Safe Haskell | None |
---|---|
Language | GHC2021 |
This module exports the Backend
type and all the available values
of that type. The type is abstract, and GHC assumes a "closed world":
all the back ends are known and are known here. The compiler driver
chooses a Backend
value based on how it is asked to generate code.
A Backend
value encapsulates the knowledge needed to take Cmm, STG,
or Core and write assembly language to a file. A back end also
provides a function that enables the compiler driver to run an
assembler on the code that is written, if any (the "post-backend
pipeline"). Finally, a back end has myriad properties. Properties
mediate interactions between a back end and the rest of the compiler,
especially the driver. Examples include the following:
- Property
backendValidityOfCImport
says whether the back end can import foreign C functions. - Property
backendForcesOptimization0
says whether the back end can be used with optimization levels higher than `-O0`. - Property
backendCDefs
tells the compiler driver, "if you're using this back end, then these are the command-line flags you should add to any invocation of the C compiler."
These properties are used elsewhere in GHC, primarily in the driver, to fine-tune operations according to the capabilities of the chosen back end. You might use a property to make GHC aware of a potential limitation of certain back ends, or a special feature available only in certain back ends. If your client code needs to know a fact that is not exposed in an existing property, you would define and export a new property. Conditioning client code on the identity or name of a back end is Not Done.
For full details, see the documentation of each property.
Synopsis
- data Backend
- ncgBackend :: Backend
- llvmBackend :: Backend
- jsBackend :: Backend
- viaCBackend :: Backend
- interpreterBackend :: Backend
- noBackend :: Backend
- allBackends :: [Backend]
- data PrimitiveImplementation
- data DefunctionalizedCodeOutput
- data DefunctionalizedPostHscPipeline
- data DefunctionalizedCDefs
- data BackendName
- backendDescription :: Backend -> String
- backendWritesFiles :: Backend -> Bool
- backendPipelineOutput :: Backend -> PipelineOutput
- backendCanReuseLoadedCode :: Backend -> Bool
- backendGeneratesCode :: Backend -> Bool
- backendGeneratesCodeForHsBoot :: Backend -> Bool
- backendSupportsInterfaceWriting :: Backend -> Bool
- backendRespectsSpecialise :: Backend -> Bool
- backendWantsGlobalBindings :: Backend -> Bool
- backendHasNativeSwitch :: Backend -> Bool
- backendPrimitiveImplementation :: Backend -> PrimitiveImplementation
- backendSimdValidity :: Backend -> Validity' String
- backendSupportsEmbeddedBlobs :: Backend -> Bool
- backendNeedsPlatformNcgSupport :: Backend -> Bool
- backendSupportsUnsplitProcPoints :: Backend -> Bool
- backendSwappableWithViaC :: Backend -> Bool
- backendUnregisterisedAbiOnly :: Backend -> Bool
- backendGeneratesHc :: Backend -> Bool
- backendSptIsDynamic :: Backend -> Bool
- backendSupportsBreakpoints :: Backend -> Bool
- backendForcesOptimization0 :: Backend -> Bool
- backendNeedsFullWays :: Backend -> Bool
- backendSpecialModuleSource :: Backend -> Bool -> Maybe String
- backendSupportsHpc :: Backend -> Bool
- backendSupportsCImport :: Backend -> Bool
- backendSupportsCExport :: Backend -> Bool
- backendCDefs :: Backend -> DefunctionalizedCDefs
- backendCodeOutput :: Backend -> DefunctionalizedCodeOutput
- backendUseJSLinker :: Backend -> Bool
- backendPostHscPipeline :: Backend -> DefunctionalizedPostHscPipeline
- backendNormalSuccessorPhase :: Backend -> Phase
- backendName :: Backend -> BackendName
- backendValidityOfCImport :: Backend -> Validity' [Backend]
- backendValidityOfCExport :: Backend -> Validity' [Backend]
- platformDefaultBackend :: Platform -> Backend
- platformNcgSupported :: Platform -> Bool
The Backend
type
A value of type Backend
represents one of GHC's back ends.
The set of back ends cannot be extended except by modifying the
definition of Backend
in this module.
The Backend
type is abstract; that is, its value constructors are
not exported. It's crucial that they not be exported, because a
value of type Backend
carries only the back end's name, not its
behavior or properties. If Backend
were not abstract, then code
elsewhere in the compiler could depend directly on the name, not on
the semantics, which would make it challenging to create a new back end.
