[CodeGenPrep] Skip merging empty case blocks
This is recommit of r287553 after fixing the invalid loop info after eliminating an empty block and unit test failures in AVR and WebAssembly :
Summary: Merging an empty case block into the header block of switch could cause ISel to add COPY instructions in the header of switch, instead of the case block, if the case block is used as an incoming block of a PHI. This could potentially increase dynamic instructions, especially when the switch is in a loop. I added a test case which was reduced from the benchmark I was targetting.
Reviewers: t.p.northover, mcrosier, manmanren, wmi, joerg, davidxl
Subscribers: joerg, qcolombet, danielcdh, hfinkel, mcrosier, llvm-commits
Differential Revision: https://reviews.llvm.org/D22696
llvm-svn: 289988
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 0ff0993..1e5d682 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -17,6 +17,8 @@
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/BlockFrequencyInfo.h"
+#include "llvm/Analysis/BranchProbabilityInfo.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
@@ -124,6 +126,11 @@
"profile-guided-section-prefix", cl::Hidden, cl::init(true),
cl::desc("Use profile info to add section prefix for hot/cold functions"));
+static cl::opt<unsigned> FreqRatioToSkipMerge(
+ "cgp-freq-ratio-to-skip-merge", cl::Hidden, cl::init(2),
+ cl::desc("Skip merging empty blocks if (frequency of empty block) / "
+ "(frequency of destination block) is greater than this ratio"));
+
namespace {
typedef SmallPtrSet<Instruction *, 16> SetOfInstrs;
typedef PointerIntPair<Type *, 1, bool> TypeIsSExt;
@@ -136,6 +143,8 @@
const TargetTransformInfo *TTI;
const TargetLibraryInfo *TLInfo;
const LoopInfo *LI;
+ std::unique_ptr<BlockFrequencyInfo> BFI;
+ std::unique_ptr<BranchProbabilityInfo> BPI;
/// As we scan instructions optimizing them, this is the next instruction
/// to optimize. Transforms that can invalidate this should update it.
@@ -182,8 +191,11 @@
private:
bool eliminateFallThrough(Function &F);
bool eliminateMostlyEmptyBlocks(Function &F);
+ BasicBlock *findDestBlockOfMergeableEmptyBlock(BasicBlock *BB);
bool canMergeBlocks(const BasicBlock *BB, const BasicBlock *DestBB) const;
void eliminateMostlyEmptyBlock(BasicBlock *BB);
+ bool isMergingEmptyBlockProfitable(BasicBlock *BB, BasicBlock *DestBB,
+ bool isPreheader);
bool optimizeBlock(BasicBlock &BB, bool& ModifiedDT);
bool optimizeInst(Instruction *I, bool& ModifiedDT);
bool optimizeMemoryInst(Instruction *I, Value *Addr,
@@ -230,6 +242,8 @@
// Clear per function information.
InsertedInsts.clear();
PromotedInsts.clear();
+ BFI.reset();
+ BPI.reset();
ModifiedDT = false;
if (TM)
@@ -382,6 +396,38 @@
return Changed;
}
+/// Find a destination block from BB if BB is mergeable empty block.
+BasicBlock *CodeGenPrepare::findDestBlockOfMergeableEmptyBlock(BasicBlock *BB) {
+ // If this block doesn't end with an uncond branch, ignore it.
+ BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
+ if (!BI || !BI->isUnconditional())
+ return nullptr;
+
+ // If the instruction before the branch (skipping debug info) isn't a phi
+ // node, then other stuff is happening here.
+ BasicBlock::iterator BBI = BI->getIterator();
+ if (BBI != BB->begin()) {
+ --BBI;
+ while (isa<DbgInfoIntrinsic>(BBI)) {
+ if (BBI == BB->begin())
+ break;
+ --BBI;
+ }
+ if (!isa<DbgInfoIntrinsic>(BBI) && !isa<PHINode>(BBI))
+ return nullptr;
+ }
+
+ // Do not break infinite loops.
+ BasicBlock *DestBB = BI->getSuccessor(0);
+ if (DestBB == BB)
+ return nullptr;
+
+ if (!canMergeBlocks(BB, DestBB))
+ DestBB = nullptr;
+
+ return DestBB;
+}
+
/// Eliminate blocks that contain only PHI nodes, debug info directives, and an
/// unconditional branch. Passes before isel (e.g. LSR/loopsimplify) often split
/// edges in ways that are non-optimal for isel. Start by eliminating these
@@ -400,48 +446,108 @@
// Note that this intentionally skips the entry block.
for (Function::iterator I = std::next(F.begin()), E = F.end(); I != E;) {
BasicBlock *BB = &*I++;
-
- // If this block doesn't end with an uncond branch, ignore it.
- BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
- if (!BI || !BI->isUnconditional())
+ BasicBlock *DestBB = findDestBlockOfMergeableEmptyBlock(BB);
+ if (!DestBB ||
+ !isMergingEmptyBlockProfitable(BB, DestBB, Preheaders.count(BB)))
continue;
- // If the instruction before the branch (skipping debug info) isn't a phi
- // node, then other stuff is happening here.
- BasicBlock::iterator BBI = BI->getIterator();
- if (BBI != BB->begin()) {
- --BBI;
- while (isa<DbgInfoIntrinsic>(BBI)) {
- if (BBI == BB->begin())
- break;
- --BBI;
- }
- if (!isa<DbgInfoIntrinsic>(BBI) && !isa<PHINode>(BBI))
- continue;
- }
-
- // Do not break infinite loops.
