Resolve warning caused by using values from two different enums in a
conditional expression which was deprecated in c++20.
The warning in question is clang -Wdeprecated-anon-enum-enum-conversion
and gcc -Wenum-compare.
We had one implementation of this type of data structure in Arm64Emitter
and one in VideoCommon. This moves the Arm64Emitter implementation to
its own file and adds begin and end functions to it, so that VideoCommon
can use it.
You may notice that the license header for the new file is CC0. I wrote
the Arm64Emitter implementation of SmallVector, so this should be no
problem.
Modify PPCSTATE_OFF and PPCSTATE_OFF_ARRAY macros when using GCC to
avoid useless log spam. Specifically, use a consteval lambda with gcc
_Pragma statements to disable the -Winvalid-offsetof warning inside the
macros.
Each successful build (and many failing ones) on the Android buildbot
generates almost 300 cases of -Winvalid-offsetof, resulting in thousands
of lines of log spam per build. In addition to bloating the log filesize
these spurious warnings make it harder to find actual warnings.
These warnings are generated by calls to the macros PPCSTATE_OFF and
PPCSTATE_OFF_ARRAY, which in turn are used by many other macros used by
the JIT. The ultimate cause is that offsetof is only conditionally
supported on non-standard-layout types, which includes the PowerPCState
struct.
To address potential questions of whether there's a better way to handle
this:
The obvious solution would be to modify PowerPCState so that it does
have a standard layout. This is unfortunately impractical.
To have a standard layout a type can only contain other types with
standard layouts. None of the stl containers are guaranteed to have
standard layouts, and PowerPCState contains a std::tuple and std::array.
PowerPCState also contains a PowerPC::Cache and InstructionCache which
themselves contain std:arrays and std::vectors.
Furthermore InstructionCache derives from Cache, and a derived class can
only have standard layout if at most one class in its hierarchy has a
non-static data member, but both classes have such members. Making
InstructionCache have a standard layout would require duplicating all
the functionality of Cache so it no longer derived from it, as well as
replacing the stl containers. This might require having a raw pointer to
said containers, with the manual memory management that implies.
All of that would be much more disruptive than would be justified to get
rid of some warnings (however annoying they might be). This is
compounded by the fact that PowerPCState hasn't had a standard layout
for a long time, if ever, and if the PPCSTATE_OFF macros weren't working
reliably it would have become obvious a long time ago.
As to why I picked the lambda solution over other potential changes:
- Keeping the define as-is and wrapping some gcc #pragmas around it
doesn't work because the pragmas don't get included when the define is
substituted to the call site.
- Keeping the define as a non-lambda expression and using inline
_Pragma() statements would ideally be better and works fine for msvc,
but fails for GCC with "'#pragma' is not allowed here".
- Turning off -Winvalid-offsetof globally for gcc would work, but there
might be other contexts where offsetof is problematic and GCC seems to
be the only compiler warning about it.
