>> Has anyone compared the efficiency of the code produced by GCC and the
>> Plan 9 compiler?

I'm not sure that this is a very important issue, whichever is better.

If the Plan 9 C compiler produced better code, would that immediately
causes the free-nix community to change compiler? Of course not, there
are many considerations other than efficiency.

The answer is that gcc is probably/possibly/allegedly more efficient,
but not by a substantial degree.

So, lets say the code is X% faster, and even X% smaller.  How does
this help?  If the code you want to run is within X% of catastrophe,
then squeezing the code with the aid of a compiler is not the only
solution.  Throwing away a lot of the code is quite a good one too.

And, before I get flamed that this is not a commercially minded
answer, a substantial part of my employ has been spent building small
embedded systems.  When the code didn't fit, we invariably played with
the compiler, decided it didn't help enough, and then started removing
code.

What value of X makes changing compiler worthwhile?