Opened 7 months ago

Last modified 7 months ago

#13698 new task

Add a more complete example for the special SPEC argument to the user guide

Reported by: mpickering Owned by:
Priority: normal Milestone:
Component: Compiler Version: 8.0.1
Keywords: newcomer Cc:
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: SpecConstr Differential Rev(s):
Wiki Page:

Description

In the section for -fspec-constr in the user guide there is an example of how to use SPEC to force a loop to be unrolled by spec constr.

However, this argument doesn't do anything until foldl is called with a specific argument. It wasn't clear to me exactly how this worked as the example was incomplete. Below is a complete file which shows how the optimisation unrolls the list completely such that res2 = 6 in core.

{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE ExistentialQuantification #-}
module Foo where
import GHC.Types

data Stream a = forall s.
                Stream !(s -> Step a s)  -- a stepper function
                !s                       -- an initial state

data Step a s = Yield a !s
               | Skip    !s
               | Done

s2 = Stream s [1,2,3]

s [] = Done
s (x:xs) = Yield x xs

res2 = Foo.foldl (+) 0 s2


foldl :: (a -> b -> a) -> a -> Stream b -> a
{-# INLINE foldl #-}
foldl f z (Stream step s) = foldl_loop SPEC z s
    where
      foldl_loop !sPEC z s = case step s of
                              Yield x s' -> foldl_loop sPEC (f z x) s'
                              Skip  s'     -> foldl_loop sPEC z s'
                              Done -> z

Change History (1)

comment:1 Changed 7 months ago by simonpj

Well the manual does say "After GHC inlines the body of foldl to a call site...". But I agree that the entire description is unclear; hardly a specification of the SPEC business.

Could you improve it?

Simon

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