Uses the new liveness behaviour. This also removes useless calls
to `processDeath` on branches that were just initialized. Branch
consolidation and processing deaths on branches inside `condbr`
is still a TODO, just like before.
This also skips var_args on other native backends as they do not
support this feature yet.
Backends want to avoid emitting unused instructions which do not have
side effects: to that end, they all have `Liveness.isUnused` checks for
many instructions. However, checking this in the backends avoids a lot
of potential optimizations. For instance, if a nested field is loaded,
then the first field access would still be emitted, since its result is
used by the next access (which is then unreferenced).
To elide more instructions, Liveness can track this data instead. For
operands which do not have to be lowered (i.e. are not side effecting
and are not something special like `arg), Liveness can ignore their
operand usages, and push the unused information further up, potentially
marking many more instructions as unreferenced.
In doing this, I also uncovered a bug in the LLVM backend relating to
discarding the result of `@cVaArg`, which this change fixes. A behaviour
test has been added to cover it.
This is a partial rewrite of Liveness, so has some other notable changes:
- A proper multi-pass system to prevent code duplication
- Better logging
- Minor bugfixes
Also remove an incorrect piece of logic which allowed fetching the 'len'
property on non-single-ptrs (e.g. many-ptrs) and add a corresponding
compile error test case.
Resolves: #4765
The issue with just passing `-gcodeview` is that it's not part of
the `OPT_g_Group` in `clang/Driver/Options.td`, so it doesn't trigger
debug info to be generated.
Passing only `-g` is sufficient on MSVC, as there is logic in `Clang.cpp`
which enables codeview if that is the default for the toolchain. Since
the default for -gnu is not codeview, we do pass `-gcodeview` in that case.
