Opened 18 years ago

Closed 5 years ago

Last modified 13 months ago

#17 closed feature request (fixed)

Separate warnings for unused local and top-level bindings

Reported by: magunter Owned by:
Priority: lowest Milestone: 8.0.1
Component: Compiler Version: None
Keywords: -fwarn-unused-binds newcomer Cc: mboes@…
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Test Case:
Blocked By: Blocking:
Related Tickets: #3283 Differential Rev(s): Phab:D591
Wiki Page:

Description (last modified by igloo)

I'd like separate warnings for local and top-level
unused binds.  I often have "unused" top-level
functions which I use from ghci.  I'd like to avoid
warnings for
these while retaining the warnings for unused local
bindings (which can always be eliminated by prefix the
name with an underscore.)

Change History (20)

comment:1 Changed 18 years ago by magunter

Summary: RFE:Separate unused-binds local/top-levlRFE:Separate unused-binds local/top-level

comment:2 Changed 17 years ago by simonmar

Logged In: YES 

Moved to feature requests

comment:3 Changed 13 years ago by igloo

Architecture: Unknown
Description: modified (diff)
difficulty: Unknown
Milestone: _|_
Operating System: Unknown

comment:4 Changed 13 years ago by SamB

Keywords: -fwarn-unused-binds added

comment:5 Changed 11 years ago by simonmar

Architecture: UnknownUnknown/Multiple

comment:6 Changed 11 years ago by simonmar

Operating System: UnknownUnknown/Multiple

comment:7 Changed 10 years ago by igloo

Owner: changed from nobody to igloo
Status: assignednew

comment:8 Changed 10 years ago by igloo

Owner: igloo deleted

comment:9 Changed 10 years ago by simonmar

Component: NoneCompiler

comment:10 Changed 10 years ago by byorgey

Summary: RFE:Separate unused-binds local/top-levelSeparate warnings for unused local and top-level bindings
Type of failure: None/Unknown

Would any developers care to guess at the difficulty of this feature? Could this make a good project for someone just starting to get their feet wet hacking on ghc?

comment:11 Changed 10 years ago by mboes

Cc: mboes@… added

I doubt the usefulness of this feature since any top-level function that is mentioned in the export list of the module won't trigger a warning. It is confusing to see dead code in a module and so top-level functions that aren't used should probably just be commented out.

However, this feature does come handy when using records: it often happens that a field name isn't used in a module, hence triggering a warning.

comment:12 in reply to:  10 Changed 10 years ago by igloo

Replying to byorgey:

Would any developers care to guess at the difficulty of this feature? Could this make a good project for someone just starting to get their feet wet hacking on ghc?

I'd say so. It would probably be worth discussing the design on the cvs-ghc list first, though.

comment:13 Changed 5 years ago by errge

Note that you can simply prefix those "used from GHCi only top-level bindings" with an underscore as a workaround until this gets implemented.

comment:14 Changed 5 years ago by nomeata

comment:15 Changed 5 years ago by nomeata

Keywords: newcomer added

comment:16 Changed 5 years ago by thomie

Differential Rev(s): Phab:D591
Milestone: 7.12.1

comment:17 Changed 5 years ago by Austin Seipp <austin@…>

In aead01902e1c41e85b758dbafd15e60d08956374/ghc:

driver: split -fwarn-unused-binds into 3 flags (fixes #17)

Summary: New flags:


Test Plan: `tests/rename/should_compile/T17` tests

Correct other tests

Reviewers: austin, rwbarton

Reviewed By: austin, rwbarton

Subscribers: rwbarton, carter, thomie

Differential Revision:

GHC Trac Issues: #17

comment:18 Changed 5 years ago by thoughtpolice

Resolution: Nonefixed
Status: newclosed

Merged, thanks!

comment:19 Changed 4 years ago by thoughtpolice


Milestone renamed

comment:20 Changed 13 months ago by Ömer Sinan Ağacan <omeragacan@…>

In c6fbac6/ghc:

Fix a race between GC threads in concurrent scavenging

While debugging #15285 I realized that free block lists (free_list in
BlockAlloc.c) get corrupted when multiple scavenge threads allocate and
release blocks concurrently. Here's a picture of one such race:

