[llvm-dev] [RFC] Requiring explicit address space arguments for PointerType

[Alexander Richardson via llvm-dev]

Hello all,

I recently finished merging the last 3.5 months of upstream changes
into our CHERI LLVM fork and would like to suggest something to both
simplify our future merges and also avoid bugs for targets such as AVR
that use non-zero pointer address spaces.

We make extensive use of address spaces: all our pointers (CHERI
capabilities) use address space 200 instead of the default zero to
ensure that we generated CHERI instructions instead of integer
arithmetic.
The problem we have is that almost every use of
PointerType::getUnqual() or ->getPointerTo() without an address space
parameter will result in an assertion failure/"cannot select" or even
a miscompilation due to creating an addrspace(0) pointer.
After every upstream merge I have to fix the new uses that have been
introduced since the last merge and I get lots of merge conflicts if
code that we have modified is moved around.

In our fork I have conditionally removed the default zero value for
address space parameters to catch these issues at compile time instead
of run time. Unfortunately, PointerType::getUnqual() and
getPointerTo()
are used so frequently that I wasn't able to just remove the default
argument, but had to resort to conditionally allowing it and disabling
it on a directory-by-directory granularity by setting an #ifdef
So far I've changed all default-zero calls for clang and some parts of
LLVM, but we haven't touched passes/targets that we don't add CHERI
support for [1].


I have uploaded these changes as https://reviews.llvm.org/D78491 which
effectively replaces
`unsigned AddressSpace = 0` with LLVM_DEFAULT_AS_PARAM(AddressSpace).
This macro expands to an argument with a default of zero unless
LLVM_NO_IMPLICIT_ADDRESS_SPACE is defined, in which case the argument
is required.
This allows incrementally removing the default zero from individual
files/directories by adding
add_definitions(-DLLVM_NO_IMPLICIT_ADDRESS_SPACE=1) to a single
CMakeLists.txt and eventually doing it for the root CMakeLists.txt.

If this sounds like a reasonable approach for avoiding default-zero
address spaces, I'd like to propose the following steps:

1. Clean up and commit https://reviews.llvm.org/D78491 and the
dependency https://reviews.llvm.orgD70947

2. Start migrating more directories by adding more
add_definitions(-DLLVM_NO_IMPLICIT_ADDRESS_SPACE=1) lines.

3. Set -DLLVM_NO_IMPLICIT_ADDRESS_SPACE=1 in the LLVM and Clang root
CMakeLists.txt to prevent future uses of PointerType::getUnqual()
inside LLVM/Clang.

(4?) Change the behaviour from opt-in to opt-out by requiring
something like -DLLVM_IMPLICIT_DEFAULT_ADDRESS_SPACE=0.

(5?) Depending on how we feel about out-of-tree consumers, remove the
LLVM_DEFAULT_AS_PARAM(AddressSpace) macros and require the argument to
always be present. Adding the argument is compatible with older LLVM
versions so it should be possible to do so without version #ifdefs. I
think we should probably reduce the amount of changes required by
out-of-tree consumers and keep the optional default, but it would
obviously be nice to eventually remove the #ifdefs from inside LLVM.

Does this sound like a sensible approach? Should I also attempt to
implement steps 4 and 5 or is 1-3 sufficient?

I would be great if we could land this upstream as it will
significantly reduce the maintenance burden for our CHERI LLVM and
should also prevent AVR issues such as e.g.
https://reviews.llvm.org/rG215dc2e203341f7d1edc4c4a191b048af4ace43d

Thanks,
Alex

[1] CHERI is currently available for MIPS and RISC-V and will soon be
available on the upcoming Arm Morello board
(https://developer.arm.com/architectures/cpu-architecture/a-profile/morello).