Because Backend
is abstract, all the obligations of a new back
end are enumerated in this module, in the form of functions that
take Backend
as an argument.
The issue of abstraction is discussed at great length in #20927 and !7442.
Available back ends
ncgBackend :: Backend Source #
The native code generator. Compiles Cmm code into textual assembler, then relies on an external assembler toolchain to produce machine code.
Only supports a few platforms (X86, PowerPC, SPARC).
See GHC.CmmToAsm.
llvmBackend :: Backend Source #
The LLVM backend.
Compiles Cmm code into LLVM textual IR, then relies on LLVM toolchain to produce machine code.
It relies on LLVM support for the calling convention used by the NCG backend to produce code objects ABI compatible with it (see "cc 10" or "ghccc" calling convention in https://llvm.org/docs/LangRef.html#calling-conventions).
Supports a few platforms (X86, AArch64, s390x, ARM).
See GHC.CmmToLlvm
viaCBackend :: Backend Source #
Via-C ("unregisterised") backend.
Compiles Cmm code into C code, then relies on a C compiler to produce machine code.
It produces code objects that are not ABI compatible with those produced by NCG and LLVM backends.
Produced code is expected to be less efficient than the one produced by NCG and LLVM backends because STG registers are not pinned into real registers. On the other hand, it supports more target platforms (those having a valid C toolchain).
See GHC.CmmToC
interpreterBackend :: Backend Source #
The ByteCode interpreter.
Produce ByteCode objects (BCO, see GHC.ByteCode) that can be interpreted. It is used by GHCi.
Currently some extensions are not supported (foreign primops).
A dummy back end that generates no code.
Use this back end to disable code generation. It is particularly useful when GHC is used as a library for other purpose than generating code (e.g. to generate documentation with Haddock) or when the user requested it (via `-fno-code`) for some reason.
allBackends :: [Backend] Source #
A list of all back ends. They are ordered as we wish them to appear when they are enumerated in error messages.
Types used to specify properties of back ends
data PrimitiveImplementation Source #
This enumeration type specifies how the back end wishes GHC's
primitives to be implemented. (Module GHC.StgToCmm.Prim provides
a generic implementation of every primitive, but some primitives,
like IntQuotRemOp
, can be implemented more efficiently by
certain back ends on certain platforms. For example, by using a
machine instruction that simultaneously computes quotient and remainder.)
For the meaning of each alternative, consult
GHC.StgToCmm.Config. (In a perfect world, type
PrimitiveImplementation
would be defined there, in the module
that determines its meaning. But I could not figure out how to do
it without mutual recursion across module boundaries.)
LlvmPrimitives | Primitives supported by LLVM |
NcgPrimitives | Primitives supported by the native code generator |
JSPrimitives | Primitives supported by JS backend |
GenericPrimitives | Primitives supported by all back ends |
Instances
Show PrimitiveImplementation Source # | |
Defined in GHC.Driver.Backend showsPrec :: Int -> PrimitiveImplementation -> ShowS # show :: PrimitiveImplementation -> String # showList :: [PrimitiveImplementation] -> ShowS # |
Properties that stand for functions
Back-end function for code generation
data DefunctionalizedCodeOutput Source #
Names a function that generates code and writes the results to a file, of this type:
Logger -> DynFlags -> Module -- ^ module being compiled -> ModLocation -> FilePath -- ^ Where to write output -> Set UnitId -- ^ dependencies -> Stream IO RawCmmGroup a -- results from `StgToCmm` -> IO a
The functions so named are defined in GHC.Driver.CodeOutput.
We expect one function per back end—or more precisely, one function for each back end that writes code to a file. (The interpreter does not write to files; its output lives only in memory.)
Back-end functions for assembly
data DefunctionalizedPostHscPipeline Source #
Names a function that tells the driver what should happen after assembly code is written. This might include running a C compiler, running LLVM, running an assembler, or various similar activities. The function named normally has this type:
TPipelineClass TPhase m => PipeEnv -> HscEnv -> Maybe ModLocation -> FilePath -> m (Maybe FilePath)
The functions so named are defined in GHC.Driver.Pipeline.
