December 2017 - ghc-tickets

[GHC] #14483: getExecutablePath from System.Environment in base should follow symlinks properly.
by GHC 25 Aug '18

25 Aug '18

#14483: getExecutablePath from System.Environment in base should follow symlinks properly. -------------------------------------+------------------------------------- Reporter: angerman | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- During the implementation of relocation capabilities in GHC for linux and macOS it came to light, that windows (which has had this support for quite some time), uses a custom implementation of `getExecutablePath`. Ideally this logic should be moved into `base`. Phyx- noted in D4121 that this should be possible for 8.6, as at that point Win32 should be new enough to be usable in base for this. The result would be that the custom code in GHC (SysTools) and the older copy in GhcPkg of the same code can be unified with the linux and macOS CPP conditional branch. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14483> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 9

[GHC] #14460: Can't decompose ghc.exe path
by GHC 25 Aug '18

25 Aug '18

#14460: Can't decompose ghc.exe path -------------------------------------+------------------------------------- Reporter: astert | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Windows Architecture: | Type of failure: GHC doesn't work Unknown/Multiple | at all Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I downloaded the haskell platform, but when I try to use ghci I get this message: ghc.exe: panic!(the 'impossible' happened) (GHC version 8.2.1 for x86_64-unknown-mingw32): can't decompose ghc.exe path: "H:\\Programs\\Haskell\\8.2.1\\bin\\ghc.exe" -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14460> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 25

[GHC] #14064: Compiling problem on FreeBSD 11 ("failed in phase")
by GHC 25 Aug '18

