These turn out to be useful for spec2017/fotonik3d and safe so long as
they are not used along side TBAA tags for local allocations. LLVM may
be able to figure out local allocations by itself anyway.
PR #68727
See RFC at
https://discourse.llvm.org/t/rfc-propagate-fir-alias-analysis-information-using-tbaa/73755
This pass adds TBAA tags to all accesses to non-pointer/target dummy
arguments. These TBAA tags tell LLVM that these accesses cannot alias:
allowing better dead code elimination, hoisting out of loops, and
vectorization.
Each function has its own TBAA tree so that accesses between funtions
MayAlias after inlining.
I also included code for adding tags for local allocations and for
global variables. Enabling all three kinds of tag is known to produce a
miscompile and so these are disabled by default. But it isn't much
code and I thought it could be interesting to play with these later if
one is looking at a benchmark which looks like it would benefit from
more alias information. I'm open to removing this code too.
TBAA tags are also added separately by TBAABuilder during CodeGen.
TBAABuilder has to run during CodeGen because it adds tags to box
accesses, many of which are implicit in FIR. This pass cannot (easily)
run in CodeGen because fir::AliasAnalysis has difficulty tracing values
between blocks, and by the time CodeGen runs, structured control flow
has already been lowered.
Coming in follow up patches
- Change CodeGen/TBAABuilder to use TBAAForest to add tags within the
same per-function trees as are used here (delayed to a later patch
to make it easier to revert)
- Command line argument processing to actually enable the pass
