[Codegen] Merge tail blocks with no successors after block placement
Summary:
I found the following case having tail blocks with no successors merging opportunities after block placement.
Before block placement:
bb0:
...
bne a0, 0, bb2:
bb1:
mv a0, 1
ret
bb2:
...
bb3:
mv a0, 1
ret
bb4:
mv a0, -1
ret
The conditional branch bne in bb0 is opposite to beq.
After block placement:
bb0:
...
beq a0, 0, bb1
bb2:
...
bb4:
mv a0, -1
ret
bb1:
mv a0, 1
ret
bb3:
mv a0, 1
ret
After block placement, that appears new tail merging opportunity, bb1 and bb3 can be merged as one block. So the conditional constraint for merging tail blocks with no successors should be removed. In my experiment for RISC-V, it decreases code size.
Author of original patch: Jim Lin
Reviewers: haicheng, aheejin, craig.topper, rnk, RKSimon, Jim, dmgreen
Reviewed By: Jim, dmgreen
Subscribers: xbolva00, dschuff, javed.absar, sbc100, jgravelle-google, aheejin, kito-cheng, dmgreen, PkmX, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D54411
llvm-svn: 363284
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 93fd6f9..fb54b5d 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -1070,31 +1070,29 @@
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
bool MadeChange = false;
- if (!EnableTailMerge) return MadeChange;
+ if (!EnableTailMerge)
+ return MadeChange;
// First find blocks with no successors.
- // Block placement does not create new tail merging opportunities for these
- // blocks.
- if (!AfterBlockPlacement) {
- MergePotentials.clear();
- for (MachineBasicBlock &MBB : MF) {
- if (MergePotentials.size() == TailMergeThreshold)
- break;
- if (!TriedMerging.count(&MBB) && MBB.succ_empty())
- MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
- }
-
- // If this is a large problem, avoid visiting the same basic blocks
- // multiple times.
+ // Block placement may create new tail merging opportunities for these blocks.
+ MergePotentials.clear();
+ for (MachineBasicBlock &MBB : MF) {
if (MergePotentials.size() == TailMergeThreshold)
- for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
- TriedMerging.insert(MergePotentials[i].getBlock());
-
- // See if we can do any tail merging on those.
- if (MergePotentials.size() >= 2)
- MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
+ break;
+ if (!TriedMerging.count(&MBB) && MBB.succ_empty())
+ MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
}
+ // If this is a large problem, avoid visiting the same basic blocks
+ // multiple times.
+ if (MergePotentials.size() == TailMergeThreshold)
+ for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
+ TriedMerging.insert(MergePotentials[i].getBlock());
+
+ // See if we can do any tail merging on those.
+ if (MergePotentials.size() >= 2)
+ MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
+
// Look at blocks (IBB) with multiple predecessors (PBB).
// We change each predecessor to a canonical form, by
// (1) temporarily removing any unconditional branch from the predecessor