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Odd PCM generated code

 
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dave3
Guest
Odd PCM generated code
PostPosted: Sun Apr 18, 2004 8:50 pm     Reply with quote
In the process of studying the PCM output for function pointers, I came across the following generated code, which appears redundant.
Code:

....................   case 1:
....................     ret= fn1;
0013:  MOVLW  00
0014:  MOVWF  7A
0015:  MOVLW  2B
0016:  MOVWF  25
0017:  MOVF   7A,W
0018:  MOVWF  26


Since 7A isn't used anywhere else, why isn't a
Code:
CLRF 26
used instead? Am I missing something? As you may infer, fn1() is at 002B. The variable 'ret' is the return value for a function which returns a pointer to one of two functions. 'ret' is declared an fptr, defined as:
Code:

typedef int8 (*fptr)();
Interestingly, if I tried returning the 'fn1' pointer directly from the switch statement, the compiler generates a warning. I'd be happy to post the entire .LST file if anyone's interested. Thanks for any insight.
Ttelmah
Guest
Re: Odd PCM generated code
PostPosted: Mon Apr 19, 2004 2:05 am     Reply with quote
dave3 wrote:
In the process of studying the PCM output for function pointers, I came across the following generated code, which appears redundant.
Code:

....................   case 1:
....................     ret= fn1;
0013:  MOVLW  00
0014:  MOVWF  7A
0015:  MOVLW  2B
0016:  MOVWF  25
0017:  MOVF   7A,W
0018:  MOVWF  26


Since 7A isn't used anywhere else, why isn't a
Code:
CLRF 26
used instead? Am I missing something? As you may infer, fn1() is at 002B. The variable 'ret' is the return value for a function which returns a pointer to one of two functions. 'ret' is declared an fptr, defined as:
Code:

typedef int8 (*fptr)();
Interestingly, if I tried returning the 'fn1' pointer directly from the switch statement, the compiler generates a warning. I'd be happy to post the entire .LST file if anyone's interested. Thanks for any insight.

Remember a pointer, is a two byte value. Though in your case, the address of fn1, is in the low bank, and hence the high byte is '0', this won't allways be the case. The compiler potentially could optimise this specific case (of a funtion in the low 256 bytes), to use a clrf for the high byte, but the code is generic, and transfers the high byte of the address, using a 'scratch' register (7A should be defined as 'scratch' if you look in the variable declaration listing).

Best Wishes
dave3
Guest
PostPosted: Mon Apr 19, 2004 12:53 pm     Reply with quote
But why isn't the scratch register optimized away?
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