09f4e5e3
by fendor at 2026-04-02T15:01:30+02:00
Make HPC work with bytecode interpreter
Add support to generate .tix files from bytecode objects and the
bytecode interpreter.
Conceptually, we insert HPC ticks into the bytecode similar to how we insert
breakpoints.
HPC and breakpoints do not share the same tick array but we use a separate
tick-array for hpc/breakpoint ticks during bytecode generation.
We teach the bytecode interpreter to handle hpc ticks.
The implementation is quite trivial, simply increment the counter in the
global hpc_ticks array for the respective module.
This hpc_ticks array is generated as part of the `CStub`, so we can rely
on it existing.
A tricky bit is "registering" a bytecode object for HPC instrumentation.
In the compiled case, this is achieved via CStub and initializer/finalizers
`.init` sections which are called when the executable is run.
After the initializers have been invoked, which is before `hs_init_ghc`,
we then call `startup_hpc` in `hs_init_ghc` iff any modules were "registered"
for hpc instrumentation via `hs_hpc_module`.
Since bytecode objects are loaded after starting up GHCi, this workflow
doesn't work for supporting `hpc` and the `hpc` run-time is never
started, even if a module is added for instrumentation.
We fix this issue by employing the same technique as is for `SptEntry`s:
* We introduce a new field to `CompiledByteCode`, called `ByteCodeHpcInfo`
which contains enough information to call `hs_hpc_module`, allowing us to
register the module for `hpc` instrumentation`.
* After registering the module, we unconditionally call `startupHpc`, to make
sure the .tix file is written.
Calling `startupHpc` multiple times is safe.
Calling `hs_hpc_module` multiple times for the same module is also safe.
If we didn't register the hpc module in this way, evaluating a bytecode object
instrumented with `-fhpc` without registering it in the `hpc` run-time will
simply not generate any `.tix` files for this bytecode object.
However, this shouldn't happen if everything is set up correctly.
Closes #27036