[llvm] [SimplifyCFG] Cache unique predecessors in `simplifyDuplicateSwitchArms` (PR #147384)
Antonio Frighetto via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 8 10:53:12 PDT 2025
https://github.com/antoniofrighetto updated https://github.com/llvm/llvm-project/pull/147384
>From d3f804084deeab0a8dcc828605280dd7d2ea2e04 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Mon, 7 Jul 2025 15:58:03 +0200
Subject: [PATCH 1/3] [SimplifyCFG] Precommit test (NFC)
---
.../Transforms/SimplifyCFG/switch-dup-bbs.ll | 43 +++++++++++++++++++
1 file changed, 43 insertions(+)
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll b/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll
index 32581bbf8f141..c59beb5460220 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll
@@ -199,3 +199,46 @@ exit:
%ret = phi i64 [ 0, %default ], [ 0, %bb1 ], [ 1, %entry ], [ 1, %bb2 ]
ret i64 %ret
}
+
+define i32 @switch_dup_unbounded_predecessors(i32 %val) {
+; SIMPLIFY-CFG-LABEL: define i32 @switch_dup_unbounded_predecessors(
+; SIMPLIFY-CFG-SAME: i32 [[VAL:%.*]]) {
+; SIMPLIFY-CFG-NEXT: [[ENTRY:.*]]:
+; SIMPLIFY-CFG-NEXT: switch i32 [[VAL]], label %[[EXIT:.*]] [
+; SIMPLIFY-CFG-NEXT: i32 99, label %[[BB1:.*]]
+; SIMPLIFY-CFG-NEXT: i32 115, label %[[BB1]]
+; SIMPLIFY-CFG-NEXT: i32 102, label %[[BB2:.*]]
+; SIMPLIFY-CFG-NEXT: i32 70, label %[[BB2]]
+; SIMPLIFY-CFG-NEXT: i32 101, label %[[BB2]]
+; SIMPLIFY-CFG-NEXT: i32 69, label %[[BB2]]
+; SIMPLIFY-CFG-NEXT: i32 103, label %[[BB2]]
+; SIMPLIFY-CFG-NEXT: ]
+; SIMPLIFY-CFG: [[BB1]]:
+; SIMPLIFY-CFG-NEXT: br label %[[EXIT]]
+; SIMPLIFY-CFG: [[BB2]]:
+; SIMPLIFY-CFG-NEXT: br label %[[EXIT]]
+; SIMPLIFY-CFG: [[EXIT]]:
+; SIMPLIFY-CFG-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ 1, %[[BB1]] ], [ 1, %[[BB2]] ]
+; SIMPLIFY-CFG-NEXT: ret i32 [[PHI]]
+;
+entry:
+ switch i32 %val, label %exit [
+ i32 99, label %bb1
+ i32 115, label %bb1
+ i32 102, label %bb2
+ i32 70, label %bb2
+ i32 101, label %bb2
+ i32 69, label %bb2
+ i32 103, label %bb2
+ ]
+
+bb1:
+ br label %exit
+
+bb2:
+ br label %exit
+
+exit:
+ %phi = phi i32 [ 0, %entry ], [ 1, %bb1 ], [ 1, %bb2 ]
+ ret i32 %phi
+}
>From f66669070cbc572c9a9d587dcf881835a6f5284b Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Mon, 7 Jul 2025 15:59:39 +0200
Subject: [PATCH 2/3] [SimplifyCFG] Cache unique predecessors in
`simplifyDuplicateSwitchArms`
Avoid repeatedly querying `getUniquePredecessor` for already-visited
switch successors so as not to incur quadratic runtime.
Fixes: https://github.com/llvm/llvm-project/issues/147239.
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 28 ++++++++++---------
.../Transforms/SimplifyCFG/switch-dup-bbs.ll | 14 ++++------
.../SimplifyCFG/switch-range-to-icmp.ll | 2 +-
3 files changed, 22 insertions(+), 22 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a75f29000ca18..30e62367d32f1 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -7493,7 +7493,7 @@ bool SimplifyCFGOpt::simplifyDuplicateSwitchArms(SwitchInst *SI,
SmallPtrSet<PHINode *, 8> Phis;
SmallPtrSet<BasicBlock *, 8> Seen;
DenseMap<PHINode *, SmallDenseMap<BasicBlock *, Value *, 8>> PhiPredIVs;
- DenseMap<BasicBlock *, SmallVector<unsigned, 4>> BBToSuccessorIndexes;
+ DenseMap<BasicBlock *, SmallVector<unsigned, 32>> BBToSuccessorIndexes;
SmallVector<SwitchSuccWrapper> Cases;
Cases.reserve(SI->getNumSuccessors());
@@ -7505,12 +7505,6 @@ bool SimplifyCFGOpt::simplifyDuplicateSwitchArms(SwitchInst *SI,
if (BB->size() != 1)
continue;
- // FIXME: This case needs some extra care because the terminators other than
- // SI need to be updated. For now, consider only backedges to the SI.
