Re: [PATCH] target/riscv: Fix a uninitialized variable warning

[Daniel Henrique Barboza <dbarboza@ventanamicro.com>]

On 10/19/25 5:19 AM, Akihiko Odaki wrote:
> riscv_cpu_validate_v() left its variable, min_vlen, uninitialized if
> no vector extension is available, causing a compiler warning. Avoid
> the warning by calling g_assert_not_reached() in the case.

For the compiler point of view the variable will be left uninitialized.
In reality we'll always set it to at least '32' in validate_v(). This
is how the function is being called:

     if (cpu->cfg.ext_zve32x) {
         riscv_cpu_validate_v(env, &cpu->cfg, &local_err);
         if (local_err != NULL) {
             error_propagate(errp, local_err);
             return;
         }
     }

This means that inside the function we guarantee that min_vlen will be
at least set to 32 because cfg->ext_zve32x will always be true:

     if (riscv_has_ext(env, RVV)) {
         min_vlen = 128;
     } else if (cfg->ext_zve64x) {
         min_vlen = 64;
     } else if (cfg->ext_zve32x) {
         min_vlen = 32;
     }


To make the compiler happy and the code a bit clearer I suggest initializing
min_vlen = 32 and folding the "if (cpu->cfg.ext_zve32x)" check inside
validate_v() for an early exit. Something like this:


@@ -417,15 +417,19 @@ static void riscv_cpu_validate_misa_priv(CPURISCVState *env, Error **errp)
  static void riscv_cpu_validate_v(CPURISCVState *env, RISCVCPUConfig *cfg,
                                   Error **errp)
  {
-    uint32_t min_vlen;
-    uint32_t vlen = cfg->vlenb << 3;
+    uint32_t min_vlen, vlen;
+
+    if (cfg->ext_zve32x) {
+        return;
+    }
+
+    min_vlen = 32;
+    vlen = cfg->vlenb << 3;
  
      if (riscv_has_ext(env, RVV)) {
          min_vlen = 128;
      } else if (cfg->ext_zve64x) {
          min_vlen = 64;
-    } else if (cfg->ext_zve32x) {
-        min_vlen = 32;
      }
  
      if (vlen > RV_VLEN_MAX || vlen < min_vlen) {
@@ -676,12 +680,10 @@ void riscv_cpu_validate_set_extensions(RISCVCPU *cpu, Error **errp)
          return;
      }
  
-    if (cpu->cfg.ext_zve32x) {
-        riscv_cpu_validate_v(env, &cpu->cfg, &local_err);
-        if (local_err != NULL) {
-            error_propagate(errp, local_err);
-            return;
-        }
+    riscv_cpu_validate_v(env, &cpu->cfg, &local_err);
+    if (local_err != NULL) {
+        error_propagate(errp, local_err);
+        return;
      }


Note: I wonder why we're allowing settings of VLEN and so on when we do
not have RVV set. Seems like a bug ...


Thanks,

Daniel




> 
> Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
> ---
>   target/riscv/tcg/tcg-cpu.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/target/riscv/tcg/tcg-cpu.c b/target/riscv/tcg/tcg-cpu.c
> index 1150bd14697c..acbfac5d9e2c 100644
> --- a/target/riscv/tcg/tcg-cpu.c
> +++ b/target/riscv/tcg/tcg-cpu.c
> @@ -426,6 +426,8 @@ static void riscv_cpu_validate_v(CPURISCVState *env, RISCVCPUConfig *cfg,
>           min_vlen = 64;
>       } else if (cfg->ext_zve32x) {
>           min_vlen = 32;
> +    } else {
> +        g_assert_not_reached();
>       }
>   
>       if (vlen > RV_VLEN_MAX || vlen < min_vlen) {
> 
> ---
> base-commit: c85ba2d7a4056595166689890285105579db446a
> change-id: 20251019-vlen-30a57c03bd93
> 
> Best regards,
> --
> Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
>