From mboxrd@z Thu Jan  1 00:00:00 1970
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From: presotto@plan9.bell-labs.com
To: 9fans@cse.psu.edu
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Subject: [9fans] (no subject)
Date: Wed, 27 Feb 2002 17:47:15 -0500
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This is what I got out of Cliff Young:

Do you mean about the general 9fans topic of 
whether optimizations are dangerous, or about
the specific issue of when you can registerize
a global?

On the former, one person made the rarely-made point
that many people write programs that depend on undefined
behavior in the C standard, then gripe when their bugs
that are masked with -O0 are exposed with -O2.  I think
this point isn't made often enough.

On the other hand, I'm entirely willing to admit that
buggy optimizations passes are the reason why people
turn optimizations off.  It's hard to write optimizations
that are as reliable as, say, hardware.  Part of this is
that the verification technology is way better on the 
hardware side, and another part is that writing a 
good optimizing compiler is a never-ending time sink,
and it's hard to get users to send you examples that
exhibit bugs, and then even harder to track them down.
And this in a program that isn't multithreaded.

On registerizing globals, it seems to be that it's 
entirely safe to registerize as long as control flow
doesn't leave your function.  I.e., don't expect
registerized globals not to mutate across function
calls.  Threaded programmers know what they're getting
themselves into, and ought to be using locks or
condition variables.