C code optimization for a = (b < c);

[WILCOX at nosc-tecr.arpa]

Assuming that the target machine implements two's compliment arithmetic
and doesn't have store condition code instructions, there is a simple
optimization that few C compilers seem to use.  Given:

     int a;
     int b, c;
     a = (b < c);

the usual approach is something like:

     a = 0;
     if( ! (b < c)) goto label;
     a = 1;
label:

A better approach would seem to be:

a = (unsigned)((c - b) >> (WIDTH_OF_INT_IN_BITS - 1));

In other words, subtract rather than compare, and shift the resulting sign
bit into the least-significant bit position with zero extend.  There is
also a variation using signed rather than unsigned shift, which generates
either -1 or 0, followed by an increment to produce either 0 or 1.

     --Dwight Wilcox
       Code 412
       Naval Ocean Systems Center
       San Diego, CA  92152-5000