CODEWITHSUNDEEP
cpointersassemblykeil8051
Gamcheong
Gamcheong

Reputation: 43

Why compiler assembly C program into this?

I don't understand why the C51 compiler (in Keil) convert C source to this assembly language code: (As p is a pointer to unsigned char)

;   p++;
            ; SOURCE LINE # 216
    MOV     R3,pDisplay?253
    INC     pDisplay?253+02H
    MOV     A,pDisplay?253+02H
    MOV     R2,pDisplay?253+01H
    JNZ     ?C0090
    INC     pDisplay?253+01H

As R3 and R2 wasn't used in the next lines of the program.
Why do compiler make these lines
MOV R3,pDisplay?253,
MOV R2,pDisplay?253+01H?

Upvotes: 3

Views: 136

Answers (1)

Mike Housky
Mike Housky

Reputation: 4069

Welcome to 1980s "state of the art" code generation for 8-bit target processors. What the code is doing is "noting the value" of p before incrementing it. That's necessary when the postincrement operator is used in a surrounding expression; and not optimized away by subsequent passes of the compiler.

Try p += 1;, or even ++p; instead. Odds are that one or both of those will generate better code because there is no "note the value before" semantics to get in the code generator's way.

[This is how I got into the minority, by the way, using ++i in for () loops rather than the more common i++.]

Upvotes: 9

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