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...
Thanks,
Radu Rendec
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