>> WTF is going on with the Hi-Tech compiler that ships with MPLAB....

It used to be that one was taught to have their compiler output  
"standard format intermediate code" (usually triple operand things  
that matched no real machines at all:  "operation source1, source2 ==>  
destination".)   Then, "later", one would write a code generator that  
would optimize that in ways specific to a destination CPU.  (actually,  
I guess you get at least three chances to optimize; you can optimized  
the intermediate code, optimize the generation of real code, and then  
optimize the resulting machine code.)

The Hi-tech compiler *is* (was?) multi-target, so it probably has some  
sort of common back-end and does something very like this sort of code  
generation.


I wasn't impressed, back when I was taught this. :-(

I'm not impressed now, when it sort-of looks like Hi-tech has  
implemented their free compiler by replacing the code generator with  
something that is at best trivial and perhaps intentionally bad...

> 365:               irrxsum1+=irbyte1;
>   115    087A     MOVF 0x7a, W
>   116    00C3     MOVWF 0x43
>   117    0843     MOVF 0x43, W
>   118    07F4     ADDWF 0x74, F
>
> Should be 2 instructions

This matches up well with "add irrxsum1, irbyte1 ==> irrxsum1 ?

Also don't forget that C expressions like the source code here are  
defined to "return" a value, so it's not beyond reasonableness that  
completely unoptimized code would want to leave the value of each  
expression calculation in W.

> 366:               ircnt1=0;
>   119    1003     BCF 0x3, 0
>   11A    3000     MOVLW 0
>   11B    1803     BTFSC 0x3, 0
>   11C    3001     MOVLW 0x1
>   11D    00FB     MOVWF 0x7b
>
> This is especially bizarre - should be 1 CLRF instruction

Yeah.  Perhaps the compiler has special code to handle common small  
constants like zero and one, but without optimization the test stays  
in the code?

I think this was a really bad move on hi-tech's part.  It makes them  
look bad, it makes compilers in general look bad.  I applaud (sort of)  
the attempt at a new way of handling "free" versions of commercial  
compilers, but I really hope that Microsoft decides that this is a  
failed experiment and does something else in the future.

BillW

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist