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[Caml-list] assertions and branch prediction
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Date: 2002-02-15 (21:02)
From: james woodyatt <jhw@w...>
Subject: [Caml-list] assertions and branch prediction

After awhile, I noticed I was writing a fair amount of code that looks 
like this:

	match p with
	| A -> ...; ()
	| B -> ...; ()
	| _ -> assert false

Then I hit upon the idea of rewriting it this way:

	assert (p = A || p = B);
	match p with
	| A -> ...; ()
	| B -> ...; ()
	| C -> ()

My thinking was that I would rather pay up front with a more expensive 
assertion, one that can be stripped out later with the -noassert option, 
rather than pay at deployment with code (for raising the Assert_failure 
exception) that I've eventually proven will not be executed.

This morning, I realized I might be defeating the branch prediction 
optimizer in ocamlopt by doing it this way.

So.  I wonder: is branch prediction affected by the assert construct?  
If so, how can I use that to my advantage?  Does anyone here know?

While we are on the subject of the assert function, it occurs to me that 
it might be nice if -noassert together with "assert false" were to 
bypass all exception handling and go right into program termination.  It 
seems to me that if I've used the -noassert option, I've made a promise 
that the code compiled with it will never raise the Assert_failure 

Or am I not sufficiently educated about how exception flow works in 
Ocaml?  I haven't looked at the generated code, because that would mean 
having to learn another machine assembly language... and I am too old to 
do that for fun anymore.

j h woodyatt <>
"...the antidote to misinformation is more information, not less."
                                                      --vinton cerf

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