• Simon Peyton Jones's avatar
    Improve code generation for conditionals · 6d14c148
    Simon Peyton Jones authored
    This patch in in preparation for the fix to Trac #13397
    
    The code generator has a special case for
      case tagToEnum (a>#b) of
        False -> e1
        True  -> e2
    
    but it was not doing nearly so well on
      case a>#b of
        DEFAULT -> e1
        1#      -> e2
    
    This patch arranges to behave essentially identically in
    both cases.  In due course we can eliminate the special
    case for tagToEnum#, once we've completed Trac #13397.
    
    The changes are:
    
    * Make CmmSink swizzle the order of a conditional where necessary;
      see Note [Improving conditionals] in CmmSink
    
    * Hack the general case of StgCmmExpr.cgCase so that it use
      NoGcInAlts for conditionals.  This doesn't seem right, but it's
      the same choice as the tagToEnum version. Without it, code size
      increases a lot (more heap checks).
    
      There's a loose end here.
    
    * Add comments in CmmOpt.cmmMachOpFoldM
    6d14c148
CmmOpt.hs 17.3 KB