Opened 9 months ago

Last modified 9 months ago

#16018 new bug

Disabling core optimizations ignores code that would otherwise warn.

Reported by: dmjio Owned by:
Priority: normal Milestone: 8.6.2
Component: Compiler Version: 8.4.3
Keywords: core, optimizations, Wall Cc: bgamari
Operating System: Linux Architecture: x86
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:

Description (last modified by bgamari)

Given the below code snippet,

module Main where

main :: IO ()
main = putStrLn "hey"

data StopLight
  = Red
  | Green
  | Yellow
  deriving (Show, Eq)

data Intersection
  = Intersection
  { light :: {-# UNPACK #-} !StopLight
  } deriving (Show, Eq)

The above code will warn with optimizations enabled (ghc -Wall -O2 Main.hs), message below:

Main.hs:13:5: warning:
    • Ignoring unusable UNPACK pragma
        on the first argument of ‘Intersection’
    • In the definition of data constructor ‘Intersection’
      In the data type declaration for ‘Intersection’
   |
13 |   = Intersection

Without optimizations, no warnings are emitted ghc -Wall -O0 Main.hs.

The desired behavior should be a warning emitted in both cases.

Change History (2)

comment:1 Changed 9 months ago by dmjio

Operating System: Unknown/MultipleLinux

comment:2 Changed 9 months ago by bgamari

Description: modified (diff)

I believe this may be similar to #9370. In short, whether we represent field unpackedness (and strictness) in the AST is determined by whether we have compiled with -O. More concretely, we drop the unpack pragma in MkId.dataConSrcToImplBang in the case of -O0.

This is really an awful design and should be changed. As described in #9370, we should rather always include these sorts of pragmas in the AST and ignore them in the simplifier if optimisation is disabled.

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