The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Friday, May 01, 2020

It seems that C's bit-fields are more of a pessimization than an optimization

A few days ago, maybe a few weeks ago, I don't know, the days are all just merging together into one long undifferentiated timey wimey blob but I'm digress, I had the odd thought that maybe, perhaps, I could make my Motorola 6809 emulator faster by using a bit-field for the condition codes instead of the individual booleans I'm using now. The thought was to get rid of the somewhat expensive routines to convert the flags to a byte value and back. I haven't used bit-fields all that much in 30 years of C programming as they tend to be implementation dependent:

C99 standard, annex J.3.9

But I could at least see how gcc deals with them and see if there is indeed a performance increase. I converted the definition of the condition codes from:

  bool e;
  bool f;
  bool h;
  bool i;
  bool n;
  bool z;
  bool v;
  bool c;
} cc;


  ; I determined this ordering of the bits empirically. 
    bool c : 1;
    bool v : 1;
    bool z : 1;
    bool n : 1;
    bool i : 1;
    bool h : 1;
    bool f : 1;
    bool e : 1;
  } f;
  mc6809byte__t b;

(Yes, by using a union I'm inviting “unspecified behavior”—from the C99 standard: “[t]he value of a union member other than the last one stored into (”), but at least gcc does the sane thing in this case.)

The code thus modified, I ran some tests to see the speed up and the results were rather disappointing—it was slower using bit-fields than with 8 separate boolean values. My guess is that the code used to set and check bits, especially in an expression like (cpu->cc.f.n == cpu->cc.f.v) && !cpu->cc.f.z was larger (and thus slower) than just using plain bool for each field.

So the upshot—by changing the code to use an implementation-defined detail and invoking unspecified behavior, thus making the resulting program less portable, I was able to slow the program down enough to see it wasn't worth the effort.


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