simonpj 2003/06/23 04:46:41 PDT
Modified files:
ghc/compiler/stranal DmdAnal.lhs
Log:
------------------------------------------------------
Make the strictness analyser more conservative for I/O
------------------------------------------------------
Consider
do { let len = <expensive> ;
; when (...) (exitWith ExitSuccess)
; print len }
Is it safe to evaluate <expensive> before doing the 'when'? Remember,
<expensive> might raise an exception etc as well.
Normal strictness analysis answer: yes, because either the when... diverges
or raises an exception, or the print will happen.
Correct I/O answer: no, because it's perfectly OK to terminate the program
successfully. And don't say the 'len' could be pushed down, because (a) sometimes
it can't and (b) sometimes the compiler might float it out.
This commit adds a hack to the demand analyser, so that it treats a case that
looks like I/O (unboxed pair, real-world as first bindre) specially, by lub'ing
the returned strictness type with TopType. A bit like adding a dummy never-taken
branch. This seems a bit hack-oid, but it's quick and it works. Not clear
how to do it 'right', either.
Test is in stranal/should_run/strun003.
Revision Changes Path
1.47 +27 -3 fptools/ghc/compiler/stranal/DmdAnal.lhs
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