[GVN] Do PHI translations across all edges between the load and the unavailable pred.
Currently we do not properly translate addresses with PHIs if LoadBB !=
LI->getParent(), because PHITranslateAddr expects a direct predecessor as argument,
because it considers all instructions outside of the current block to
not requiring translation.
The amount of cases that trigger this should be very low, as most single
predecessor blocks should be folded into their predecessor by GVN before
we actually start with value numbering. It is still not guaranteed to
happen, so we should do PHI translation along all edges between the
loads' block and the predecessor where we have to place a load.
There are a few test cases showing current limits of the PHI translation, which
could be improved later.
Reviewers: spatel, reames, efriedma, john.brawn
Reviewed By: efriedma
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65020
llvm-svn: 369570
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 29911a4..65ba0bb 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1164,15 +1164,30 @@
// Do PHI translation to get its value in the predecessor if necessary. The
// returned pointer (if non-null) is guaranteed to dominate UnavailablePred.
+ // We do the translation for each edge we skipped by going from LI's block
+ // to LoadBB, otherwise we might miss pieces needing translation.
// If all preds have a single successor, then we know it is safe to insert
// the load on the pred (?!?), so we can insert code to materialize the
// pointer if it is not available.
- PHITransAddr Address(LI->getPointerOperand(), DL, AC);
- Value *LoadPtr = nullptr;
- LoadPtr = Address.PHITranslateWithInsertion(LoadBB, UnavailablePred,
- *DT, NewInsts);
+ Value *LoadPtr = LI->getPointerOperand();
+ BasicBlock *Cur = LI->getParent();
+ while (Cur != LoadBB) {
+ PHITransAddr Address(LoadPtr, DL, AC);
+ LoadPtr = Address.PHITranslateWithInsertion(
+ Cur, Cur->getSinglePredecessor(), *DT, NewInsts);
+ if (!LoadPtr) {
+ CanDoPRE = false;
+ break;
+ }
+ Cur = Cur->getSinglePredecessor();
+ }
+ if (LoadPtr) {
+ PHITransAddr Address(LoadPtr, DL, AC);
+ LoadPtr = Address.PHITranslateWithInsertion(LoadBB, UnavailablePred, *DT,
+ NewInsts);
+ }
// If we couldn't find or insert a computation of this phi translated value,
// we fail PRE.
if (!LoadPtr) {
@@ -1187,8 +1202,12 @@
if (!CanDoPRE) {
while (!NewInsts.empty()) {
- Instruction *I = NewInsts.pop_back_val();
- markInstructionForDeletion(I);
+ // Erase instructions generated by the failed PHI translation before
+ // trying to number them. PHI translation might insert instructions
+ // in basic blocks other than the current one, and we delete them
+ // directly, as markInstructionForDeletion only allows removing from the
+ // current basic block.
+ NewInsts.pop_back_val()->eraseFromParent();
}
// HINT: Don't revert the edge-splitting as following transformation may
// also need to split these critical edges.