target/riscv/vector_helpers: do early exit when vstart >= vl

We're going to make changes that will required each helper to be
responsible for the 'vstart' management, i.e. we will relieve the
'vstart < vl' assumption that helpers have today.

Helpers are usually able to deal with vstart >= vl, i.e. doing nothing
aside from setting vstart = 0 at the end, but the tail update functions
will update the tail regardless of vstart being valid or not. Unifying
the tail update process in a single function that would handle the
vstart >= vl case isn't trivial (see [1] for more info).

This patch takes a blunt approach: do an early exit in every single
vector helper if vstart >= vl, unless the helper is guarded with
vstart_eq_zero in the translation. For those cases the helper is ready
to deal with cases where vl might be zero, i.e. throwing exceptions
based on it like vcpop_m() and first_m().

Helpers that weren't changed:

- vcpop_m(), vfirst_m(), vmsetm(), GEN_VEXT_VIOTA_M(): these are guarded
  directly with vstart_eq_zero;

- GEN_VEXT_VCOMPRESS_VM(): guarded with vcompress_vm_check() that checks
  vstart_eq_zero;

- GEN_VEXT_RED(): guarded with either reduction_check() or
  reduction_widen_check(), both check vstart_eq_zero;

- GEN_VEXT_FRED(): guarded with either freduction_check() or
  freduction_widen_check(), both check vstart_eq_zero.

Another exception is vext_ldst_whole(), who operates on effective vector
length regardless of the current settings in vtype and vl.

[1] https://lore.kernel.org/qemu-riscv/1590234b-0291-432a-a0fa-c5a6876097bc@linux.alibaba.com/

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240314175704.478276-7-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/vector_internals.c

@@ -44,6 +44,8 @@ void do_vext_vv(void *vd, void *v0, void *vs1, void *vs2,
     uint32_t vma = vext_vma(desc);
     uint32_t i;
 
+    VSTART_CHECK_EARLY_EXIT(env);
+
     for (i = env->vstart; i < vl; i++) {
         if (!vm && !vext_elem_mask(v0, i)) {
             /* set masked-off elements to 1s */
@@ -68,6 +70,8 @@ void do_vext_vx(void *vd, void *v0, target_long s1, void *vs2,
     uint32_t vma = vext_vma(desc);
     uint32_t i;
 
+    VSTART_CHECK_EARLY_EXIT(env);
+
     for (i = env->vstart; i < vl; i++) {
         if (!vm && !vext_elem_mask(v0, i)) {
             /* set masked-off elements to 1s */