Introduce a pass which statically demotes some helpers from candidates
for inlining:
1) Helpers which are part of SCC on the callgraph.
2) Helpers which contains an `insertvalue`.
3) Helpers which _fail_ the `CriticalArguments` criterion.
A critical argument is defined starting from the critical operand
definition.
A critical operand is either the condition of a `switch` instruction or
the indices of a `getelementptr` instruction.
If the dataflow computing the the operand trace back to a formal
parameter of the function, that parameter will be considered critical
when attempting the inlining of the helper.
In case each condition is satisfied, the `revng.inline.policy` metadata
is attached to the function, so that it can be used at _inline_ time to
take the decision based on the critical arguments constantness.
The metadata is encoded as an `iN` integer with N = `arg_size() + 1`:
the extra most-significant bit is always zero so LLVM's signed-decimal
`iN` printer renders the value positively (avoiding e.g. `!{i2 -2}`).
The (de)serialisation primitives live in `revng/Support/IRHelpers.h`
as `serializeInliningPolicy` / `deserializeInliningPolicy` so the
runtime `revngFunctionIsolation` doesn't need to link the build-time
`revngHelperInliningAnalyses` to read the metadata back.
Introduce a build-time pass that tags every QEMU helper transitively
reaching a function defined under one of the configured runtime-library
directories (currently `fpu/` only) as `revng_inline`. The intent is to
make the lift pipeline surface "leaf-level" runtime-library calls (e.g.
softfloat ops) directly in the lifted IR by inlining away every wrapper
helper that sits between QEMU's `helper_*` boundary and the leaf.
We also gate the tagging by a per-function body-size budget.
The pass is run in the pipeline building the `libtcg-helpers-full-*.bc`
artifacts.
Andrea Gussoni