- if (BB->hasNPredecessorsOrMore(4) ||
- BB->getUniquePredecessor() != SI->getParent())
- continue;
-
// FIXME: Relax that the terminator is a BranchInst by checking for equality
// on other kinds of terminators. We decide to only support unconditional
// branches for now for compile time reasons.
@@ -7518,14 +7512,22 @@ bool SimplifyCFGOpt::simplifyDuplicateSwitchArms(SwitchInst *SI,
if (!BI || BI->isConditional())
continue;
- if (Seen.insert(BB).second) {
- // Keep track of which PHIs we need as keys in PhiPredIVs below.
- for (BasicBlock *Succ : BI->successors())
- Phis.insert_range(llvm::make_pointer_range(Succ->phis()));
- // Add the successor only if not previously visited.
- Cases.emplace_back(SwitchSuccWrapper{BB, &PhiPredIVs});
+ if (!Seen.insert(BB).second) {
+ BBToSuccessorIndexes[BB].emplace_back(I);
+ continue;
}
+ // FIXME: This case needs some extra care because the terminators other than
+ // SI need to be updated. For now, consider only backedges to the SI.
+ if (BB->getUniquePredecessor() != SI->getParent())
+ continue;
+
+ // Keep track of which PHIs we need as keys in PhiPredIVs below.
+ for (BasicBlock *Succ : BI->successors())
+ Phis.insert_range(llvm::make_pointer_range(Succ->phis()));
+
+ // Add the successor only if not previously visited.
+ Cases.emplace_back(SwitchSuccWrapper{BB, &PhiPredIVs});
BBToSuccessorIndexes[BB].emplace_back(I);
}
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll b/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll
index c59beb5460220..d2d917de11897 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-dup-bbs.ll
@@ -207,18 +207,16 @@ define i32 @switch_dup_unbounded_predecessors(i32 %val) {
; SIMPLIFY-CFG-NEXT: switch i32 [[VAL]], label %[[EXIT:.*]] [
; SIMPLIFY-CFG-NEXT: i32 99, label %[[BB1:.*]]
; SIMPLIFY-CFG-NEXT: i32 115, label %[[BB1]]
-; SIMPLIFY-CFG-NEXT: i32 102, label %[[BB2:.*]]
-; SIMPLIFY-CFG-NEXT: i32 70, label %[[BB2]]
-; SIMPLIFY-CFG-NEXT: i32 101, label %[[BB2]]
-; SIMPLIFY-CFG-NEXT: i32 69, label %[[BB2]]
-; SIMPLIFY-CFG-NEXT: i32 103, label %[[BB2]]
+; SIMPLIFY-CFG-NEXT: i32 102, label %[[BB1]]
+; SIMPLIFY-CFG-NEXT: i32 70, label %[[BB1]]
+; SIMPLIFY-CFG-NEXT: i32 101, label %[[BB1]]
+; SIMPLIFY-CFG-NEXT: i32 69, label %[[BB1]]
+; SIMPLIFY-CFG-NEXT: i32 103, label %[[BB1]]
; SIMPLIFY-CFG-NEXT: ]
; SIMPLIFY-CFG: [[BB1]]:
; SIMPLIFY-CFG-NEXT: br label %[[EXIT]]
-; SIMPLIFY-CFG: [[BB2]]:
-; SIMPLIFY-CFG-NEXT: br label %[[EXIT]]
; SIMPLIFY-CFG: [[EXIT]]:
-; SIMPLIFY-CFG-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ 1, %[[BB1]] ], [ 1, %[[BB2]] ]
+; SIMPLIFY-CFG-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ 1, %[[BB1]] ]
; SIMPLIFY-CFG-NEXT: ret i32 [[PHI]]
;
entry:
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-range-to-icmp.ll b/llvm/test/Transforms/SimplifyCFG/switch-range-to-icmp.ll
index 4136f33983a2b..8f2ae2d054f1e 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-range-to-icmp.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-range-to-icmp.ll
@@ -149,7 +149,7 @@ unreach2:
define void @pr53208_single_reachable_dest(i8 %sw, ptr %p0) {
; CHECK-LABEL: @pr53208_single_reachable_dest(
-; CHECK-NEXT: group2:
+; CHECK-NEXT: exit:
; CHECK-NEXT: call void @bar(ptr [[P0:%.*]])
; CHECK-NEXT: ret void
;
>From 9cb4f541ddc7ee6adccf9e2df1dab179219e9961 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Tue, 8 Jul 2025 19:51:58 +0200
Subject: [PATCH 3/3] !fixup do not create an entry if does not already exist
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 30e62367d32f1..75c96503d556d 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -7513,7 +7513,9 @@ bool SimplifyCFGOpt::simplifyDuplicateSwitchArms(SwitchInst *SI,
continue;
if (!Seen.insert(BB).second) {
- BBToSuccessorIndexes[BB].emplace_back(I);
+ auto It = BBToSuccessorIndexes.find(BB);
+ if (It != BBToSuccessorIndexes.end())
+ It->second.emplace_back(I);
continue;
}
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