[llvm] 9a17bff - [LoopNest] Allow empty basic blocks without loops
Whitney Tsang via llvm-commits
llvm-commits at lists.llvm.org
Mon Jan 4 12:00:06 PST 2021
Author: Whitney Tsang
Date: 2021-01-04T19:59:50Z
New Revision: 9a17bff4f715a9f3ec89f4eacae8fdea1b74fe79
URL: https://github.com/llvm/llvm-project/commit/9a17bff4f715a9f3ec89f4eacae8fdea1b74fe79
DIFF: https://github.com/llvm/llvm-project/commit/9a17bff4f715a9f3ec89f4eacae8fdea1b74fe79.diff
LOG: [LoopNest] Allow empty basic blocks without loops
Allow loop nests with empty basic blocks without loops in different
levels as perfect.
Reviewers: Meinersbur
Differential Revision: https://reviews.llvm.org/D93665
Added:
Modified:
llvm/include/llvm/Analysis/LoopNestAnalysis.h
llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h
llvm/lib/Analysis/LoopNestAnalysis.cpp
llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
llvm/test/Analysis/LoopNestAnalysis/perfectnest.ll
Removed:
################################################################################
diff --git a/llvm/include/llvm/Analysis/LoopNestAnalysis.h b/llvm/include/llvm/Analysis/LoopNestAnalysis.h
index 4d77d735819f..692909db8341 100644
--- a/llvm/include/llvm/Analysis/LoopNestAnalysis.h
+++ b/llvm/include/llvm/Analysis/LoopNestAnalysis.h
@@ -128,6 +128,12 @@ class LoopNest {
[](const Loop *L) { return L->isLoopSimplifyForm(); });
}
+ /// Return true if all loops in the loop nest are in rotated form.
+ bool areAllLoopsRotatedForm() const {
+ return std::all_of(Loops.begin(), Loops.end(),
+ [](const Loop *L) { return L->isRotatedForm(); });
+ }
+
StringRef getName() const { return Loops.front()->getName(); }
protected:
diff --git a/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h b/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h
index 64c569de1f58..fd5a7daf3add 100644
--- a/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h
+++ b/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h
@@ -244,6 +244,12 @@ unsigned SplitAllCriticalEdges(Function &F,
const CriticalEdgeSplittingOptions &Options =
CriticalEdgeSplittingOptions());
+/// Recursivelly traverse all empty 'single successor' basic blocks of \p From
+/// (if there are any). Return the last basic block found or \p End if it was
+/// reached during the search.
+const BasicBlock &skipEmptyBlockUntil(const BasicBlock *From,
+ const BasicBlock *End);
+
/// Split the edge connecting the specified blocks, and return the newly created
/// basic block between \p From and \p To.
BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To,
diff --git a/llvm/lib/Analysis/LoopNestAnalysis.cpp b/llvm/lib/Analysis/LoopNestAnalysis.cpp
index ef10b7e97461..abc219a8bd32 100644
--- a/llvm/lib/Analysis/LoopNestAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopNestAnalysis.cpp
@@ -16,6 +16,7 @@
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;
@@ -253,49 +254,66 @@ static bool checkLoopsStructure(const Loop &OuterLoop, const Loop &InnerLoop,
// Ensure the only branch that may exist between the loops is the inner loop
// guard.
if (OuterLoopHeader != InnerLoopPreHeader) {
- const BranchInst *BI =
- dyn_cast<BranchInst>(OuterLoopHeader->getTerminator());
-
- if (!BI || BI != InnerLoop.getLoopGuardBranch())
- return false;
-
- bool InnerLoopExitContainsLCSSA = ContainsLCSSAPhi(*InnerLoopExit);
-
- // The successors of the inner loop guard should be the inner loop
- // preheader and the outer loop latch.
- for (const BasicBlock *Succ : BI->successors()) {
- if (Succ == InnerLoopPreHeader)
- continue;
- if (Succ == OuterLoopLatch)
- continue;
-
- // If `InnerLoopExit` contains LCSSA Phi instructions, additional block
- // may be inserted before the `OuterLoopLatch` to which `BI` jumps. The
- // loops are still considered perfectly nested if the extra block only
- // contains Phi instructions from InnerLoopExit and OuterLoopHeader.
