Tool/software: Code Composer Studio
win7 32 bit, CCS V6, F28023.
Hello:
I have two code snippets that compile into different assembly language code, and I'd like to know how the compiler decides which to use. I'd like to utilize this technique in my own code.
Snippet 1:
memset((char *) RootName, 0, sizeof(RootName));
This statement assembles into the following INLINE code...
3f50aa: 8F000096 MOVL XAR4, #0x000096 3f50ac: F618 RPT #24 3f50ad: 2B84 || MOV *XAR4++, #0
Nice and tight. I understand the compiler knows ahead of time that sizeof distills down to a number less then 255 (the max size of the RPT instruction.)
Snippet 2:
memset(buffer, 0, sizeof(buffer));
and it's assembly code...
3f42a7: D500 MOVB XAR5, #0x0 3f42a8: 76800400 MOVL XAR6, #0x000400 3f42aa: 8F000400 MOVL XAR4, #0x000400 3f42ac: 06A6 MOVL ACC, @XAR6 3f42ad: 767F5EE8 LCR memset
and then the actual work routine...
memset():
3f5ee8: FF58 TEST ACC
396 register char *m = (char *)mem;
3f5ee9: C5A4 MOVL XAR7, @XAR4
398 while (length--) *m++ = ch;
3f5eea: EC09 SBF C$L2, EQ
3f5eeb: 1901 SUBB ACC, #1
3f5eec: 1EA6 MOVL @XAR6, ACC
C$L1:
3f5eed: 0200 MOVB ACC, #0
3f5eee: DE81 SUBB XAR6, #1
3f5eef: 7D87 MOV *XAR7++, AR5
3f5ef0: 1901 SUBB ACC, #1
3f5ef1: 0FA6 CMPL ACC, @XAR6
3f5ef2: EDFB SBF C$L1, NEQ
C$L2:
3f5ef3: 0006 LRETR
Again, I understand the compiler knows the sizeof is larger then 255
Another example uses the second snippet when the compiler does not know the size ahead of time, as expected.
My question...
How does the compiler know which code to use?
Here is the code I see when debugging (copied from the debugger during a debugging session...)
#if ((defined(_INLINE) || defined(_MEMSET)) && !defined(_TMS320C6X)) && !defined(__TMS470__) && !defined(__ARP32__)
_OPT_IDEFN void *memset(void *mem, register int ch, register size_t length)
{
register char *m = (char *)mem;
while (length--) *m++ = ch;
return mem;
}
#endif /* _INLINE || _MEMSET */
As far as I can tell, the 3 processor types are not defined (so, true), and I do not know how the other two are set (_INLINE_ and _MEMSET)
Is there more to this that I am not seeing?
Thanks, Mark.
PS. This is unrelated, but snippet 2 work code seems VERY inefficient, I wrote this in assembly and I got it to two instructions. I may ask another thread on that later. (of course, I have optimization set low.)
