Re: [Haskell-cafe] pointer equality

Carl Howells wrote:

On Tue, Jul 19, 2011 at 11:14 PM, yi huang wrote:

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2011/7/20 Eugene Kirpichov

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reallyUnsafePointerEq#, and it really is as unsafe as it sounds :)

Why is it so unsafe? i can't find any documentation on it. I think always compare pointer first is a good optimization.

False positives and false negatives are both possible, depending on GC timing.

At the moment, as implemented in ghc, false positives are not possible, because GC only happens on allocation [*], and there is no allocation happening in that primitive operation. I don't think this is going to change without a total rewrite of ghc, since allowing GC (i.e., moving pointers) at arbitrary times would be a fundamental change to the STG execution model. Pretty much everything else imaginable can happen; in particular, if two variables a and b compared equal at one point, they may later become different pointers again. In the parallel RTS, if you're unlucky, this may even be a permanent effect. Best regards, Bertram [*] we'll have thread-local GC for the first generation soon, but a lot of effort went into ensuring consistentcy of pointers seen by other threads.