RV64: Make cast from unsigned->signed int explicit in rej_uniform

This is required by `-Wconversion`, and is indeed worth a comment
since a cast from `unsigned` to `int` can in theory overflow.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker

mlkem/src/native/riscv64/meta.h

@@ -61,7 +61,10 @@ static MLK_INLINE int mlk_rej_uniform_native(int16_t *r, unsigned len,
                                              const uint8_t *buf,
                                              unsigned buflen)
 {
-  return mlk_rv64v_rej_uniform(r, len, buf, buflen);
+  /* The cast from unsigned to signed integer is safe
+   * because the return value is <= len, which we asssume
+   * to be bound by 4096 and hence <= INT_MAX. */
+  return (int) mlk_rv64v_rej_uniform(r, len, buf, buflen);
 }
 
 static MLK_INLINE int mlk_poly_reduce_native(int16_t data[MLKEM_N])

mlkem/src/native/riscv64/src/rv64v_poly.c

@@ -734,7 +734,7 @@ void mlk_rv64v_poly_reduce(int16_t *r)
 unsigned int mlk_rv64v_rej_uniform(int16_t *r, unsigned int len,
                                    const uint8_t *buf, unsigned int buflen)
 {
-  size_t n, ctr, pos;
+  unsigned n, ctr, pos;
   vuint16m1_t x, y;
   vbool16_t lt;
 
@@ -743,8 +743,8 @@ unsigned int mlk_rv64v_rej_uniform(int16_t *r, unsigned int len,
 
   while (ctr < len && pos < buflen)
   {
-    const size_t vl = __riscv_vsetvl_e16m1((buflen - pos) * 8 / 12);
-    const size_t vl23 = (vl * 24) / 32;
+    const unsigned vl = __riscv_vsetvl_e16m1((buflen - pos) * 8 / 12);
+    const unsigned vl23 = (vl * 24) / 32;
 
     const vuint16m1_t vid = __riscv_vid_v_u16m1(vl);
     const vuint16m1_t srl12v = __riscv_vmul_vx_u16m1(vid, 12, vl);