[GHC] #9736: Constant folding rules are wrong for GHCJS

[GHC]

#9736: Constant folding rules are wrong for GHCJS
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              Reporter:  luite       |            Owner:  luite
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by luite):

 I've made the `removeOp32` operation target word size dependent (from
 `DynFlags`) and `rightShiftLogical` use a correctly sized word for the
 target.

 I'm not 100% sure what's the best way to do `removeOp32` for GHCJS, since
 `ArchJavaScript` has to be treated as 32 bit (that's the only word size
 available for bitwise operations), but numbers can have a bigger range. In
 most cases, the primops already handle truncation, and the dataflow
 analysis in the optimizer removes the superfluous narrowing operations. In
 some situations, narrowing is still needed, for example when a value is
 obtained from a foreign operation.

 Since most `NarrowInt32` operations are actually unnecessary for GHCJS,
 and I'm not sure how well the optimizer will be able to strip the
 `NarrowInt32Op` from for example `Int32` operations, I'm not adding an
 exception for `ArchJavaScript` in `removeOp32` and deal with exceptional
 situations like foreign return values individually in the desugarer.

 If it turns out that there are situations that can't be handled this way,
 I'll send another change request for that, like below:

 {{{#!hs
 removeOp32 :: RuleM CoreExpr
 removeOp32 = do
   dflags <- getDynFlags
   if wordSizeInBits dflags == 32 &&
      -- JavaScript has 32 bit bitwise operations
      -- but numbers have a greater range
      platformArch (targetPlatform dflags) /= ArchJavaScript
   then do
     [e] <- getArgs
     return e
   else mzero
 }}}

 (and unfortunately I'd have to copy/paste the whole PrelRules module into
 GHCJS again if I'm too late)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9736#comment:3>
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