Re: [9fans] plan or side effect

[Sean Quinlan <seanq@research.bell-labs.com>]

My experience is that most, if not all, optimizing C
compilers will make optimistic assumptions about aliasing.
Given that C allows arbitrary cast and application of the &
operator, it is pretty much impossible to excatly determine
what can be aliased.  The compiler takes a shot at figuring this
out, but in general, it has to be optimistic if it is going
to find many variables that can be registerised.
For example, I believe the plan 9 compilers is a little
optimistic with globals.  The compiler will registerise
such variables within a function even though it is possible
that the variable is aliased via a pointer.  I recall
Ken saying he never gets caught, but obviously it is not
hard to contrive examples when he will get the "wrong"
answer.


presotto@plan9.bell-labs.com wrote:
> 
> Bushnell et al,
> 
> By the way, I'm not ignoring Thomas' question.
> I'm querying compiler designers I know about my
> statement that they know that they might be breaking
> programs, i.e. generating incorrect code,
> with some optimizations and find that acceptable.
> It was definitely a true statement when I was at
> DG in the compiler group, but that was a long
> time ago.  It could be that I'm assuming a plan when
> all I'm seeing is the inevitable bugs.