On May 10, 2009, at 1:05 AM, Gwynne Raskind wrote:
The smallest (and likely fastest) would probably be the lookup table
version:
The smallest in terms of code, and it is probably the fasted method.
But it is also the most error prone ;)
And the switch-case version compiles to three pages of assembly full
of addls, addbs, hlts, jmps, and so forth.
Actually, the number of machine instructions equals 6 plus 2 times the
number of case clauses.
I ran some (very) simple benchmarks (full source code at the end of
this message).
...
Food for thought :).
My results:
GCC 4.0, 32-bit, -O3:
Switch version: 0.19739 sec
Non-Standard switch: 0.184204 sec
If-else version: 0.262892 sec
Lookup version: 0.114981 sec
GCC 4.0, 32-bit, -Os:
Switch version: 0.321028 sec
Non-Standard switch: 0.28091 sec
If-else version: 0.417788 sec
Lookup version: 0.102283 sec
GCC 4.2, 32-bit, -O3:
Switch version: 0.269755 sec
Non-Standard switch: 0.196217 sec
If-else version: 0.270493 sec
Lookup version: 0.106891 sec
GCC 4.2, 32-bit, -Os:
Switch version: 0.302562 sec
Non-Standard switch: 0.185923 sec
If-else version: 0.380549 sec
Lookup version: 0.098202 sec
Strict Aliasing does not have an effect in this case, so Objective-C,
C and C++ should perform equally (and it did).
Test code is basically:
volatile int c;
clock_t t0;
t0 = clock();
for (int i = 0; i < 100000000; ++i)
c = hexDigitToInt1(i & 255);
fprintf(stdout, "hexDigitToInt1: elapsed time: %g sec\n", (double)
(clock() - t0) / CLOCKS_PER_SEC);
I guess in Gwynne's test code the modulo operator has a very high
impact.
Regards
Andreas
Results:
GCC 4.0, 32-bit, -O3:
If-else version: 0.64217
Switch version: 0.60569
Lookup version: 0.41606
GCC 4.0, 32-bit, -Os:
If-else version: 0.70390
Switch version: 0.78775
Lookup version: 0.36639
GCC 4.2, 32-bit, -O3:
If-else version: 0.54521
Switch version: 0.59065
Lookup version: 0.42472
GCC 4.2, 32-bit, -Os:
If-else version: 0.71297
Switch version: 0.71112
Lookup version: 0.40114
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