- BasicBlock *DestBB = BI->getSuccessor(0);
- if (DestBB == BB)
- continue;
-
- if (!canMergeBlocks(BB, DestBB))
- continue;
-
- // Do not delete loop preheaders if doing so would create a critical edge.
- // Loop preheaders can be good locations to spill registers. If the
- // preheader is deleted and we create a critical edge, registers may be
- // spilled in the loop body instead.
- if (!DisablePreheaderProtect && Preheaders.count(BB) &&
- !(BB->getSinglePredecessor() && BB->getSinglePredecessor()->getSingleSuccessor()))
- continue;
-
eliminateMostlyEmptyBlock(BB);
MadeChange = true;
}
return MadeChange;
}
+bool CodeGenPrepare::isMergingEmptyBlockProfitable(BasicBlock *BB,
+ BasicBlock *DestBB,
+ bool isPreheader) {
+ // Do not delete loop preheaders if doing so would create a critical edge.
+ // Loop preheaders can be good locations to spill registers. If the
+ // preheader is deleted and we create a critical edge, registers may be
+ // spilled in the loop body instead.
+ if (!DisablePreheaderProtect && isPreheader &&
+ !(BB->getSinglePredecessor() &&
+ BB->getSinglePredecessor()->getSingleSuccessor()))
+ return false;
+
+ // Try to skip merging if the unique predecessor of BB is terminated by a
+ // switch or indirect branch instruction, and BB is used as an incoming block
+ // of PHIs in DestBB. In such case, merging BB and DestBB would cause ISel to
+ // add COPY instructions in the predecessor of BB instead of BB (if it is not
+ // merged). Note that the critical edge created by merging such blocks wont be
+ // split in MachineSink because the jump table is not analyzable. By keeping
+ // such empty block (BB), ISel will place COPY instructions in BB, not in the
+ // predecessor of BB.
+ BasicBlock *Pred = BB->getUniquePredecessor();
+ if (!Pred ||
+ !(isa<SwitchInst>(Pred->getTerminator()) ||
+ isa<IndirectBrInst>(Pred->getTerminator())))
+ return true;
+
+ if (BB->getTerminator() != BB->getFirstNonPHI())
+ return true;
+
+ // We use a simple cost heuristic which determine skipping merging is
+ // profitable if the cost of skipping merging is less than the cost of
+ // merging : Cost(skipping merging) < Cost(merging BB), where the
+ // Cost(skipping merging) is Freq(BB) * (Cost(Copy) + Cost(Branch)), and
+ // the Cost(merging BB) is Freq(Pred) * Cost(Copy).
+ // Assuming Cost(Copy) == Cost(Branch), we could simplify it to :
+ // Freq(Pred) / Freq(BB) > 2.
+ // Note that if there are multiple empty blocks sharing the same incoming
+ // value for the PHIs in the DestBB, we consider them together. In such
+ // case, Cost(merging BB) will be the sum of their frequencies.
+
+ if (!isa<PHINode>(DestBB->begin()))
+ return true;
+
+ SmallPtrSet<BasicBlock *, 16> SameIncomingValueBBs;
+
+ // Find all other incoming blocks from which incoming values of all PHIs in
+ // DestBB are the same as the ones from BB.
+ for (pred_iterator PI = pred_begin(DestBB), E = pred_end(DestBB); PI != E;
+ ++PI) {
+ BasicBlock *DestBBPred = *PI;
+ if (DestBBPred == BB)
+ continue;
+
+ bool HasAllSameValue = true;
+ BasicBlock::const_iterator DestBBI = DestBB->begin();
+ while (const PHINode *DestPN = dyn_cast<PHINode>(DestBBI++)) {
+ if (DestPN->getIncomingValueForBlock(BB) !=
+ DestPN->getIncomingValueForBlock(DestBBPred)) {
+ HasAllSameValue = false;
+ break;
+ }
+ }
+ if (HasAllSameValue)
+ SameIncomingValueBBs.insert(DestBBPred);
+ }
+
+ // See if all BB's incoming values are same as the value from Pred. In this
+ // case, no reason to skip merging because COPYs are expected to be place in
+ // Pred already.
+ if (SameIncomingValueBBs.count(Pred))
+ return true;
+
+ if (!BFI) {
+ Function &F = *BB->getParent();
+ LoopInfo LI{DominatorTree(F)};
+ BPI.reset(new BranchProbabilityInfo(F, LI));
+ BFI.reset(new BlockFrequencyInfo(F, *BPI, LI));
+ }
+
+ BlockFrequency PredFreq = BFI->getBlockFreq(Pred);
+ BlockFrequency BBFreq = BFI->getBlockFreq(BB);
+
+ for (auto SameValueBB : SameIncomingValueBBs)
+ if (SameValueBB->getUniquePredecessor() == Pred &&
+ DestBB == findDestBlockOfMergeableEmptyBlock(SameValueBB))
+ BBFreq += BFI->getBlockFreq(SameValueBB);
+
+ return PredFreq.getFrequency() <=
+ BBFreq.getFrequency() * FreqRatioToSkipMerge;
+}
+
/// Return true if we can merge BB into DestBB if there is a single
/// unconditional branch between them, and BB contains no other non-phi
/// instructions.