class initialization check overhead

Wed Oct 23 19:41:00 GMT 2002

On 23 Oct 2002, Adam Megacz wrote:
> Jeff Sturm <jsturm@one-point.com> writes:
> > Or are you suggesting something like PROT_NONE, as in a guard page?
>> Whoops, yes, that's what I meant. Would this work?

I don't know.
> Could _Jv_InitClass be re-jiggered to perform all the register saves
> itself (rather than asking the caller to do it)? In other words, put
> a PUSHALL at the beginning and a POPALL at the end?

Not without doing away with the calling convention, CALL_EXPR, etc.
Some trapping instruction might be better, like below. This example only
clobbers eax and memory, and overwrites the single-byte INT3 with one NOP.
#include <stdio.h>
#include <sys/mman.h>
#include <sys/signal.h>
#include <asm/param.h>
static void handler(int signum, struct sigcontext sc) {
 unsigned char *ip = (unsigned char *)sc.eip - 1;
 *((int *)sc.eax) += 1;
 if (*ip == 0xcc) {
 if (mprotect ((void *)(((unsigned long)ip) & ~(EXEC_PAGESIZE-1)),
 EXEC_PAGESIZE, PROT_READ | PROT_WRITE | PROT_EXEC) < 0)
 abort();
 *ip = 0x90;
 }
}
static inline void trap(volatile int *p) {
 asm volatile("int 3ドル" : : "a" (p) : "memory");
}
int main(void) {
 struct sigaction sa;
 int n;
 static volatile int x = 0;
 sa.sa_flags = SA_RESTART;
 sa.sa_handler = handler;
 sigaction(SIGTRAP, &sa, NULL);
 printf("%d\n", x);
 for (n = 0; n < 10; n++) trap(&x);
 printf("%d\n", x);
}