Re: Pointer arithmetic error

[David Given <dg@cowlark.com>]

Bernd Petrovitsch wrote:
[...]
> In C, there is no type "byte" (unless you typedef oder #define it).
> "byte" is usually (but not necessarily) meant as "unsigned char".

The issue here is that sparse is helpful and converts pointer offsets 
into byte offsets when generating code for pointer arithmetic. So:

const int* p = (const char*) 1234;
p += 10;

-->

set.32 %p0 <- 1234
add.32 %p1 <- %p0, 40
(In fact, in this example it'd collapse these together and use 1274 
instead.)

This appears to be done using hard-coded knowledge that a byte (one unit 
of addressingness) is 8 bits wide.

Chris Li wrote:
> Byte need to big enough to hold the char. Using bits_in_byte is better.
> There might be other place in sparse assume byte is 8 bits.

IIRC C specifies that sizeof() returns values measured in chars, but I 
don't believe it specifies any mapping between the size of chars and the 
underlying addressing units --- it should be possible to use 16-bit 
chars, for example, on an 8-bit byte system. Using 32-bit ints, 
sizeof(int) would then return 2; but you wouldn't be able to access 
individual bytes from C.

That's assuming I've remembered the spec correctly, of course --- this 
stuff is annoying subtle.

If you like I'll go and have a look to see if this can be easily fixed 
--- I need to do it anyway. In fact, I have vague recollections that 
load and store were actually using offsets like 0, 1, 2, etc (the 
'right' values), which means they must have been using bits_per_char to 
do their calculations; I need to check up on this.

-- 
David Given
dg@cowlark.com