Re: [PATCH 03/13] target/hexagon/cpu: add HVX IEEE FP extension

[Brian Cain <brian.cain@oss.qualcomm.com>]

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On Tue, Mar 24, 2026 at 1:48 PM Taylor Simpson <ltaylorsimpson@gmail.com>
wrote:

>
>
> On Tue, Mar 24, 2026 at 10:52 AM Matheus Bernardino <
> matheus.bernardino@oss.qualcomm.com> wrote:
>
>> On Mon, Mar 23, 2026 at 4:33 PM Taylor Simpson <ltaylorsimpson@gmail.com>
>> wrote:
>> >
>> >
>> >
>> > On Mon, Mar 23, 2026 at 7:15 AM Matheus Tavares Bernardino <
>> matheus.bernardino@oss.qualcomm.com> wrote:
>> >>
>> >> This flag will be used to control the HVX IEEE float instructions,
>> which
>> >> are only available at some Hexagon cores. When unavailable, the
>> >> instruction is essentially treated as a no-op.
>> >>
>> >> Signed-off-by: Matheus Tavares Bernardino <
>> matheus.bernardino@oss.qualcomm.com>
>> >> ---
>> >>  target/hexagon/cpu.h             |  1 +
>> >>  target/hexagon/translate.h       |  1 +
>> >>  target/hexagon/attribs_def.h.inc |  3 +++
>> >>  target/hexagon/cpu.c             |  1 +
>> >>  target/hexagon/decode.c          | 22 ++++++++++++++++++++++
>> >>  target/hexagon/translate.c       |  1 +
>> >>  6 files changed, 29 insertions(+)
>> >>
>> >>
>> >> diff --git a/target/hexagon/decode.c b/target/hexagon/decode.c
>> >> index dbc9c630e8..d832a64a17 100644
>> >> --- a/target/hexagon/decode.c
>> >> +++ b/target/hexagon/decode.c
>> >> @@ -696,6 +696,18 @@ static bool pkt_has_write_conflict(Packet *pkt)
>> >>      return !bitmap_empty(conflict, 32);
>> >>  }
>> >>
>> >> +static void convert_to_nop(Insn *insn)
>> >> +{
>> >> +    bool is_endloop = insn->is_endloop;
>> >> +    memset(insn, 0, sizeof(*insn));
>> >> +    insn->opcode = A2_nop;
>> >> +    insn->new_read_idx = -1;
>> >> +    insn->dest_idx = -1;
>> >> +    insn->generate = opcode_genptr[insn->opcode];
>> >> +    insn->iclass = 0b111;
>> >> +    insn->is_endloop = is_endloop;
>> >> +}
>> >> +
>> >>  /*
>> >>   * decode_packet
>> >>   * Decodes packet with given words
>> >> @@ -746,6 +758,16 @@ int decode_packet(DisasContext *ctx, int
>> max_words, const uint32_t *words,
>> >>          /* Ran out of words! */
>> >>          return 0;
>> >>      }
>> >> +
>> >> +    /* Disable HVX IEEE instruction if extension is disabled. */
>> >> +    if (!ctx->ieee_fp_extension) {
>> >> +        for (i = 0; i < num_insns; i++) {
>> >> +            if (GET_ATTRIB(pkt->insn[i].opcode, A_HVX_IEEE_FP)) {
>> >> +                convert_to_nop(&pkt->insn[i]);
>> >> +            }
>> >> +        }
>> >> +    }
>> >> +
>> >
>> >
>> > Better to leave the instruction alone and turn it into a nop by not
>> generating any TCG.
>> >
>> > That way, the disassembly (-d in_asm) will still show what's actually
>> in the binary.  You could add the check in gen_tcg_funcs.py.
>> >
>> > You could also consider adding some sort of marker in the disassembly
>> to indicate that the flag is needed for the instruction to do anything.
>>
>> Ah, good idea. Will do both for the next round, thanks.
>>
>
> Note that we'll need to be careful with packets that use the result vector
> in a .new context.  For example
>     { V0.sf = vadd(V1.sf,V2.sf)
>       vmem(R19+#0x0) = V0.new }
> The problem is that the store wants to read the value from future_VRegs.
> However, if the vadd is  nop, there is junk in future_VRegs.  So, we'll
> either have to get the store to read from the real VRegs or have the vadd
> copy the old value of the destination into the future_VRegs value.  The
> first option will be more efficient because it will avoid the vector copy.
>
>
For the sake of ease-of-verification we'll want to do whatever the ISS
does.  It's not very obvious to me what it would do in this packet context
based on the description of the nop-like behavior, but we'll follow the
ISS' lead.  In practical terms the garbage in future_VRegs is probably just
as bad or good as any other value - if you bothered to execute this packet
on the target w/o support for this opcode you probably don't care much
about the result.



> We should also add a test to fp_hvx_disabled for this case.
>
> HTH,
> Taylor
>

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