25 Aug '18

#14064: Compiling problem on FreeBSD 11 ("failed in phase") -------------------------------------+------------------------------------- Reporter: ohho | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: FreeBSD Architecture: x86_64 | Type of failure: GHC doesn't work (amd64) | at all Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The scenario showed as below: {{{ # This is a FreeBSD Jail for testing $ freebsd-version 11.1-RELEASE $ pkg info # (related packages only) binutils-2.28,1 GNU binary tools gcc-ecj-4.5 Eclipse Java Compiler used to build GCC Java gcc5-5.4.0_2 GNU Compiler Collection 5 gmake-4.2.1_1 GNU version of 'make' utility gmp-6.1.2 Free library for arbitrary precision arithmetic libiconv-1.14_10 Character set conversion library libunistring-0.9.7 Unicode string library mpc-1.0.3 Library of complex numbers with arbitrarily high precision mpfr-3.1.5_1 Library for multiple-precision floating- point computations perl5-5.24.1_1 Practical Extraction and Report Language $ ls -l /usr/local/lib/lib{iconv,charset,gmp}.* -rw-r--r-- 1 root wheel 6864 Jul 13 09:10 /usr/local/lib/libcharset.a lrwxr-xr-x 1 root wheel 19 Jul 13 09:10 /usr/local/lib/libcharset.so -> libcharset.so.1.0.0 lrwxr-xr-x 1 root wheel 19 Jul 13 09:10 /usr/local/lib/libcharset.so.1 -> libcharset.so.1.0.0 -rw-r--r-- 1 root wheel 10733 Jul 13 09:10 /usr/local/lib/libcharset.so.1.0.0 -rw-r--r-- 1 root wheel 1252196 Jul 13 09:23 /usr/local/lib/libgmp.a lrwxr-xr-x 1 root wheel 16 Jul 13 09:23 /usr/local/lib/libgmp.so -> libgmp.so.10.3.2 lrwxr-xr-x 1 root wheel 16 Jul 13 09:23 /usr/local/lib/libgmp.so.10 -> libgmp.so.10.3.2 -rwxr-xr-x 1 root wheel 531728 Jul 13 09:23 /usr/local/lib/libgmp.so.10.3.2 -rw-r--r-- 1 root wheel 1110392 Jul 13 09:10 /usr/local/lib/libiconv.a lrwxr-xr-x 1 root wheel 17 Jul 13 09:10 /usr/local/lib/libiconv.so -> libiconv.so.2.5.1 lrwxr-xr-x 1 root wheel 17 Jul 13 09:10 /usr/local/lib/libiconv.so.2 -> libiconv.so.2.5.1 -rw-r--r-- 1 root wheel 1076449 Jul 13 09:10 /usr/local/lib/libiconv.so.2.5.1 # installed using # env CC=/usr/local/bin/gcc5 LDFLAGS='-Wl,-rpath=/usr/local/lib/gcc5' ./configure --prefix=/home/ghc $ /home/ghc/bin/ghc --numeric-version 8.2.1 $ cat /home/ghc/lib/ghc-8.2.1/settings [("GCC extra via C opts", " -fwrapv -fno-builtin"), ("C compiler command", "/usr/local/bin/gcc5"), ("C compiler flags", " -fno-stack-protector"), ("C compiler link flags", " -fuse-ld=gold"), ("C compiler supports -no-pie", "NO"), ("Haskell CPP command","/usr/local/bin/gcc5"), ("Haskell CPP flags","-E -undef -traditional"), ("ld command", "ld.gold"), ("ld flags", ""), ("ld supports compact unwind", "YES"), ("ld supports build-id", "YES"), ("ld supports filelist", "NO"), ("ld is GNU ld", "YES"), ("ar command", "/usr/local/bin/ar"), ("ar flags", "q"), ("ar supports at file", "YES"), ("touch command", "touch"), ("dllwrap command", "/bin/false"), ("windres command", "/bin/false"), ("libtool command", "libtool"), ("perl command", "/usr/local/bin/perl"), ("cross compiling", "NO"), ("target os", "OSFreeBSD"), ("target arch", "ArchX86_64"), ("target word size", "8"), ("target has GNU nonexec stack", "True"), ("target has .ident directive", "True"), ("target has subsections via symbols", "False"), ("target has RTS linker", "@HaskellHaveRTSLinker@"), ("Unregisterised", "NO"), ("LLVM llc command", "llc"), ("LLVM opt command", "opt") ] $ /home/ghc/bin/ghci GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Prelude> (!!6) $ scanl (*) 1 [1..] 720 $ cat Main.hs module Main (main) where main :: IO () main = putStrLn "hello world" $ /home/ghc/bin/ghc -O0 --make Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... In file included from /home/ghc/lib/ghc-8.2.1/include/rts/Threads.h:19:0: error: 0, from /home/ghc/lib/ghc-8.2.1/include/Rts.h:173, from /tmp/ghc28144_0/ghc_4.c:1: /usr/local/lib/gcc5/gcc/x86_64-portbld-freebsd11.0/5.4.0/include- fixed/sys/types.h:266:9: error: error: unknown type name '__vm_ooffset_t' typedef __vm_ooffset_t vm_ooffset_t; ^ | 266 | typedef __vm_ooffset_t vm_ooffset_t; | ^ /usr/local/lib/gcc5/gcc/x86_64-portbld-freebsd11.0/5.4.0/include- fixed/sys/types.h:268:9: error: error: unknown type name '__vm_pindex_t' typedef __vm_pindex_t vm_pindex_t; ^ | 268 | typedef __vm_pindex_t vm_pindex_t; | ^ `gcc5' failed in phase `C Compiler'. (Exit code: 1) }}} On my FreeBSD 10.3 box: {{{ $ freebsd-version 10.3-STABLE $ pkg info binutils-2.28,1 GNU binary tools gcc-5.4.0_2 Meta-port for the default version of the GNU Compiler Collection gcc-ecj-4.5 Eclipse Java Compiler used to build GCC Java gcc5-5.4.0_3 GNU Compiler Collection 5 gmake-4.2.1_1 GNU version of 'make' utility gmp-6.1.2 Free library for arbitrary precision arithmetic libffi-3.2.1 Foreign Function Interface libiconv-1.14_10 Character set conversion library perl5-5.24.1_1 Practical Extraction and Report Language $ cat ./ghc-8.2.1-bin/lib/ghc-8.2.1/settings [("GCC extra via C opts", " -fwrapv -fno-builtin"), ("C compiler command", "gcc"), ("C compiler flags", " -fno-stack-protector"), ("C compiler link flags", " -fuse-ld=gold"), ("C compiler supports -no-pie", "NO"), ("Haskell CPP command","gcc"), ("Haskell CPP flags","-E -undef -traditional"), ("ld command", "ld.gold"), ("ld flags", ""), ("ld supports compact unwind", "YES"), ("ld supports build-id", "YES"), ("ld supports filelist", "NO"), ("ld is GNU ld", "YES"), ("ar command", "/usr/bin/ar"), ("ar flags", "clqs"), ("ar supports at file", "NO"), ("touch command", "touch"), ("dllwrap command", "/bin/false"), ("windres command", "/bin/false"), ("libtool command", "libtool"), ("perl command", "/usr/local/bin/perl"), ("cross compiling", "NO"), ("target os", "OSFreeBSD"), ("target arch", "ArchX86_64"), ("target word size", "8"), ("target has GNU nonexec stack", "True"), ("target has .ident directive", "True"), ("target has subsections via symbols", "False"), ("target has RTS linker", "@HaskellHaveRTSLinker@"), ("Unregisterised", "NO"), ("LLVM llc command", "llc"), ("LLVM opt command", "opt") ] $ ./ghc-8.2.1-bin/bin/ghc -O0 --make Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... $ ./Main hello, world! }}} I also noticed that using FreeBSD's strip (from elftoolchain), the {{{ Installing library in /home/ghc/lib/ghc-8.2.1/ghc-prim-0.5.1.0 "utils/ghc-cabal/dist-install/build/tmp/ghc-cabal-bindist" copy libraries /integer-gmp dist-install "strip" '' '/home/ghc' '/home/ghc/lib/ghc-8.2.1' '/home/ghc/share/doc/ghc- 8.2.1/html/libraries' 'v p dyn' }}} phase will be a bit longer than GNU's strip. (Tested by put my $LOCALBASE/bin ahead in $PATH). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14064> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 14

