Re: [PULL v2 25/30] Hexagon HVX (target/hexagon) instruction decoding

[Peter Maydell]

On Wed, 3 Nov 2021 at 21:17, Taylor Simpson <tsimpson@quicinc.com> wrote:
>
> Add new file to target/hexagon/meson.build
>
> Acked-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>

Hi; Coverity points out a variable written to and then
overwritten before it's ever used in this function (CID 1527408):




> +static void
> +check_new_value(Packet *pkt)
> +{
> +    /* .new value for a MMVector store */
> +    int i, j;
> +    const char *reginfo;
> +    const char *destletters;
> +    const char *dststr = NULL;
> +    uint16_t def_opcode;
> +    char letter;
> +    int def_regnum;

def_regnum has function level scope...

> +
> +    for (i = 1; i < pkt->num_insns; i++) {
> +        uint16_t use_opcode = pkt->insn[i].opcode;
> +        if (GET_ATTRIB(use_opcode, A_DOTNEWVALUE) &&
> +            GET_ATTRIB(use_opcode, A_CVI) &&
> +            GET_ATTRIB(use_opcode, A_STORE)) {
> +            int use_regidx = strchr(opcode_reginfo[use_opcode], 's') -
> +                opcode_reginfo[use_opcode];
> +            /*
> +             * What's encoded at the N-field is the offset to who's producing
> +             * the value.
> +             * Shift off the LSB which indicates odd/even register.
> +             */
> +            int def_off = ((pkt->insn[i].regno[use_regidx]) >> 1);
> +            int def_oreg = pkt->insn[i].regno[use_regidx] & 1;
> +            int def_idx = -1;
> +            for (j = i - 1; (j >= 0) && (def_off >= 0); j--) {
> +                if (!GET_ATTRIB(pkt->insn[j].opcode, A_CVI)) {
> +                    continue;
> +                }
> +                def_off--;
> +                if (def_off == 0) {
> +                    def_idx = j;
> +                    break;
> +                }
> +            }
> +            /*
> +             * Check for a badly encoded N-field which points to an 
> instruction
> +             * out-of-range
> +             */
> +            g_assert(!((def_off != 0) || (def_idx < 0) ||
> +                       (def_idx > (pkt->num_insns - 1))));
> +
> +            /* def_idx is the index of the producer */
> +            def_opcode = pkt->insn[def_idx].opcode;
> +            reginfo = opcode_reginfo[def_opcode];
> +            destletters = "dexy";
> +            for (j = 0; (letter = destletters[j]) != 0; j++) {
> +                dststr = strchr(reginfo, letter);
> +                if (dststr != NULL) {
> +                    break;
> +                }
> +            }
> +            if ((dststr == NULL)  && GET_ATTRIB(def_opcode, A_CVI_GATHER)) {
> +                def_regnum = 0;

In this half of the if() we set it to 0...

> +                pkt->insn[i].regno[use_regidx] = def_oreg;
> +                pkt->insn[i].new_value_producer_slot = 
> pkt->insn[def_idx].slot;
> +            } else {
> +                if (dststr == NULL) {
> +                    /* still not there, we have a bad packet */
> +                    g_assert_not_reached();
> +                }
> +                def_regnum = pkt->insn[def_idx].regno[dststr - reginfo];
> +                /* Now patch up the consumer with the register number */
> +                pkt->insn[i].regno[use_regidx] = def_regnum ^ def_oreg;
> +                /* special case for (Vx,Vy) */
> +                dststr = strchr(reginfo, 'y');
> +                if (def_oreg && strchr(reginfo, 'x') && dststr) {
> +                    def_regnum = pkt->insn[def_idx].regno[dststr - reginfo];
> +                    pkt->insn[i].regno[use_regidx] = def_regnum;
> +                }

...but the only place we read def_regnum is in this other half of the
if(), and if we get here then we've set it to something out of pxt->insn.

Were we supposed to do something with def_regnum outside this if(),
or could we instead drop the initialization in the first half of the if()
and move its declaration inside this else {} block ?

> +                /*
> +                 * We need to remember who produces this value to later
> +                 * check if it was dynamically cancelled
> +                 */
> +                pkt->insn[i].new_value_producer_slot = 
> pkt->insn[def_idx].slot;
> +            }
> +        }
> +    }
> +}

thanks
-- PMM