Summary:
Loop unswitch hoists branches on loop-invariant conditions. However, if this
condition is poison/undef and the branch wasn't originally reachable, loop
unswitch introduces UB (since the optimized code will branch on poison/undef and
the original one didn't)).
We fix this problem by freezing the condition to ensure we don't introduce UB.
We will now transform the following:
while (...) {
if (C) { A }
else { B }
}
Into:
C' = freeze(C)
if (C') {
while (...) { A }
} else {
while (...) { B }
}
This patch fixes the root cause of the following bug reports (which use the old loop unswitch, but can be reproduced with minor changes in the code and -enable-nontrivial-unswitch):
- https://llvm.org/bugs/show_bug.cgi?id=27506
- https://llvm.org/bugs/show_bug.cgi?id=31652
Reviewers: reames, majnemer, chenli, sanjoy, hfinkel
Reviewed By: reames
Subscribers: hiraditya, jvesely, nhaehnle, filcab, regehr, trentxintong, nlopes, llvm-commits, mzolotukhin
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D29015
As it's causing some bot failures (and per request from kbarton).
This reverts commit r358543/ab70da07286e618016e78247e4a24fcb84077fda.
llvm-svn: 358546
making it no longer even remotely simple.
The pass will now be more of a "full loop unswitching" pass rather than
anything substantively simpler than any other approach. I plan to rename
it accordingly once the dust settles.
The key ideas of the new loop unswitcher are carried over for
non-trivial unswitching:
1) Fully unswitch a branch or switch instruction from inside of a loop to
outside of it.
2) Update the CFG and IR. This avoids needing to "remember" the
unswitched branches as well as avoiding excessively cloning and
reliance on complex parts of simplify-cfg to cleanup the cfg.
3) Update the analyses (where we can) rather than just blowing them away
or relying on something else updating them.
Sadly, #3 is somewhat compromised here as the dominator tree updates
were too complex for me to want to reason about. I will need to make
another attempt to do this now that we have a nice dynamic update API
for dominators. However, we do adhere to #3 w.r.t. LoopInfo.
This approach also adds an important principls specific to non-trivial
unswitching: not *all* of the loop will be duplicated when unswitching.
This fact allows us to compute the cost in terms of how much *duplicate*
code is inserted rather than just on raw size. Unswitching conditions
which essentialy partition loops will work regardless of the total loop
size.
Some remaining issues that I will be addressing in subsequent commits:
- Handling unstructured control flow.
- Unswitching 'switch' cases instead of just branches.
- Moving to the dynamic update API for dominators.
Some high-level, interesting limitationsV that folks might want to push
on as follow-ups but that I don't have any immediate plans around:
- We could be much more clever about not cloning things that will be
deleted. In fact, we should be able to delete *nothing* and do
a minimal number of clones.
- There are many more interesting selection criteria for which branch to
unswitch that we might want to look at. One that I'm interested in
particularly are a set of conditions which all exit the loop and which
can be merged into a single unswitched test of them.
Differential revision: https://reviews.llvm.org/D34200
llvm-svn: 318549
Juneyoung Lee