[GHC] #14012: External interpreter fails on FreeBSD
by GHC 25 Aug '18

25 Aug '18

#14012: External interpreter fails on FreeBSD ----------------------------------------+--------------------------------- Reporter: bgamari | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.1 Keywords: | Operating System: FreeBSD Architecture: Unknown/Multiple | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+--------------------------------- {{{ $ inplace/bin/ghc-stage2 -fexternal-interpreter --interactive GHCi, version 8.3.20170722: http://www.haskell.org/ghc/ :? for help {handle: <file descriptor: 12>}: GHCi.Message.remoteCall: end of file }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14012> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 5

[GHC] #11470: Support changing cross compiler target at runtime
by GHC 24 Aug '18

24 Aug '18

#11470: Support changing cross compiler target at runtime -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Relevant mailing list thread: https://mail.haskell.org/pipermail/ghc- devs/2016-January/011064.html At the moment, GHC's cross-compiling support requires you to build a cross-compiler for every target platform from scratch. (https://ghc.haskell.org/trac/ghc/wiki/CrossCompilation) There are two reasons for this: 1. GHC uses a configure script to interrogate the GCC toolchain about the vagaries of cross-compilation, some parameters of which get baked into various build system scripts and header files and such. Some of this information wends its way into the final built GHC executable. 2. GHC needs to build the boot libraries, using the stage 1 compiler, targeting the cross compiler. It would be very useful to move this configuration from configure time to run time, so that a user can use GHC as a cross-compiler without having to recompile GHC. That is, we would like to (1) ensure that any information from the configure script can alternately be supplemented at runtime, (2) that users can cross-compile the boot libraries on-demand (since GHC won't ship with the ARM versions of the boot libraries), and (3) rearchitect GHC internally so that it handles knows to keep separate the interface files/package database for various cross compilation targets. Such a change will make cross-compiling more convenient (the iOS cross- compiler currently requires GHC to be built twice, once for ARM and once for the simulator!) and will also pave the way for supporting Template Haskell and compiler plugins in the cross-compiler, since a GHC that knows how to deal with both the target and host platforms can simply ensure that it only loads code built for the host platform (a user can then, if necessary, build code twice, once for the host for Template Haskell, and once for the target). Unclear points of design: * How should GHC get the information about the GCC cross-compiler toolchain (which currently is gotten by `configure`?) Preferably, not by running a `configure` script every time you invoke it. What is this information anyway? * How should GHC rebuild the boot libraries? Maybe `cabal-install` can simply handle this for you. Note: please update the top of this ticket as the design becomes clearer. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11470> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #10346: Cross-module SpecConstr
by GHC 24 Aug '18

