patch applied (ghc): Haskell Program Coverage

[Simon Peyton-Jones]

Andy,

| > instead of
| >
| >   Note (Tick "M" 23) e
| >
| > how about
| >
| >   case tick# "M"# 23# of DEFAULT -> e
| >
| > where tick# is a PrimOp, and the case expression is compiled to
| > inline code (ie. no extra info table / return address for e).  The
| > PrimOp would be marked as having side effects, so the simplifier
| > wouldn't rearrange code around it.  There would be nothing HPC-
| > specific in CoreSyn, CorePrep, CoreToStg, or StgSyn.  All you need
| > to do is provide an implementation for tick# in CgPrimOp.  I'm
| > probably missing something!
| >
| > The "M"# is a Literal - a MachStr.  I imagine you don't need the
| > package name, because it is always the current package name (the
| > code generator for tick# can prepend the package name if it needs
| > to).  A bit of cleverness in CgPrimOp is required to common up all
| > the module names, I guess.
| >
| 
| I tried this, and the tick# M# 23# got lifted out into a CAF, and was
| evaluated only once :-) Perhaps the PrimOp's side_effect flag
| was off on that build, though.  Does the side_effect flag stop this?

What you want is
 a) Return an unboxed result (can be State#)
 b) the side-effect flag

If both these hold, then the primop won't be floated out anywhere. 

Please do try this.  I would much, much rather use a PrimOp than a Note.
Indeed I think we should do the SCC stuff via primops too.

Simon