8. Hacking on the Futhark Compiler¶
The Futhark compiler is a significant body of code with a not entirely
straightforward design. The main source of documentation is the
Haddock comments in the source code itself. You can generate
hyperlinked reference documentation by running
stack haddock or
cabal haddock, depending on your preference of build system.
There is also possibly-outdated documentation on Hackage
If you feel that the documentation is incomplete, or something lacks an explanation, then feel free to report it as an issue on the GitHub page. Documentation bugs are bugs too.
The Futhark compiler is usually built using Stack. It’s a good idea to familiarise yourself with how it works. As a starting point, here are a few hints:
When testing, pass
stackto disable the GHC optimiser. This speeds up builds considerably (although it still takes a while). The resulting Futhark compiler will run slower, but it is not something you will notice for small test programs.
When debugging, pass
stack. This will build the Futhark compiler with debugging information (not just profiling). In particular, hard crashes will print a stack trace. You can also get actual profiling information by passing
+RTS -pprof-all -RTSto the Futhark compiler. This asks the Haskell runtime to print profiling information to a file. For more information, see the Profiling chapter in the GHC User Guide.
You may wish to set the environment variable
FUTHARK_COMPILER_DEBUGGING=1. Currently this only has the effect of making the frontend print internal names, but it may control more things in the future.
8.1. Debugging Internal Type Errors¶
The Futhark compiler uses a typed core language, and the type checker
is run after every pass. If a given pass produces a program with
inconsistent typing, the compiler will report an error and abort.
While not every compiler bug will manifest itself as a core language
type error (unfortunately), many will. To write the erroneous core
program to a file in case of type error, pass
-v filename to the
compiler. This will also enable verbose output, so you can tell which
pass fails. The
-v option is also useful when the compiler itself
crashes, as you can at least tell where in the pipeline it got to.
8.2. Checking Generated Code¶
Hacking on the compiler will often involve inspecting the quality of the generated code. The recommended way to do this is to use futhark-c or futhark-opencl to compile a Futhark program to an executable. These backends insert various forms of instrumentation that can be enabled by passing run-time options to the generated executable.
As a first resort, use
-toption to use the built-in runtime measurements. A nice trick is to pass
-t /dev/stderr, while redirecting standard output to
/dev/null. This will print the runtime on the screen, but not the execution result.
-rto ask for several runs, e.g.
-r 10. If combined with
-t, this will cause several runtimes to be printed (one per line). The futhark-bench tool itself uses
-rto perform its measurements.
-Dto have the program print information on allocation and deallocation of memory.
(futhark-opencl only) Use the
-Doption to enable synchronous execution.
clFinish()will be called after most OpenCL operations, and a running log of kernel invocations will be printed. At the end of execution, the program prints a table summarising all kernels and their total runtime and average runtime.
For debugging specific compiler passes, the
futhark dev subcommand
allows you to tailor your own compilation pipeline using command line
options. It is also useful for seeing what the AST looks like after
8.4. When you are about to have a bad day¶
When using the
cuda backend, you can use the
runtime option to dump PTX, a kind of high-level assembly for NVIDIA
GPUs, corresponding to the GPU kernels. This can be used to
investigate why the generated code isn’t running as fast as you expect
(not fun), or even whether NVIDIAs compiler is miscompiling something
(extremely not fun). With the OpenCL backend,
--dump-opencl-binary does the same thing.
On AMD platforms,
--dump-opencl-binary tends to produce an actual
binary of some kind, and it is pretty tricky to obtain a debugger for
it (they are available and open source, but the documentation and
installation instructions are terrible). Instead, AMDs OpenCL kernel
compiler accepts a
-save-temps=foo build option, which will make
it write certain intermediate files, prefixed with
particular, it will write an
.s file that contains what appears to
be HSA assembly (at least when using ROCm). If you find yourself
having to do do this, then you are definitely going to have a bad day,
and probably evening and night as well.