24 Aug '18

#10346: Cross-module SpecConstr -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Type-class specialisation now happens flawlessly across modules. That is, if I define {{{ module DefineF where f :: Num a => a -> a {-# INLINEABLE f #-} f x = ...f x'.... }}} then modules that import `DefineF` and call `f` at some particular type (say `Int`) will generate a specialised copy of `f`'s code. But this does not happen for `SpecConstr`; we only specialise a function for calls made in the same module. All the infrastructure is in place to allow cross-module `SpecConstr`; it just hasn't been done yet. This ticket is to record the idea. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10346> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 19

[GHC] #12695: Build failure due to MAP_NORESERVE being removed in FreeBSD 11.x and later
by GHC 23 Aug '18

23 Aug '18

#12695: Build failure due to MAP_NORESERVE being removed in FreeBSD 11.x and later -------------------------------------+------------------------------------- Reporter: abbe | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: FreeBSD Architecture: | Type of failure: Building GHC Unknown/Multiple | failed Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- While trying to build GHC 8.0.1 on FreeBSD 11.x, I come across following compiler error: {{{ /usr/local/bin/clang-cpp38 -DPROFILING -DTHREADED_RTS -DDEBUG -fno- stack-protector -O2 -pipe -Qunused-arguments -I/usr/local/include -fstack-protector -fno-strict-aliasing -Wall -Wall -Wextra -Wstrict- prototypes -Wmissing-prototypes -Wmissing-declarations -Winline -Waggregate-return -Wpointer-arith -Wmissing-noreturn -Wnested-externs -Wredundant-decls -Iincludes -Iincludes/dist -Iincludes/dist- derivedconstants/header -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build -DCOMPILING_RTS -fno-strict-aliasing -fno-common -Irts/dist/build/autogen -Wno-unknown-pragmas -O2 -fomit-frame- pointer -g -DRtsWay=\"rts_v\" -MM -x c rts/posix/OSMem.c -MF rts/dist/build/.depend-v-p-dyn-l-debug-thr-thr_debug-thr_l-thr_p- debug_dyn-thr_dyn-thr_debug_dyn-l_dyn-thr_l_dyn.c_asm.bit rts/posix/OSMem.c:158:5: error: USE_LARGE_ADDRESS_SPACE needs MAP_NORESERVE # error USE_LARGE_ADDRESS_SPACE needs MAP_NORESERVE ^ 1 error generated. }}} From some [https://svnweb.freebsd.org/base/head/sys/sys/mman.h?r1=273249&r2=273250& investigation], it turns out when it was there, it was not even implemented. The value of `__FreeBSD_version` (in case you plan to add conditional based on it), where it is [https://svnweb.freebsd.org/base/head/sys/sys/param.h?r1=273250&r2=273249&pa… changed] is `1100039`. Let me know if you need more information. Thanks for looking -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12695> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #11474: incorrect redundant-constraints warning
by GHC 23 Aug '18

