[MSSA] Fix PR28880 by fixing use optimizer's lower bound tracking behavior.
Summary:
In the use optimizer, we need to keep of whether the lower bound still
dominates us or else we may decide a lower bound is still valid when it
is not due to intervening pushes/pops. Fixes PR28880 (and probably a
bunch of other things).
Reviewers: george.burgess.iv
Subscribers: MatzeB, llvm-commits, sebpop
Differential Revision: https://reviews.llvm.org/D23237
llvm-svn: 277978
diff --git a/llvm/lib/Transforms/Utils/MemorySSA.cpp b/llvm/lib/Transforms/Utils/MemorySSA.cpp
index 8455f79..234d1af 100644
--- a/llvm/lib/Transforms/Utils/MemorySSA.cpp
+++ b/llvm/lib/Transforms/Utils/MemorySSA.cpp
@@ -1288,6 +1288,7 @@
// Note: Correctness depends on this being initialized to 0, which densemap
// does
unsigned long LowerBound;
+ const BasicBlock *LowerBoundBlock;
// This is where the last walk for this memory location ended.
unsigned long LastKill;
bool LastKillValid;
@@ -1333,7 +1334,6 @@
VersionStack.pop_back();
++PopEpoch;
}
-
for (MemoryAccess &MA : *Accesses) {
auto *MU = dyn_cast<MemoryUse>(&MA);
if (!MU) {
@@ -1355,13 +1355,24 @@
if (LocInfo.PopEpoch != PopEpoch) {
LocInfo.PopEpoch = PopEpoch;
LocInfo.StackEpoch = StackEpoch;
- // If the lower bound was in the info we popped, we have to reset it.
- if (LocInfo.LowerBound >= VersionStack.size()) {
+ // If the lower bound was in something that no longer dominates us, we
+ // have to reset it.
+ // We can't simply track stack size, because the stack may have had
+ // pushes/pops in the meantime.
+ // XXX: This is non-optimal, but only is slower cases with heavily
+ // branching dominator trees. To get the optimal number of queries would
+ // be to make lowerbound and lastkill a per-loc stack, and pop it until
+ // the top of that stack dominates us. This does not seem worth it ATM.
+ // A much cheaper optimization would be to always explore the deepest
+ // branch of the dominator tree first. This will guarantee this resets on
+ // the smallest set of blocks.
+ if (LocInfo.LowerBoundBlock && LocInfo.LowerBoundBlock != BB &&
+ !DT->dominates(LocInfo.LowerBoundBlock, BB)){
// Reset the lower bound of things to check.
// TODO: Some day we should be able to reset to last kill, rather than
// 0.
-
LocInfo.LowerBound = 0;
+ LocInfo.LowerBoundBlock = VersionStack[0]->getBlock();
LocInfo.LastKillValid = false;
}
} else if (LocInfo.StackEpoch != StackEpoch) {
@@ -1437,6 +1448,7 @@
MU->setDefiningAccess(VersionStack[LocInfo.LastKill]);
}
LocInfo.LowerBound = VersionStack.size() - 1;
+ LocInfo.LowerBoundBlock = BB;
}
}