Rewrite with faster clz instruction

Author: jserv

common.h

@@ -10,3 +10,9 @@
 #define MASK(n) (~((~0U << (n))))
 
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof(*(a)))
+
+/* Ensure that __builtin_clz is never called with 0 argument */
+static inline int ilog2(int x)
+{
+    return 31 - __builtin_clz(x | 1);
+}

riscv.c

@@ -763,9 +763,7 @@ void vm_step(vm_t *vm)
 
     if ((vm->sstatus_sie || !vm->s_mode) && (vm->sip & vm->sie)) {
         uint32_t applicable = (vm->sip & vm->sie);
-        uint8_t idx = 32;
-        while (!(applicable & (1 << --idx)))
-            ; /* TODO: Rewrite with faster a lookup table. */
+        uint8_t idx = ilog2(applicable);
         vm->exc_cause = (1U << 31) | idx;
         vm->stval = 0;
         vm_trap(vm);

uart.c

@@ -48,11 +48,8 @@ void u8250_update_interrupts(u8250_state_t *uart)
     uart->pending_ints &= uart->ier;
 
     /* Update current interrupt (higher bits -> more priority) */
-    if (uart->pending_ints) {
-        uart->current_int = 8;
-        while (!(uart->pending_ints & (1 << --uart->current_int)))
-            ; /* TODO: Rewrite with faster lookup table. */
-    }
+    if (uart->pending_ints)
+        uart->current_int = ilog2(uart->pending_ints);
 }
 
 void u8250_check_ready(u8250_state_t *uart)