23 Aug '18

#11474: incorrect redundant-constraints warning -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code when compiled with GHC 8 {{{#!hs {-# LANGUAGE Haskell2010, FunctionalDependencies, KindSignatures, ScopedTypeVariables, TypeOperators, UndecidableInstances #-} import GHC.Generics data Options data Value newtype Tagged s b = Tagged {unTagged :: b} class GToJSON f where gToJSON :: Options -> f a -> Value class SumToJSON f allNullary where sumToJSON :: Options -> f a -> Tagged allNullary Value class AllNullary (f :: * -> *) allNullary | f -> allNullary instance ( AllNullary (a :+: b) allNullary, -- <- removing this line causes a compile error SumToJSON (a :+: b) allNullary ) => GToJSON (a :+: b) where gToJSON opts = (unTagged :: Tagged allNullary Value -> Value) . sumToJSON opts }}} emits a warning {{{ bug.hs:19:10: warning: • Redundant constraint: AllNullary (a :+: b) allNullary • In the instance declaration for ‘GToJSON (a :+: b)’ }}} when commenting out the `AllNullary` constraint, this however results the compile error {{{ bug.hs:19:10: error: • Could not deduce (SumToJSON (a :+: b) allNullary0) from the context: SumToJSON (a :+: b) allNullary bound by an instance declaration: SumToJSON (a :+: b) allNullary => GToJSON (a :+: b) at redconstr.hs:(19,10)-(22,24) The type variable ‘allNullary0’ is ambiguous • In the ambiguity check for an instance declaration To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the instance declaration for ‘GToJSON (a :+: b)’ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11474> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 12

[GHC] #9688: Improve the interaction between CSE and the join point transformation
by GHC 23 Aug '18

