This relands commit 99108c791de0285ee726a10e8274772b18cee73c (D95026) which was
reverted by 8d5a981a135a0f0ae0a10c59b7c8093aae1c28de because the underlying
problem (https://llvm.org/pr49495) is fixed.
This reverts commit 99108c791de0285ee726a10e8274772b18cee73c.
Clang is miscompiling LLVM with this change, a stage-2 build hits
multiple failures.
As a repro, I built clang in a stage1 directory and used it this way:
cmake -G Ninja ../llvm \
-DCMAKE_CXX_COMPILER=`pwd`/../build-stage1/bin/clang++ \
-DCMAKE_C_COMPILER=`pwd`/../build-stage1/bin/clang \
-DLLVM_TARGETS_TO_BUILD="X86;NVPTX;AMDGPU" \
-DLLVM_ENABLE_PROJECTS=mlir \
-DLLVM_BUILD_EXAMPLES=ON \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_ENABLE_ASSERTIONS=On
ninja check-mlir
This patch makes FoldBranchToCommonDest merge branch conditions into `select i1` rather than `and/or i1` when it is called by SimplifyCFG.
It is known that merging conditions into and/or is poison-unsafe, and this is towards making things *more* correct by removing possible miscompilations.
Currently, InstCombine simply consumes these selects into and/or of i1 (which is also unsafe), so the visible effect would be very small. The unsafe select -> and/or transformation will be removed in the future.
There has been efforts for updating optimizations to support the select form as well, and they are linked to D93065.
The safe transformation is fired when it is called by SimplifyCFG only. This is done by setting the new `PoisonSafe` argument as true.
Another place that calls FoldBranchToCommonDest is LoopSimplify. `PoisonSafe` flag is set to false in this case because enabling it has a nontrivial impact in performance because SCEV is more conservative with select form and InductiveRangeCheckElimination isn't aware of select form of and/or i1.
Reviewed By: nikic
Differential Revision: https://reviews.llvm.org/D95026
Currently SimplifyCFG drops the debug locations of 'bonus' instructions.
Such instructions are moved before the first branch. The reason for the
current behavior is that this could lead to surprising debug stepping,
if the block that's folded is dead.
In case the first branch and the instructions to be folded have the same
debug location, this shouldn't be an issue and we can keep the debug
location.
Reviewed By: vsk
Differential Revision: https://reviews.llvm.org/D93662
And that exposes that a number of tests don't *actually* manage to
maintain DomTree validity, which is inline with my observations.
Once again, SimlifyCFG pass currently does not require/preserve DomTree
by default, so this is effectively NFC.
... so just ensure that we pass DomTreeUpdater it into it.
Fixes DomTree preservation for a large number of tests,
all of which are marked as such so that they do not regress.
