[PATCH v1 1/2] target/riscv: fix vector register address calculation in strided LD/ST

[Chao Liu]

his patch fixes a critical bug in the RISC-V vector instruction translation that
caused incorrect data handling in strided load operations (e.g., vlsseg8e32).

Problem Description:

The `get_log2` function in `trans_rvv.c.inc` returned a value 1 higher than the
actual log2 value. For example, get_log2(4) incorrectly returned 3 instead of 2.

This led to erroneous vector register offset calculations, resulting in data
overlap where bytes 32-47 were incorrectly copied to positions 16-31 in ChaCha20
encryption code.

rvv_test_func:
    vsetivli    zero, 1, e32, m1, ta, ma
    li          t0, 64

    vlsseg8e32.v v0, (a0), t0
    addi        a0, a0, 32
    vlsseg8e32.v v8, (a0), t0

    vssseg8e32.v v0, (a1), t0
    addi        a1, a1, 32
    vssseg8e32.v v8, (a1), t0
    ret


Analysis:

The original implementation counted the number of right shifts until
zero, including the final shift that reduced the value to zero:

static inline uint32_t get_log2(uint32_t a)
{
    uint32_t i = 0;
    for (; a > 0;) {
        a >>= 1;
        i++;
    }
    return i; // Returns 3 for a=4 (0b100 → 0b10 → 0b1 → 0b0)
}


Fix:

The corrected function stops shifting when only the highest bit remains
and handles the special case of a=0:

static inline uint32_t get_log2(uint32_t a)
{
    uint32_t i = 0;
    if (a == 0) {
        return i; // Handle edge case
    }
    for (; a > 1; a >>= 1) {
        i++;
    }
    return i; // Now returns 2 for a=4
}

Fixes: 28c12c1f2f ("Generate strided vector loads/stores with tcg nodes.")

Signed-off-by: Chao Liu <chao....@yeah.net>
---
 target/riscv/insn_trans/trans_rvv.c.inc | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/target/riscv/insn_trans/trans_rvv.c.inc 
b/target/riscv/insn_trans/trans_rvv.c.inc
index 2b6077ac06..f50b62b1d8 100644
--- a/target/riscv/insn_trans/trans_rvv.c.inc
+++ b/target/riscv/insn_trans/trans_rvv.c.inc
@@ -877,7 +877,10 @@ static inline uint32_t MAXSZ(DisasContext *s)
 static inline uint32_t get_log2(uint32_t a)
 {
     uint32_t i = 0;
-    for (; a > 0;) {
+    if (a == 0) {
+        return i;
+    }
+    for (; a > 1;) {
         a >>= 1;
         i++;
     }
-- 
2.50.1