NcgPostHscPipeline | |
ViaCPostHscPipeline | |
LlvmPostHscPipeline | |
JSPostHscPipeline | |
NoPostHscPipeline | After code generation, nothing else need happen. |
Other back-end functions
data DefunctionalizedCDefs Source #
Names a function that tells the driver what command-line options
to include when invoking a C compiler. It's meant for -D
options that
define symbols for the C preprocessor. Because the exact symbols
defined might depend on versions of tools located in the file
system (cough LLVM cough), the function requires an IO
action.
The function named has this type:
Logger -> DynFlags -> IO [String]
Names of back ends (for API clients of version 9.4 or earlier)
data BackendName Source #
Instances
Show BackendName Source # | |
Defined in GHC.Driver.Backend.Internal showsPrec :: Int -> BackendName -> ShowS # show :: BackendName -> String # showList :: [BackendName] -> ShowS # | |
Eq BackendName Source # | |
Defined in GHC.Driver.Backend.Internal (==) :: BackendName -> BackendName -> Bool # (/=) :: BackendName -> BackendName -> Bool # |
Properties of back ends
backendDescription :: Backend -> String Source #
An informal description of the back end, for use in issuing warning messages only. If code depends on what's in the string, you deserve what happens to you.
backendWritesFiles :: Backend -> Bool Source #
This flag tells the compiler driver whether the back end will write files: interface files and object files. It is typically true for "real" back ends that generate code into the filesystem. (That means, not the interpreter.)
backendPipelineOutput :: Backend -> PipelineOutput Source #
When the back end does write files, this value tells the compiler in what manner of file the output should go: temporary, persistent, or specific.
backendCanReuseLoadedCode :: Backend -> Bool Source #
This flag tells the driver whether the back end can reuse code (bytecode or object code) that has been loaded dynamically. Likely true only of the interpreter.
backendGeneratesCode :: Backend -> Bool Source #
It is is true of every back end except -fno-code
that it "generates code." Surprisingly, this property
influences the driver in a ton of ways. Some examples:
- If the back end does not generate code, then the driver needs to turn on code generation for Template Haskell (because that code needs to be generated and run at compile time).
- If the back end does not generate code, then the driver does not need to deal with an output file.
- If the back end does generated code, then the
driver supports
HscRecomp
. If not, recompilation does not need a linkable (and is automatically up to date).
backendSupportsInterfaceWriting :: Backend -> Bool Source #
When set, this flag turns on interface writing for
Backpack. It should probably be the same as
backendGeneratesCode
, but it is kept distinct for
reasons described in Note [-fno-code mode].
backendRespectsSpecialise :: Backend -> Bool Source #
When preparing code for this back end, the type
checker should pay attention to SPECIALISE pragmas. If
this flag is False
, then the type checker ignores
SPECIALISE pragmas (for imported things?).
backendWantsGlobalBindings :: Backend -> Bool Source #
This back end wants the mi_globals
field of a
ModIface
to be populated (with the top-level bindings
of the original source). Only true for the interpreter.
backendHasNativeSwitch :: Backend -> Bool Source #
The back end targets a technology that implements
switch
natively. (For example, LLVM or C.) Therefore
it is not necessary for GHC to ccompile a Cmm Switch
form into a decision tree with jump tables at the
leaves.
backendPrimitiveImplementation :: Backend -> PrimitiveImplementation Source #
As noted in the documentation for
PrimitiveImplementation
, certain primitives have
multiple implementations, depending on the capabilities
of the back end. This field signals to module
GHC.StgToCmm.Prim what implementations to use with
this back end.
backendSupportsEmbeddedBlobs :: Backend -> Bool Source #
This flag says whether the back end supports large binary blobs. See Note [Embedding large binary blobs] in GHC.CmmToAsm.Ppr.
backendNeedsPlatformNcgSupport :: Backend -> Bool Source #
This flag tells the compiler driver that the back end does not support every target platform; it supports only platforms that claim NCG support. (It's set only for the native code generator.) Crufty. If the driver tries to use the native code generator without platform support, the driver fails over to the LLVM back end.
backendSupportsUnsplitProcPoints :: Backend -> Bool Source #
This flag is set if the back end can generate code for proc points. If the flag is not set, then a Cmm pass needs to split proc points (that is, turn each proc point into a standalone procedure).
backendSwappableWithViaC :: Backend -> Bool Source #
This flag guides the driver in resolving issues about API support on the target platform. If the flag is set, then these things are true:
- When the target platform supports only an unregisterised API, this backend can be replaced with compilation via C.
