Debugging the compilerdebugging options (for GHC)HACKER TERRITORY. HACKER TERRITORY. (You were warned.)Dumping out compiler intermediate structuresdumping GHC intermediatesintermediate passes, outputpass optionsMake 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
, or most of them with
. Some of the most useful ones
are:
:
parser output
:
renamer output
:
typechecker output
:
Dump Template Haskell expressions that we splice in,
and what Haskell code the expression evaluates to.
:
Dump a type signature for each value defined at
the top level of the module. The list is sorted
alphabetically. Using
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 specified in this module;
see .
:
dumps the names of all rules that fired in this module
:
dumps detailed information about all rules that fired in
this module
:
dumps the output of the vectoriser.
:
simplifier output (Core-to-Core passes)
:
inlining info from the simplifier
:
CPR analyser output
:
strictness analyser output
:
CSE pass output
:
worker/wrapper split output
:
`occurrence analysis' output
:
output of core preparation pass
:
output of STG-to-STG passes
:
flattened Abstract C
:
Print the C-- code out.
:
Dump the results of C-- to C-- optimising passes.
:
assembly language from the native-code generator
:
byte code compiler output
:
dump foreign export stubs
:
Show the output of each run of the simplifier. Used when even
doesn't cut it.
:
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 .Dump statistics about how many of each kind of
transformation too place. If you add
you get more detailed
information.Make the interface loader be *real* chatty about what it is
upto.Make the type checker be *real* chatty about what it is
upto.Make the vectoriser be *real* chatty about what it is
upto.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.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
Print out each pass name as it happens.Print a one-line summary of the size of the Core program
at the end of the optimisation pipeline.Show statistics for the usage of fast strings by the
compiler.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.Suppress the printing of uniques in debugging output. 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 Suppress the printing of coercions in Core dumps to make them
shorter.Suppress the printing of module qualification prefixes in Core dumps to make them easier to read.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.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.Checking for consistencyconsistency checkslintTurn 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).
:
Ditto for C-- level.How to read Core syntax (from some
flags)reading Core syntaxCore syntax, how to readLet's do this by commenting an example. It's from doing
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.)Unregisterised compilationunregisterised compilationThe 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.Unregisterised compilation cannot be selected at
compile-time; you have to build GHC with the appropriate options
set. Consult the GHC Building Guide for details.