On Fri, Aug 03, 2007 at 09:01:18PM -0300, Isaac Dupree wrote: > Tim Chevalier wrote: >> On 8/3/07, Simon Peyton-Jones <[EMAIL PROTECTED]> wrote: >>> Stefan is right here. >>> >>> - It's not surprising that with -Onot you get different code from >>> different source programs, even if one can readily be transformed into >>> the other. That's what -O does. >>> >> Yes, but I found it surprising that just removing a type signature >> should result in markedly different code. Are there other known >> situations where that can happen? > > It is not _just_ removing a type signature, it is also changing the type > from `Bool` to `forall a. a`. An explicit type signature of the latter > would have produced the same results as no type signature, I believe. The > surprise is that an unconstrained type-variable being variable rather than > instantiated to an arbitrary type, makes any difference (since it doesn't, > normally, at runtime). I would guess the programs `Bool` and `a` are the > same once optimizations are turned on? Maybe GHC could avoid the creation > of type-lambdas that are unused (in some sense)... with -Onot... I'm > dubious about that.
It *does not change the designed semantics at all*. It *tickles a bug*. In the *absence of code generator bugs* it generates *semantically equivalent code*. Do you want inserting or removing type signatures to always tickle the same bugs? Seems like an awfully constraining desgin choice to me. > Inserting a preemption test in non-allocating loops seems like a good idea > to me (I hate the invisible threat that my program might not thread as > threading should work)... any idea how bad the performance impact could be > (I guess the test could be specified to branch-predict that the loop > wouldn't be interrupted), and whether there could be a pragma to disable > that test in certain loops? Is -threaded versus not, relevant here?
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