- if (InnerLoopExitContainsLCSSA && IsExtraPhiBlock(*Succ) &&
- Succ->getSingleSuccessor() == OuterLoopLatch) {
- // Points to the extra block so that we can reference it later in the
- // final check. We can also conclude that the inner loop is
- // guarded and there exists LCSSA Phi node in the exit block later if we
- // see a non-null `ExtraPhiBlock`.
- ExtraPhiBlock = Succ;
- continue;
- }
+ const BasicBlock &SingleSucc =
+ skipEmptyBlockUntil(OuterLoopHeader, InnerLoopPreHeader);
- DEBUG_WITH_TYPE(VerboseDebug, {
- dbgs() << "Inner loop guard successor " << Succ->getName()
- << " doesn't lead to inner loop preheader or "
- "outer loop latch.\n";
- });
- return false;
+ // no conditional branch present
+ if (&SingleSucc != InnerLoopPreHeader) {
+ const BranchInst *BI = dyn_cast<BranchInst>(SingleSucc.getTerminator());
+
+ if (!BI || BI != InnerLoop.getLoopGuardBranch())
+ return false;
+
+ bool InnerLoopExitContainsLCSSA = ContainsLCSSAPhi(*InnerLoopExit);
+
+ // The successors of the inner loop guard should be the inner loop
+ // preheader or the outer loop latch possibly through empty blocks.
+ for (const BasicBlock *Succ : BI->successors()) {
+ const BasicBlock *PotentialInnerPreHeader = Succ;
+ const BasicBlock *PotentialOuterLatch = Succ;
+
+ // Ensure the inner loop guard successor is empty before skipping
+ // blocks.
+ if (Succ->getInstList().size() == 1) {
+ PotentialInnerPreHeader =
+ &skipEmptyBlockUntil(Succ, InnerLoopPreHeader);
+ PotentialOuterLatch = &skipEmptyBlockUntil(Succ, OuterLoopLatch);
+ }
+
+ if (PotentialInnerPreHeader == InnerLoopPreHeader)
+ continue;
+ if (PotentialOuterLatch == OuterLoopLatch)
+ continue;
+
+ // If `InnerLoopExit` contains LCSSA Phi instructions, additional block
+ // may be inserted before the `OuterLoopLatch` to which `BI` jumps. The
+ // loops are still considered perfectly nested if the extra block only
+ // contains Phi instructions from InnerLoopExit and OuterLoopHeader.
+ if (InnerLoopExitContainsLCSSA && IsExtraPhiBlock(*Succ) &&
+ Succ->getSingleSuccessor() == OuterLoopLatch) {
+ // Points to the extra block so that we can reference it later in the
+ // final check. We can also conclude that the inner loop is
+ // guarded and there exists LCSSA Phi node in the exit block later if
+ // we see a non-null `ExtraPhiBlock`.
+ ExtraPhiBlock = Succ;
+ continue;
+ }
+
+ DEBUG_WITH_TYPE(VerboseDebug, {
+ dbgs() << "Inner loop guard successor " << Succ->getName()
+ << " doesn't lead to inner loop preheader or "
+ "outer loop latch.\n";
+ });
+ return false;
+ }
}
}
- // Ensure the inner loop exit block leads to the outer loop latch.
- const BasicBlock *SuccInner = InnerLoopExit->getSingleSuccessor();
- if (!SuccInner ||
- (SuccInner != OuterLoopLatch && SuccInner != ExtraPhiBlock)) {
+ // Ensure the inner loop exit block lead to the outer loop latch possibly
+ // through empty blocks.
+ const BasicBlock &SuccInner =
+ skipEmptyBlockUntil(InnerLoop.getExitBlock(), OuterLoopLatch);
+ if (&SuccInner != OuterLoopLatch && &SuccInner != ExtraPhiBlock) {
DEBUG_WITH_TYPE(
VerboseDebug,
dbgs() << "Inner loop exit block " << *InnerLoopExit
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 5b8bc184daca..1b89ebe2f7db 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -494,6 +494,31 @@ void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
}
+const BasicBlock &llvm::skipEmptyBlockUntil(const BasicBlock *From,
+ const BasicBlock *End) {
+ assert(From && "Expecting valid From");
+ assert(End && "Expecting valid End");
+
+ if (From == End || !From->getSingleSuccessor())
+ return *From;
+
+ auto IsEmpty = [](const BasicBlock *BB) {
+ return (BB->getInstList().size() == 1);
+ };
+
+ // Visited is used to avoid running into an infinite loop.
