[fpc-devel] LLVM code generator

[Jonas Maebe]

Hi,

The LLVM code generator is more or less ready, including Dwarf-EH-based 
exception handling support. It's currently only supported on 
Darwin/x86-64 and Linux/x86-64, but it can do a "make all" and the 
testsuite can be finished as well. There are still some extra failures 
that do not happen with the built-in code generator, but most tests work 
fine.

What still needs to be done:
* investigate the RTE 217 when you compile a program with an error in it
* add support for a volatile() built-in function similar to how we have 
unaligned(), to fix e.g. the endless loop in tests/test/tmt1 (or in 
general: global variables used for cross-thread synchronisation)
* add support for specifying the target LLVM version. Currently it's 
hard-coded to LLVM 3.9, although it works all the way up to at least 
LLVM 7.0. Specifying the version number would mainly be necessary (for 
now) to target older LLVM versions
* check some remaining test failures
* add support for more platforms. Windows will be harder (and not 
something I plan on working any time soon), because I have not yet added 
support for generating LLVM-based exception handling code that is needed 
for SEH-based exception handling. Other 64 bit platforms should be 
"reasonably" easy (the parameter handling needs to be generalised more 
though). 32 bit platforms will probably require either some compiler 
re-engineering, hacking, or a translation of the tcg64 code generator to 
some kind of high-level code generator as well.
* add support for generating debug info
* add support for generating more meta-information for optimizations 
(e.g. range information about subrange types and enums)
* add support for saving the bitcode in addition to the native object 
files, and subsequently using link-time optimisation
* add support for some custom calling conventions
* pass on more FPC-related code generation options to LLVM (currently, 
mainly -CfXXX and -Ox get passed on)
* add support for TLS-based threadvars

Known limitations:
* hardware exceptions are unsupported. They will be caught, but 
unwinding is hit-and-miss because the state of the registers may be 
completely different than what LLVM expects (higher optimisation levels 
= more chance it will fail). This is an LLVM limitation and not 
something that can be fixed in FPC. It's also a major source of extra 
tests failures.

The support is currently only on the 
https://svn.freepascal.org/FPC/svn/fpc/branches/debug_eh branch.

To test:
* make sure you have clang on your system (the ones from Xcode work fine 
too) and that it's in your path. If not, you can download one from 
http://releases.llvm.org/download.html#7.0.0
* build FPC as follows
** Darwin (can also use a higher version than 10.6, but the built-in 
default --10.5--- won't work):
   make LOCALOPT="-dllvm -Fullvm" OPT="-WM10.6" all
** Linux: you may also have to specify the library path to libgcc_s. 
E.g. on Ubuntu 16.04:
   make LOCALOPT="-dllvm -Fullvm -Fl/usr/lib/gcc/x86_64-linux-gnu/5" 
OPT="-Fullvm -Fl/usr/lib/gcc/x86_64-linux-gnu/5" all -j 4 FPMAKEOPT="-T 
4"

The LOCALOPT flags are only used while building the compiler. They're 
separate here because otherwise you get errors while building the IDE 
(the -Fullvm doesn't work there while -dllvm would still make it use 
llvm units). This does mean that IDE will be built without the LLVM code 
generator, and hence its built-in compiler won't generate working code 
because it won't use Dwarf-EH-based exception handling, while the RTL 
will only contain support for that kind of exception handling.

Note that on Linux, all generated binaries will be dynamically linked 
and depend on libc (and libgcc_s), unlike with the regular code 
generator.


Jonas