Restrict pointers

[Jan Hoogerbrugge]

>From: Jim Wilson <wilson@specifixinc.com>
>
>Jan Hoogerbrugge wrote:
>>As far as I know, memory references via two pointers where one of them is 
>>a restrict pointer and the other pointer is not restricted and is not 
>>derived from the first one are independent.
>
>I believe the problem here is "derived from".  If two pointers are both 
>restricted, then clearly they don't alias.
>
>If one pointer is restricted, then you have to perform tests to determine 
>if the second pointer might be derived from the first.  These tests have 
>not been implemented yet, and may be infeasible to implement in the current 
>alias analysis code.  This is because the current code is based on RTL, and 
>RTL is so low level that it is very hard to track pointer relationships.
>
>It is probably more feasible to implement this in tree-ssa, where we have 
>more info about declarations and assignments.  The derived from test should 
>be much more feasible there.
>
>Compilers are allowed to ignore restrict, so there is no bug here.  The 
>only problem is that we have a missed optimization, because we don't have 
>the infrastructure needed to implement the missed optimization. tree-ssa 
>probably helps with the infrastructure problem.

It's a pitty that GCC does not exploit all power of restrict pointers. I am
working on a port of GCC to the TriMedia VLIW processor and many
of the applications that are available for this processor use restrict 
pointers
extensively. Many compute kernels run a few times faster when restrict
pointers are present and fully exploited. I guess that this will be true for
many other instruction-level parallel processors for which applications have
been tuned.

What can we / I do to improve the handling of restrict pointers? Any idea
of the difficulty and where to start?

Jan

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