HACKER TERRITORY. HACKER TERRITORY. (You were warned.)
Make a debugging dump after pass <pass> (may be common enough to need a short form…). You can get all of these at once (lots of output) by using -v5, or most of them with -v4. Some of the most useful ones are:
parser output
renamer output
typechecker output
Dump a type signature for each value defined at the top level of the module. The list is sorted alphabetically. Using -dppr-debug dumps a type signature for all the imported and system-defined things as well; useful for debugging the compiler.
derived instances
desugarer output
output of specialisation pass
dumps all rewrite rules (including those generated by the specialisation pass)
simplifer output (Core-to-Core passes)
inlining info from the simplifier
UsageSP inference pre-inf and output
CPR analyser output
strictness analyser output
CSE pass output
worker/wrapper split output
`occurrence analysis' output
output of “saturate” pass
output of STG-to-STG passes
unflattened Abstract C
flattened Abstract C
same as what goes to the C compiler
native-code generator intermediate form
assembly language from the native-code generator
byte code compiler output
dump foreign export stubs
Show the output of the intermediate Core-to-Core and STG-to-STG passes, respectively. (Lots of output!) So: when we're really desperate:
% ghc -noC -O -ddump-simpl -dverbose-simpl -dcore-lint Foo.hs |
Show the output of each iteration of the simplifier (each run of the simplifier has a maximum number of iterations, normally 4). Used when even -dverbose-simpl doesn't cut it.
Debugging output is in one of several “styles.” Take the printing of types, for example. In the “user” style (the default), the compiler's internal ideas about types are presented in Haskell source-level syntax, insofar as possible. In the “debug” style (which is the default for debugging output), the types are printed in with explicit foralls, and variables have their unique-id attached (so you can check for things that look the same but aren't). This flag makes debugging output appear in the more verbose debug style.
In error messages, expressions are printed to a certain “depth”, with subexpressions beyond the depth replaced by ellipses. This flag sets the depth.
Dump statistics about how many of each kind of transformation too place. If you add -dppr-debug you get more detailed information.
Make the renamer be *real* chatty about what it is upto.
Print out summary of what kind of information the renamer had to bring in.
Have the renamer report what imports does not contribute.
Turn on heavyweight intra-pass sanity-checking within GHC, at Core level. (It checks GHC's sanity, not yours.)
Ditto for STG level. (NOTE: currently doesn't work).
Turn on checks around UsageSP inference (-fusagesp). This verifies various simple properties of the results of the inference, and also warns if any identifier with a used-once annotation before the inference has a used-many annotation afterwards; this could indicate a non-worksafe transformation is being applied.
Let's do this by commenting an example. It's from doing -ddump-ds on this code:
skip2 m = m : skip2 (m+2) |
Remember, everything has a “Unique” and it is usually printed out when debugging, in some form or another. So here we go…
Desugared:
Main.skip2{-r1L6-} :: _forall_ a$_4 =>{{Num a$_4}} -> a$_4 -> [a$_4]
--# `r1L6' is the Unique for Main.skip2;
--# `_4' is the Unique for the type-variable (template) `a'
--# `{{Num a$_4}}' is a dictionary argument
_NI_
--# `_NI_' means "no (pragmatic) information" yet; it will later
--# evolve into the GHC_PRAGMA info that goes into interface files.
Main.skip2{-r1L6-} =
/\ _4 -> \ d.Num.t4Gt ->
let {
{- CoRec -}
+.t4Hg :: _4 -> _4 -> _4
_NI_
+.t4Hg = (+{-r3JH-} _4) d.Num.t4Gt
fromInt.t4GS :: Int{-2i-} -> _4
_NI_
fromInt.t4GS = (fromInt{-r3JX-} _4) d.Num.t4Gt
--# The `+' class method (Unique: r3JH) selects the addition code
--# from a `Num' dictionary (now an explicit lamba'd argument).
--# Because Core is 2nd-order lambda-calculus, type applications
--# and lambdas (/\) are explicit. So `+' is first applied to a
--# type (`_4'), then to a dictionary, yielding the actual addition
--# function that we will use subsequently...
--# We play the exact same game with the (non-standard) class method
--# `fromInt'. Unsurprisingly, the type `Int' is wired into the
--# compiler.
lit.t4Hb :: _4
_NI_
lit.t4Hb =
let {
ds.d4Qz :: Int{-2i-}
_NI_
ds.d4Qz = I#! 2#
} in fromInt.t4GS ds.d4Qz
--# `I# 2#' is just the literal Int `2'; it reflects the fact that
--# GHC defines `data Int = I# Int#', where Int# is the primitive
--# unboxed type. (see relevant info about unboxed types elsewhere...)
--# The `!' after `I#' indicates that this is a *saturated*
--# application of the `I#' data constructor (i.e., not partially
--# applied).
skip2.t3Ja :: _4 -> [_4]
_NI_
skip2.t3Ja =
\ m.r1H4 ->
let { ds.d4QQ :: [_4]
_NI_
ds.d4QQ =
let {
ds.d4QY :: _4
_NI_
ds.d4QY = +.t4Hg m.r1H4 lit.t4Hb
} in skip2.t3Ja ds.d4QY
} in
:! _4 m.r1H4 ds.d4QQ
{- end CoRec -}
} in skip2.t3Ja |
(“It's just a simple functional language” is an unregisterised trademark of Peyton Jones Enterprises, plc.)
The term "unregisterised" really means "compile via vanilla C", disabling some of the platform-specific tricks that GHC normally uses to make programs go faster. When compiling unregisterised, GHC simply generates a C file which is compiled via gcc.
Unregisterised compilation can be useful when porting GHC to a new machine, since it reduces the prerequisite tools to gcc, as, and ld and nothing more, and furthermore the amount of platform-specific code that needs to be written in order to get unregisterised compilation going is usually fairly small.
Compile via vanilla ANSI C only, turning off platform-specific optimisations. NOTE: in order to use -unreg, you need to have a set of libraries (including the RTS) built for unregisterised compilation. This amounts to building GHC with way "u" enabled.