🚀 go-pugleaf

In article <58n6df$rgb@atlas.xylogics.com>,
James Carlson <carlson@xylogics.com> wrote:
>
>In article <c7574ef08b6629a6c3f2218773ac27bf@wretched.mailer.com>, "Richard Kiessig" <rick@kiessig.com> writes:
>|> GCC seems to generate address comparisons in RTL as signed compares (i.e.
>|> "GT" instead of "GTU").  Is there a reason for this?  Shouldn't address
>|> compares be unsigned?  It probably doesn't matter on most machines, but it
>|> does on the one I'm working on.

This is definitely a bug on almost all architectures.. It just doesn't
show up that often on most of them.

>According to section 6.3.9 of the ANSI standard, the only requirement
>on the relational operators when used with addresses is that they
>operate correctly within the same object.  So as long as the compiler
>never allocates an object which spans the magic + to - boundary and
>malloc() never returns a block which spans that boundary, then signed
>and unsigned are equivalent, and any code which doesn't behave correctly
>with this definition isn't ANSI code.

Not just the compiler..  The OS can certainly make a difference, and
while ANSI may not mention stuff like "mmap()", a signed comparison can
certainly be a real problem. 

On many 32-bit systems the user space will contain the +/- boundary (not
all, because in many cases the +/- boundary is also the kernel/user
boundary).  On Linux/x86, for example, valid user accesses are from 0 to
0xbfffffff, and objects most certainly can span the +/- boundary. 

Simple test program to follow. It prints out

	Buggy compiler

If something is wrong, but it seems like this problem is fixed at least
in gcc-2.7.2..

		Linus

-----

#include <stdio.h>
#include <sys/mman.h>
#include <sys/types.h>

/*
 * This assumes 32-bit machines and a page-size that 
 * is <= 16384. Edit to suit..
 *
 * Run this program with stdin redirected from something
 * that can be mmap'ed - generally not the tty ;)
 */ 

#define MEM 16384
#define START ((char *)(0x80000000-MEM))
#define LEN (2*MEM)

int main(int argc, char **argv)
{
	char * p;

	p = mmap(START, LEN, PROT_READ|PROT_WRITE,
		MAP_FIXED|MAP_FILE|MAP_PRIVATE, 0, 0);
	if (p == (caddr_t) -1) {
		perror("mmap");
		exit(1);
	}
	if (p > p+LEN) {
		fprintf(stderr, "Buggy compiler\n");
		exit(1);
	}
	printf("ok\n");
	return 0;
}