+ SmallPtrSet<const BasicBlock *, 4> Visited;
+ const BasicBlock *BB = From->getSingleSuccessor();
+ const BasicBlock *PredBB = BB;
+ while (BB && BB != End && IsEmpty(BB) && !Visited.count(BB)) {
+ Visited.insert(BB);
+ PredBB = BB;
+ BB = BB->getSingleSuccessor();
+ }
+
+ return (BB == End) ? *End : *PredBB;
+}
+
BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, DominatorTree *DT,
LoopInfo *LI, MemorySSAUpdater *MSSAU) {
unsigned SuccNum = GetSuccessorNumber(BB, Succ);
diff --git a/llvm/test/Analysis/LoopNestAnalysis/perfectnest.ll b/llvm/test/Analysis/LoopNestAnalysis/perfectnest.ll
index b7b3b7a7c93e..7593d6f1748b 100644
--- a/llvm/test/Analysis/LoopNestAnalysis/perfectnest.ll
+++ b/llvm/test/Analysis/LoopNestAnalysis/perfectnest.ll
@@ -85,6 +85,55 @@ perf_nest_2D_2_loop_i_end:
ret void
}
+define void @perf_nest_2D_3(i32** %y, i32** %x, i64 signext %nx, i64 signext %ny) {
+; CHECK-LABEL: IsPerfect=true, Depth=1, OutermostLoop: perf_nest_2D_3_loop_j, Loops: ( perf_nest_2D_3_loop_j )
+; CHECK-LABEL: IsPerfect=true, Depth=2, OutermostLoop: perf_nest_2D_3_loop_i, Loops: ( perf_nest_2D_3_loop_i perf_nest_2D_3_loop_j )
+entry:
+ br label %perf_nest_2D_3_loop_i
+
+perf_nest_2D_3_loop_i:
+ %i = phi i64 [ 0, %entry ], [ %inc13, %inc_i ]
+ %cmp21 = icmp slt i64 0, %ny
+ br label %singleSucc
+
+singleSucc:
+ br i1 %cmp21, label %preheader.j, label %for.end
+
+preheader.j:
+ br label %perf_nest_2D_3_loop_j
+
+perf_nest_2D_3_loop_j:
+ %j = phi i64 [ 0, %preheader.j ], [ %inc, %inc_j ]
+ %arrayidx = getelementptr inbounds i32*, i32** %x, i64 %j
+ %0 = load i32*, i32** %arrayidx, align 8
+ %arrayidx6 = getelementptr inbounds i32, i32* %0, i64 %j
+ %1 = load i32, i32* %arrayidx6, align 4
+ %arrayidx8 = getelementptr inbounds i32*, i32** %y, i64 %j
+ %2 = load i32*, i32** %arrayidx8, align 8
+ %arrayidx11 = getelementptr inbounds i32, i32* %2, i64 %i
+ store i32 %1, i32* %arrayidx11, align 4
+ br label %inc_j
+
+inc_j:
+ %inc = add nsw i64 %j, 1
+ %cmp2 = icmp slt i64 %inc, %ny
+ br i1 %cmp2, label %perf_nest_2D_3_loop_j, label %for.exit
+
+for.exit:
+ br label %for.end
+
+for.end:
+ br label %inc_i
+
+inc_i:
+ %inc13 = add nsw i64 %i, 1
+ %cmp = icmp slt i64 %inc13, %nx
+ br i1 %cmp, label %perf_nest_2D_3_loop_i, label %perf_nest_2D_3_loop_i_end
+
+perf_nest_2D_3_loop_i_end:
+ ret void
+}
+
; Test a perfect 3-dim loop nest of the form:
; for (i=0; i<nx; ++i)
; for (j=0; j<ny; ++j)
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