HACKER TERRITORY. HACKER TERRITORY. (You were warned.)
-ddump-pass
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. You can prevent them from clogging up
your standard output by passing -ddump-to-file.
Some of the most useful ones are:
-ddump-parsed:
parser output
-ddump-rn:
renamer output
-ddump-tc:
typechecker output
-ddump-splices:
Dump Template Haskell expressions that we splice in, and what Haskell code the expression evaluates to.
-ddump-types:
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.
-ddump-deriv:
derived instances
-ddump-ds:
desugarer output
-ddump-spec:
output of specialisation pass
-ddump-rules:
dumps all rewrite rules specified in this module; see Section 7.20.6, “Controlling what's going on in rewrite rules”.
-ddump-rule-firings:
dumps the names of all rules that fired in this module
-ddump-rule-rewrites:
dumps detailed information about all rules that fired in this module
-ddump-vect:
dumps the output of the vectoriser.
-ddump-simpl:
simplifier output (Core-to-Core passes)
-ddump-inlinings:
inlining info from the simplifier
-ddump-cpranal:
CPR analyser output
-ddump-stranal:
strictness analyser output
-ddump-strsigs:
strictness signatures
-ddump-cse:
CSE pass output
-ddump-worker-wrapper:
worker/wrapper split output
-ddump-occur-anal:
`occurrence analysis' output
-ddump-prep:
output of core preparation pass
-ddump-stg:
output of STG-to-STG passes
-ddump-flatC:
flattened Abstract C
-ddump-cmm:
Print the C-- code out.
-ddump-opt-cmm:
Dump the results of C-- to C-- optimising passes.
-ddump-asm:
assembly language from the native code generator
-ddump-llvm:
LLVM code from the LLVM code generator
-ddump-bcos:
byte code compiler output
-ddump-foreign:
dump foreign export stubs
-ddump-simpl-phases:
Show the output of each run of the simplifier. Used when even
-dverbose-core2core doesn't cut it.
-ddump-simpl-iterations:
Show the output of each iteration
of the simplifier (each run of the simplifier has a maximum
number of iterations, normally 4). This outputs even more information
than -ddump-simpl-phases.
-ddump-simpl-stats
Dump statistics about how many of each kind of
transformation too place. If you add
-dppr-debug you get more detailed
information.
-ddump-if-trace
Make the interface loader be *real* chatty about what it is up to.
-ddump-tc-trace
Make the type checker be *real* chatty about what it is up to.
-ddump-vt-trace
Make the vectoriser be *real* chatty about what it is up to.
-ddump-rn-trace
Make the renamer be *real* chatty about what it is up to.
-ddump-rn-stats
Print out summary of what kind of information the renamer had to bring in.
-dverbose-core2core
,
-dverbose-stg2stg
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-core2core -dcore-lint Foo.hs
-dshow-passes
Print out each pass name as it happens.
-ddump-core-stats
Print a one-line summary of the size of the Core program at the end of the optimisation pipeline.
-dfaststring-stats
Show statistics for the usage of fast strings by the compiler.
-dppr-debug
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.
-dppr-user-length
In error messages, expressions are printed to a certain “depth”, with subexpressions beyond the depth replaced by ellipses. This flag sets the depth. Its default value is 5.
-dppr-colsNNN
Set the width of debugging output. Use this if your code is wrapping too much.
For example: -dppr-cols200.
-dppr-case-as-let
Print single alternative case expressions as though they were strict let expressions. This is helpful when your code does a lot of unboxing.
-dno-debug-output
Suppress any unsolicited debugging output. When GHC
has been built with the DEBUG option it
occasionally emits debug output of interest to developers.
The extra output can confuse the testing framework and
cause bogus test failures, so this flag is provided to
turn it off.
-dsuppress-all
Suppress everything that can be suppressed, except for unique ids as this often makes the printout ambiguous. If you just want to see the overall structure of the code, then start here.
-dsuppress-uniques
Suppress the printing of uniques. This may make
the printout ambiguous (e.g. unclear where an occurrence of 'x' is bound), but
it makes the output of two compiler runs have many fewer gratuitous differences,
so you can realistically apply diff. Once diff
has shown you where to look, you can try again without -dsuppress-uniques
-dsuppress-idinfo
Suppress extended information about identifiers where they are bound. This includes strictness information and inliner templates. Using this flag can cut the size of the core dump in half, due to the lack of inliner templates
-dsuppress-module-prefixes
Suppress the printing of module qualification prefixes.
This is the Data.List in Data.List.length.
-dsuppress-type-signatures
Suppress the printing of type signatures.
-dsuppress-type-applications
Suppress the printing of type applications.
-dsuppress-coercions
Suppress the printing of type coercions.
Let's do this by commenting an example. It's from doing
-ddump-ds on this code:
skip2 m = m : skip2 (m+2)
Before we jump in, a word about names of things. Within GHC,
variables, type constructors, etc., are identified by their
“Uniques.” These are of the form `letter' plus
`number' (both loosely interpreted). The `letter' gives some idea
of where the Unique came from; e.g., _
means “built-in type variable”; t
means “from the typechecker”; s
means “from the simplifier”; and so on. The `number'
is printed fairly compactly in a `base-62' format, which everyone
hates except me (WDP).
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 lambda'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.)