target/riscv: always clear vstart in whole vec move insns

These insns have 2 paths: we'll either have vstart already cleared if
vstart_eq_zero or we'll do a brcond to check if vstart >= maxsz to call
the 'vmvr_v' helper. The helper will clear vstart if it executes until
the end, or if vstart >= vl.

For starters, the check itself is wrong: we're checking vstart >= maxsz,
when in fact we should use vstart in bytes, or 'startb' like 'vmvr_v' is
calling, to do the comparison. But even after fixing the comparison we'll
still need to clear vstart in the end, which isn't happening too.

We want to make the helpers responsible to manage vstart, including
these corner cases, precisely to avoid these situations:

- remove the wrong vstart >= maxsz cond from the translation;
- add a 'startb >= maxsz' cond in 'vmvr_v', and clear vstart if that
  happens.

This way we're now sure that vstart is being cleared in the end of the
execution, regardless of the path taken.

Fixes: f714361ed7 ("target/riscv: rvv-1.0: implement vstart CSR")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20240314175704.478276-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/insn_trans/trans_rvv.c.inc

@@ -3664,12 +3664,9 @@ static bool trans_##NAME(DisasContext *s, arg_##NAME * a)               \
                              vreg_ofs(s, a->rs2), maxsz, maxsz);        \
             mark_vs_dirty(s);                                           \
         } else {                                                        \
-            TCGLabel *over = gen_new_label();                           \
-            tcg_gen_brcondi_tl(TCG_COND_GEU, cpu_vstart, maxsz, over);  \
             tcg_gen_gvec_2_ptr(vreg_ofs(s, a->rd), vreg_ofs(s, a->rs2), \
                                tcg_env, maxsz, maxsz, 0, gen_helper_vmvr_v); \
             mark_vs_dirty(s);                                           \
-            gen_set_label(over);                                        \
         }                                                               \
         return true;                                                    \
     }                                                                   \

target/riscv/vector_helper.c

@@ -5075,6 +5075,11 @@ void HELPER(vmvr_v)(void *vd, void *vs2, CPURISCVState *env, uint32_t desc)
     uint32_t startb = env->vstart * sewb;
     uint32_t i = startb;
 
+    if (startb >= maxsz) {
+        env->vstart = 0;
+        return;
+    }
+
     if (HOST_BIG_ENDIAN && i % 8 != 0) {
         uint32_t j = ROUND_UP(i, 8);
         memcpy((uint8_t *)vd + H1(j - 1),