    Thread 2 (Thread 32573.32601):
    #0  check_tail
        (bd=0x940d40 <stg_TSO_info>) at rts/sm/BlockAlloc.c:860
    #1  0x0000000000928ef7 in checkFreeListSanity
        () at rts/sm/BlockAlloc.c:896
    #2  0x0000000000928979 in freeGroup
        (p=0x7e998ce02880) at rts/sm/BlockAlloc.c:721
    #3  0x0000000000928a17 in freeChain
        (bd=0x7e998ce02880) at rts/sm/BlockAlloc.c:738
    #4  0x0000000000926911 in freeChain_sync
        (bd=0x7e998ce02880) at rts/sm/GCUtils.c:80
    #5  0x0000000000934720 in scavenge_capability_mut_lists
        (cap=0x1acae80) at rts/sm/Scav.c:1665
    #6  0x000000000092b411 in gcWorkerThread
        (cap=0x1acae80) at rts/sm/GC.c:1157
    #7  0x000000000090be9a in yieldCapability
        (pCap=0x7f9994e69e20, task=0x7e9984000b70, gcAllowed=true) at rts/Capability.c:861
    #8  0x0000000000906120 in scheduleYield
        (pcap=0x7f9994e69e50, task=0x7e9984000b70) at rts/Schedule.c:673
    #9  0x0000000000905500 in schedule
        (initialCapability=0x1acae80, task=0x7e9984000b70) at rts/Schedule.c:293
    #10 0x0000000000908d4f in scheduleWorker
        (cap=0x1acae80, task=0x7e9984000b70) at rts/Schedule.c:2554
    #11 0x000000000091a30a in workerStart
        (task=0x7e9984000b70) at rts/Task.c:444
    #12 0x00007f99937fa6db in start_thread
        (arg=0x7f9994e6a700) at pthread_create.c:463
    #13 0x000061654d59f88f in clone
        () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

    Thread 1 (Thread 32573.32573):
    #0  checkFreeListSanity
        () at rts/sm/BlockAlloc.c:887
    #1  0x0000000000928979 in freeGroup
        (p=0x7e998d303540) at rts/sm/BlockAlloc.c:721
    #2  0x0000000000926f23 in todo_block_full
        (size=513, ws=0x1aa8ce0) at rts/sm/GCUtils.c:264
    #3  0x00000000009583b9 in alloc_for_copy
        (size=513, gen_no=0) at rts/sm/Evac.c:80
    #4  0x000000000095850d in copy_tag_nolock
        (p=0x7e998c675f28, info=0x421d98 <Main_Large_con_info>, src=0x7e998d075d80, size=513,
        gen_no=0, tag=1) at rts/sm/Evac.c:153
    #5  0x0000000000959177 in evacuate
        (p=0x7e998c675f28) at rts/sm/Evac.c:715
    #6  0x0000000000932388 in scavenge_small_bitmap
        (p=0x7e998c675f28, size=1, bitmap=0) at rts/sm/Scav.c:271
    #7  0x0000000000934aaf in scavenge_stack
        (p=0x7e998c675f28, stack_end=0x7e998c676000) at rts/sm/Scav.c:1908
    #8  0x0000000000934295 in scavenge_one
        (p=0x7e998c66e000) at rts/sm/Scav.c:1466
    #9  0x0000000000934662 in scavenge_mutable_list
        (bd=0x7e998d300440, gen=0x1b1d880) at rts/sm/Scav.c:1643
    #10 0x0000000000934700 in scavenge_capability_mut_lists
        (cap=0x1aaa340) at rts/sm/Scav.c:1664
    #11 0x00000000009299b6 in GarbageCollect
        (collect_gen=0, do_heap_census=false, gc_type=2, cap=0x1aaa340, idle_cap=0x1b38aa0)
        at rts/sm/GC.c:378
    #12 0x0000000000907a4a in scheduleDoGC
        (pcap=0x7ffdec5b5310, task=0x1b36650, force_major=false) at rts/Schedule.c:1798
    #13 0x0000000000905de7 in schedule
        (initialCapability=0x1aaa340, task=0x1b36650) at rts/Schedule.c:546
    #14 0x0000000000908bc4 in scheduleWaitThread
        (tso=0x7e998c0067c8, ret=0x0, pcap=0x7ffdec5b5430) at rts/Schedule.c:2537
    #15 0x000000000091b5a0 in rts_evalLazyIO
        (cap=0x7ffdec5b5430, p=0x9c11f0, ret=0x0) at rts/RtsAPI.c:530
    #16 0x000000000091ca56 in hs_main
        (argc=1, argv=0x7ffdec5b5628, main_closure=0x9c11f0, rts_config=...) at rts/RtsMain.c:72
    #17 0x0000000000421ea0 in main

In particular, dbl_link_onto() which is used to add a freed block to a
doubly-linked free list is not thread safe and corrupts the list when
called concurrently.

Note that thread 1 is to blame here as thread 2 is properly taking the
spinlock. With this patch we now take the spinlock when freeing a todo
block in GC, avoiding this race.

Test Plan:
- Tried slow validate locally: this patch does not introduce new failures.
- circleci: The test got killed
  because it took 5 hours but T7919 (which was previously failing on circleci)

Reviewers: simonmar, bgamari, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

GHC Trac Issues: #15285

Differential Revision:
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