- When the target does not support an unregisterised API, this back end can replace compilation via C.
backendUnregisterisedAbiOnly :: Backend -> Bool Source #
This flag is true if the back end works *only* with the unregisterised ABI.
backendGeneratesHc :: Backend -> Bool Source #
This flag is set if the back end generates C code in
a .hc
file. The flag lets the compiler driver know
if the command-line flag -C
is meaningful.
backendSptIsDynamic :: Backend -> Bool Source #
This flag says whether SPT (static pointer table)
entries will be inserted dynamically if needed. If
this flag is False
, then GHC.Iface.Tidy should emit C
stubs that initialize the SPT entries.
backendSupportsBreakpoints :: Backend -> Bool Source #
If this flag is unset, then the driver ignores the flag -fbreak-points
,
since backends other than the interpreter tend to panic on breakpoints.
backendForcesOptimization0 :: Backend -> Bool Source #
If this flag is set, then the driver forces the optimization level to 0, issuing a warning message if the command line requested a higher optimization level.
backendNeedsFullWays :: Backend -> Bool Source #
I don't understand exactly how this works. But if
this flag is set *and* another condition is met, then
ghc/Main.hs
will alter the DynFlags
so that all the
hostFullWays
are asked for. It is set only for the interpreter.
backendSpecialModuleSource :: Backend -> Bool -> Maybe String Source #
This flag is also special for the interpreter: if a
message about a module needs to be shown, do we know
anything special about where the module came from? The
Boolean argument is a recomp
flag.
backendSupportsHpc :: Backend -> Bool Source #
This flag says whether the back end supports Haskell Program Coverage (HPC). If not, the compiler driver will ignore the `-fhpc` option (and will issue a warning message if it is used).
backendSupportsCImport :: Backend -> Bool Source #
This flag says whether the back end supports foreign
import of C functions. (Supports means "does not
barf on," so -fno-code
supports foreign C imports.)
backendSupportsCExport :: Backend -> Bool Source #
This flag says whether the back end supports foreign export of Haskell functions to C.
backendCDefs :: Backend -> DefunctionalizedCDefs Source #
When using this back end, it may be necessary or advisable to pass some `-D` options to a C compiler. This (defunctionalized) function produces those options, if any. An IO action may be necessary in order to interrogate external tools about what version they are, for example.
The function's type is
Logger -> DynFlags -> IO [String]
This field is usually defaulted.
backendCodeOutput :: Backend -> DefunctionalizedCodeOutput Source #
This (defunctionalized) function generates code and writes it to a file. The type of the function is
Logger -> DynFlags -> Module -- ^ module being compiled -> ModLocation -> FilePath -- ^ Where to write output -> Set UnitId -- ^ dependencies -> Stream IO RawCmmGroup a -- results from `StgToCmm` -> IO a
backendUseJSLinker :: Backend -> Bool Source #
backendPostHscPipeline :: Backend -> DefunctionalizedPostHscPipeline Source #
This (defunctionalized) function tells the compiler driver what else has to be run after code output. The type of the function is
TPipelineClass TPhase m => PipeEnv -> HscEnv -> Maybe ModLocation -> FilePath -> m (Maybe FilePath)
backendNormalSuccessorPhase :: Backend -> Phase Source #
Somewhere in the compiler driver, when compiling
Haskell source (as opposed to a boot file or a sig
file), it needs to know what to do with the code that
the backendCodeOutput
writes to a file. This Phase
value gives instructions like "run the C compiler",
"run the assembler," or "run the LLVM Optimizer."
backendName :: Backend -> BackendName Source #
Name of the back end, if any. Used to migrate legacy clients of the GHC API. Code within the GHC source tree should not refer to a back end's name.
backendValidityOfCImport :: Backend -> Validity' [Backend] Source #
When foreign C import or export is invalid, the carried value enumerates the valid back ends.
backendValidityOfCExport :: Backend -> Validity' [Backend] Source #
When foreign C import or export is invalid, the carried value enumerates the valid back ends.
Other functions of back ends
platformNcgSupported :: Platform -> Bool Source #
Is the platform supported by the Native Code Generator?