23 Aug '18

#9688: Improve the interaction between CSE and the join point transformation -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: CSE | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Runtime Blocked By: | performance bug Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- It appears that the join point transformation sometimes interferes with CSE when CSE would be much better. Two examples: === digitToIntMaybe === Suppose we define {{{#!hs isHexDigit :: Char -> Bool isHexDigit c = (fromIntegral (ord c - ord '0')::Word) <= 9 || (fromIntegral (ord c - ord 'a')::Word) <= 5 || (fromIntegral (ord c - ord 'A')::Word) <= 5 digitToInt c | (fromIntegral dec::Word) <= 9 = dec | (fromIntegral hexl::Word) <= 5 = hexl + 10 | (fromIntegral hexu::Word) <= 5 = hexu + 10 | otherwise = error ("Char.digitToInt: not a digit " ++ show c) -- sigh where dec = ord c - ord '0' hexl = ord c - ord 'a' hexu = ord c - ord 'A' -- We could also expand this out in cases manually, but it makes no -- difference as far as I can tell. }}} Suppose we then write a naive `digitToIntMaybe` function: {{{#!hs digitToIntMaybe c | isHexDigit c = Just (digitToInt c) | otherwise = Nothing }}} What I would want this to do is "zip" the nested cases and give Core like this: {{{ $wdigitToIntMaybe $wdigitToIntMaybe = \ ww_s2Ag -> let { x#_a2yy x#_a2yy = -# (ord# ww_s2Ag) 48 } in case tagToEnum# (leWord# (int2Word# x#_a2yy) (__word 9)) of _ { False -> let { x#1_X2z7 x#1_X2z7 = -# (ord# ww_s2Ag) 97 } in case tagToEnum# (leWord# (int2Word# x#1_X2z7) (__word 5)) of _ { False -> let { x#2_X2zh x#2_X2zh = -# (ord# ww_s2Ag) 65 } in case tagToEnum# (leWord# (int2Word# x#2_X2zh) (__word 5)) of _ { False -> Nothing; True -> Just (I# (+# x#2_X2zh 10)) }; True -> Just (I# (+# x#1_X2z7 10)) }; True -> Just (I# x#_a2yy) } digitToIntMaybe digitToIntMaybe = \ w_s2Ad -> case w_s2Ad of _ { C# ww1_s2Ag -> $wdigitToIntMaybe ww1_s2Ag } }}} But instead, the join point transformation triggers, and we get this: {{{ digitToIntMaybe1 digitToIntMaybe1 = \ ww_s2Cp -> error (unpackAppendCString# "Char.digitToInt: not a digit "# ($w$cshowsPrec15 ww_s2Cp ([]))) $wdigitToIntMaybe $wdigitToIntMaybe = \ ww_s2Cp -> let { $j_s2Bc $j_s2Bc = \ _ -> Just (let { a_s2B5 a_s2B5 = int2Word# (-# (ord# ww_s2Cp) 48) } in case tagToEnum# (leWord# a_s2B5 (__word 9)) of _ { False -> let { a1_s2B7 a1_s2B7 = int2Word# (-# (ord# ww_s2Cp) 97) } in case tagToEnum# (leWord# a1_s2B7 (__word 5)) of _ { False -> let { a2_s2B9 a2_s2B9 = int2Word# (-# (ord# ww_s2Cp) 65) } in case tagToEnum# (leWord# a2_s2B9 (__word 5)) of _ { False -> digitToIntMaybe1 ww_s2Cp; True -> I# (+# (word2Int# a2_s2B9) 10) }; True -> I# (+# (word2Int# a1_s2B7) 10) }; True -> I# (word2Int# a_s2B5) }) } in case tagToEnum# (leWord# (int2Word# (-# (ord# ww_s2Cp) 48)) (__word 9)) of _ { False -> case tagToEnum# (leWord# (int2Word# (-# (ord# ww_s2Cp) 97)) (__word 5)) of _ { False -> case tagToEnum# (leWord# (int2Word# (-# (ord# ww_s2Cp) 65)) (__word 5)) of _ { False -> Nothing; True -> $j_s2Bc void# }; True -> $j_s2Bc void# }; True -> $j_s2Bc void# } digitToIntMaybe digitToIntMaybe = \ w_s2Cm -> case w_s2Cm of _ { C# ww1_s2Cp -> $wdigitToIntMaybe ww1_s2Cp } }}} We perform the same three tests twice each, and test for an error condition that obviously can't happen. === `quotRem` and `divMod` === If we define {{{#!hs x `quot` y = fst (x `quotRem` y) x `rem` y = snd (x `quotRem` y) }}} and then write something like {{{#!hs f x y | x `rem` y == 0 = x `quot` y | otherwise = 17 }}} then CSE works some magic and we only calculate `quotRem x y` once. Unfortunately, if we do this: {{{#!hs whatever x y = if x `myRem` y == 0 then (x `myQuot` y) + 14 else x `myQuot` y }}} then the join point transformation fires, collecting the `myQuot x y` expressions in the case branches and preventing CSE from recognizing the much better opportunity to eliminate those calculations altogether. The situation with `divMod` is much worse. The join point transformation applied to the cases defining `divMod` prevents CSE from working magic on it in even simple situations, unless one of the arguments is known, making this definition unusable (the resulting Core is too horrifyingly long to paste here). It would probably be possible to improve the `divMod` situation to something close to the `quotRem` one by making `divMod` `NOINLINE` and adding special `divModLit` rules, but I'd much rather see a general solution. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9688> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 4

[GHC] #10100: Bogus "redundant constraint" warning with functional dependencies
by GHC 23 Aug '18

23 Aug '18

#10100: Bogus "redundant constraint" warning with functional dependencies -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Compiling this code with GHC HEAD: {{{#!hs {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} module T10100 where data Zero data Succ a class Add a b ab | a b -> ab, a ab -> b instance Add Zero b b instance (Add a b ab) => Add (Succ a) b (Succ ab) }}} Gives a bogus warning about redundant constraint: {{{ Redundant constraint: Add a b ab In the instance declaration for ‘Add (Succ a) b (Succ ab)’ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10100> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6