On Tue, Dec 18, 2012 at 10:16 AM, Radu Rendec <radu.ren...@cloudbit.ro>wrote:
> Hello,
>
> I noticed that mspgcc-20120406 produces strange assembly for some very
> simple pointer arithmetic. I've managed to isolate this in a tiny test
> case:
>
> ext.c:
> int foo, bar;
>
> main.c:
> extern int foo, bar;
>
> void main(void) {
> char *ptr = (char *)&foo;
> char *top = (char *)&bar;
>
> *(int *)ptr = top - ptr;
> }
>
> I'm compiling with this command:
> msp430-gcc -Os -mmcu=msp430f47173 main.c ext.c
>
> The only reason for putting foo and bar in another .c file than main()
> is to avoid "top - ptr" getting optimized out into a constant value
> determined at compile time.
>
> The assembly for the above C code looks like this:
> 313e: 3a 40 02 11 mov #4354, r10 ;#0x1102
> 3142: 0f 4a mov r10, r15
> 3144: 8f 10 swpb r15
> 3146: 8f 11 sxt r15
> 3148: 8f 10 swpb r15
> 314a: 8f 11 sxt r15
> 314c: 0b 4f mov r15, r11
> 314e: 3c 40 00 11 mov #4352, r12 ;#0x1100
> 3152: 0f 4c mov r12, r15
> 3154: 8f 10 swpb r15
> 3156: 8f 11 sxt r15
> 3158: 8f 10 swpb r15
> 315a: 8f 11 sxt r15
> 315c: 0d 4f mov r15, r13
> 315e: 0a 8c sub r12, r10
> 3160: 0b 7d subc r13, r11
> 3162: 82 4a 00 11 mov r10, &0x1100
>
> Given the above assembly, 0x1102 is the address of bar and 0x1100 the
> address of foo.
>
> Any idea why "top - ptr" is not compiled as a simple "sub" instruction?
> I'm guessing that it has something to do with operator signedness, but I
> can't figure it out...
>
Yes, it's obviously sign-extending the pointers and doing a 32-bit
subtraction. I have a recollection that how mspgcc treated pointers
(signed/unsigned) changed sometime during the 4.7 development cycle.
mspgcc-20120911 appears to do the right thing and generates a simple 16-bit
subtract.
Peter
> Thanks,
>
> Radu Rendec
>
>
>
>
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