target/riscv: Add a property to set vl to ceil(AVL/2)

RVV spec allows implementations to set vl with values within
[ceil(AVL/2),VLMAX] when VLMAX < AVL < 2*VLMAX. This commit adds a
property "rvv_vl_half_avl" to enable setting vl = ceil(AVL/2). This
behavior helps identify compiler issues and bugs.

Signed-off-by: Jason Chien <jason.chien@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Message-ID: <20240722175004.23666-1-jason.chien@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
index 4bda754..cc55525 100644
--- a/target/riscv/cpu.c
+++ b/target/riscv/cpu.c
@@ -2661,6 +2661,7 @@
 
     DEFINE_PROP_BOOL("rvv_ta_all_1s", RISCVCPU, cfg.rvv_ta_all_1s, false),
     DEFINE_PROP_BOOL("rvv_ma_all_1s", RISCVCPU, cfg.rvv_ma_all_1s, false),
+    DEFINE_PROP_BOOL("rvv_vl_half_avl", RISCVCPU, cfg.rvv_vl_half_avl, false),
 
     /*
      * write_misa() is marked as experimental for now so mark

target/riscv/cpu_cfg.h

diff --git a/target/riscv/cpu_cfg.h b/target/riscv/cpu_cfg.h
index 8b272fb..96fe26d 100644
--- a/target/riscv/cpu_cfg.h
+++ b/target/riscv/cpu_cfg.h
@@ -127,6 +127,7 @@
     bool ext_smepmp;
     bool rvv_ta_all_1s;
     bool rvv_ma_all_1s;
+    bool rvv_vl_half_avl;
 
     uint32_t mvendorid;
     uint64_t marchid;

target/riscv/vector_helper.c

diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
index 10a52ce..072bd44 100644
--- a/target/riscv/vector_helper.c
+++ b/target/riscv/vector_helper.c
@@ -75,6 +75,8 @@
     vlmax = vext_get_vlmax(cpu->cfg.vlenb, vsew, lmul);
     if (s1 <= vlmax) {
         vl = s1;
+    } else if (s1 < 2 * vlmax && cpu->cfg.rvv_vl_half_avl) {
+        vl = (s1 + 1) >> 1;
     } else {
         vl = vlmax;
     }