imul   0x909(%rax,%r31,8),%rdx,%r25
> >>>>
> >>>> What about imul by immediate? The present spec is quite unclear there:
> >>>> The insn page says {ND=ZU} and the table says 0/1 in the ND column.
> >>>>
> >>>
> >>> We don't support it yet, I put it in RFC.
> >>> ...
> >>> 2. Support APX ZU   -- In progress
> >>> 3. Support APX CCMP and CTEST -- In progress ...
> >>>
> >>> About 0/1 in the ND column, it means ZU can be 0/1.
> >>>
> >>> IMUL with opcodes 0x69 and 0x6B in map 0 and SETcc instructions
> >>> Although these instructions do not support NDD, the EVEX.ND bit is
> >>> used to control whether its destination register has its upper bits
> >>> (namely,
> >> bits [63:OSIZE]) zeroed when OSIZE is 8b or 16b.
> >>> That is, if EVEX.ND = 1, the upper bits are always zeroed;
> >>> otherwise, they keep the old values when OSIZE is 8b or 16b. For
> >>> these instructions,
> >> EVEX.[V4,V3,V2,V1,V0] must be all zero.
> >>
> >> So ZU indeed isn't just a typo there. For 32- and 64-bit forms, is
> >> EVEX.ND then simply being ignored? The ZU really is meaningful only for
> 16-bit forms, aiui ...
> >>
> >
> > EVEX.ZU should be ignored for 32-bit and 64-bit forms. For imul (in spec 6.30
> IMUL), EVEX.ND stands for ND or ZU.
> 
> In cases like this, where ignoring bits is kind of unexpected, the spec would
> better say explicitly (on the instruction page) when a meaningless bit is indeed
> ignored, rather than being reserved and causing #UD. Note how even the text
> in the APX-EVEX-INT section leaves open (or at least ambiguous, by not
> mentioning the case) whether SETcc with a memory operand ignores EVEX.ND
> or causes #UD when the bit is set.
> 

Sorry, my previous answer was inaccurate, EVEX.ZU will not be ignored in 32-bit and 64-bit forms.

Prior to Intel® APX, the following rules apply in 64-bit mode when an instruction’s destination is a GPR and
OSIZE < 64b:
1. If OSIZE is 32b, the destination GPR gets the instruction’s result in bits [31:0] and all zeros in bits
[63:32].
2. If OSIZE is 8b or 16b, the destination GPR gets the instruction’s result in bits [OSIZE-1:0] but keep its
old value in bits [63:OSIZE].

The ZU indication described in items 2.(b) of Section 3.1.2.3.1 does not introduce an NDD. For those
instructions, EVEX.ND=0 keeps the current x86 behavior, but EVEX.ND=1 forces the zeroing of bits
[63:OSIZE] for any OSIZE < 64b

> > I think ZU makes sense for both the 16-bit form (imul) and the 8-bit form
> (setcc, I'm not sure if imul supports it yet).
> 
> No, IMUL by immediate (or actually any IMUL with multiple operands) doesn't
> support byte register operands. For SETcc the ZU aspect is pretty clear and
> doesn't even need expressing by new syntax in (dis)assembly - you can simply
> distinguish the two forms by using either 8-bit registers (no ZU) or 32-/64-bit
> ones (with ZU). In principle that's possible with IMUL as well, of course, but it
> may be deemed a little odd:
> 
> 	imul	$17, %dx, %cx
> 	imul	$17, %dx, %ecx
> 
> Yet personally I'd still prefer this over adding e.g. {zu} on either the mnemonic
> or the destination operand. Question (as with the way to express
> {nf}) is how other assemblers are going to handle it. (Would be quite nice if
> the spec could at least give more clear hints towards suggested syntax, but
> that hadn't been the case already with the syntax extensions needed for
> AVX512.)
> 
 
I will add suffix “zx” (for the Intel syntax) or “zwq” (for the AT&T syntax) to the mnemonic:

Intel syntax                                              AT&T syntax
imulzx rax, word ptr[ rbx ], 0xab         imulzwq $0xab, (%rbx), %rax

Lili.