[llvm] [DFAJumpThreading] Add MaxOuterUseBlocks threshold (PR #163428)
Hongyu Chen via llvm-commits
llvm-commits at lists.llvm.org
Tue Oct 28 09:22:21 PDT 2025
https://github.com/XChy updated https://github.com/llvm/llvm-project/pull/163428
>From 2c9d30def6b0f13614c8984a0da346dac3c69c00 Mon Sep 17 00:00:00 2001
From: XChy <xxs_chy at outlook.com>
Date: Wed, 15 Oct 2025 01:30:40 +0800
Subject: [PATCH 1/2] [DFAJumpThreading] Add MaxOuterUseBlocks threshold
---
.../Transforms/Scalar/DFAJumpThreading.cpp | 38 ++++++++++++++++---
1 file changed, 33 insertions(+), 5 deletions(-)
diff --git a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
index 66e45ecbde7df..e84ca819b93d8 100644
--- a/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
@@ -122,16 +122,22 @@ static cl::opt<unsigned>
cl::desc("Maximum cost accepted for the transformation"),
cl::Hidden, cl::init(50));
-extern cl::opt<bool> ProfcheckDisableMetadataFixes;
-
-} // namespace llvm
-
static cl::opt<double> MaxClonedRate(
"dfa-max-cloned-rate",
cl::desc(
"Maximum cloned instructions rate accepted for the transformation"),
cl::Hidden, cl::init(7.5));
+static cl::opt<unsigned>
+ MaxOuterUseBlocks("dfa-max-out-use-blocks",
+ cl::desc("Maximum unduplicated blocks with outer uses "
+ "accepted for the transformation"),
+ cl::Hidden, cl::init(40));
+
+extern cl::opt<bool> ProfcheckDisableMetadataFixes;
+
+} // namespace llvm
+
namespace {
class SelectInstToUnfold {
SelectInst *SI;
@@ -965,8 +971,16 @@ struct TransformDFA {
// SLPVectorizer.
// TODO: Thread the switch partially before reaching the threshold.
uint64_t NumOrigInst = 0;
- for (auto *BB : DuplicateMap.keys())
+ uint64_t NumOuterUseBlock = 0;
+ for (auto *BB : DuplicateMap.keys()) {
NumOrigInst += BB->sizeWithoutDebug();
+ // Only unduplicated blocks with single predecessor require new phi
+ // nodes.
+ for (auto *Succ : successors(BB))
+ if (!DuplicateMap.count(Succ) && Succ->getSinglePredecessor())
+ NumOuterUseBlock++;
+ }
+
if (double(NumClonedInst) / double(NumOrigInst) > MaxClonedRate) {
LLVM_DEBUG(dbgs() << "DFA Jump Threading: Not jump threading, too much "
"instructions wll be cloned\n");
@@ -977,6 +991,20 @@ struct TransformDFA {
return false;
}
+ // Too much unduplicated blocks with outer uses may cause too much
+ // insertions of phi nodes for duplicated definitions. TODO: Drop this
+ // threshold if we come up with another way to reduce the number of inserted
+ // phi nodes.
+ if (NumOuterUseBlock > MaxOuterUseBlocks) {
+ LLVM_DEBUG(dbgs() << "DFA Jump Threading: Not jump threading, too much "
+ "blocks with outer uses\n");
+ ORE->emit([&]() {
+ return OptimizationRemarkMissed(DEBUG_TYPE, "NotProfitable", Switch)
+ << "Too much blocks with outer uses.";
+ });
+ return false;
+ }
+
InstructionCost DuplicationCost = 0;
unsigned JumpTableSize = 0;
>From c23bc4b9004c5b76ddbadf688b1b3bfa532e36c7 Mon Sep 17 00:00:00 2001
From: XChy <xxs_chy at outlook.com>
Date: Wed, 29 Oct 2025 00:22:04 +0800
Subject: [PATCH 2/2] Add test coverage
---
.../DFAJumpThreading/max-outer-uses.ll | 253 ++++++++++++++++++
1 file changed, 253 insertions(+)
create mode 100644 llvm/test/Transforms/DFAJumpThreading/max-outer-uses.ll
diff --git a/llvm/test/Transforms/DFAJumpThreading/max-outer-uses.ll b/llvm/test/Transforms/DFAJumpThreading/max-outer-uses.ll
new file mode 100644
index 0000000000000..f059754b8221c
--- /dev/null
+++ b/llvm/test/Transforms/DFAJumpThreading/max-outer-uses.ll
@@ -0,0 +1,253 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt -S -passes=dfa-jump-threading -dfa-max-out-use-blocks=5 %s | FileCheck %s
+
+define void @max_outer_uses_by_switch(i32 %cond) {
+; CHECK-LABEL: define void @max_outer_uses_by_switch(
+; CHECK-SAME: i32 [[COND:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: br label %[[SWITCH_BB:.*]]
+; CHECK: [[SWITCH_BB]]:
+; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[ONE:%.*]], %[[CASE1_SUCC:.*]] ], [ 2, %[[CASE2:.*]] ]
+; CHECK-NEXT: switch i32 [[PHI]], label %[[DEFAULT_DEST:.*]] [
+; CHECK-NEXT: i32 0, label %[[CASE1:.*]]
+; CHECK-NEXT: i32 1, label %[[CASE2]]
+; CHECK-NEXT: ]
+; CHECK: [[CASE1]]:
+; CHECK-NEXT: br label %[[CASE1_SUCC]]
+; CHECK: [[CASE1_SUCC]]:
+; CHECK-NEXT: [[ONE]] = phi i32 [ 1, %[[CASE1]] ]
+; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB]] [
+; CHECK-NEXT: i32 0, label %[[OUTER1:.*]]
+; CHECK-NEXT: i32 1, label %[[OUTER2:.*]]
+; CHECK-NEXT: i32 2, label %[[OUTER3:.*]]
+; CHECK-NEXT: i32 3, label %[[OUTER4:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[CASE2]]:
+; CHECK-NEXT: br label %[[SWITCH_BB]]
+; CHECK: [[OUTER1]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER2]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER3]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER4]]:
+; CHECK-NEXT: ret void
+; CHECK: [[DEFAULT_DEST]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %switch_bb
+
+switch_bb:
+ %phi = phi i32 [ 0, %entry ], [ %one, %case1_succ ], [ 2, %case2 ]
+ switch i32 %phi, label %default_dest [
+ i32 0, label %case1
+ i32 1, label %case2
+ ]
+
+case1:
+ br label %case1_succ
+
+case1_succ:
+ %one = phi i32 [ 1, %case1 ]
+ switch i32 %cond, label %switch_bb [
+ i32 0, label %outer1
+ i32 1, label %outer2
+ i32 2, label %outer3
+ i32 3, label %outer4
+ ]
+
+case2:
+ br label %switch_bb
+
+outer1:
+ ret void
+
+outer2:
+ ret void
+
+outer3:
+ ret void
+
+outer4:
+ ret void
+
+default_dest:
+ ret void
+}
+
+define void @less_outer_uses_by_switch(i32 %cond, i1 %c1) {
+; CHECK-LABEL: define void @less_outer_uses_by_switch(
+; CHECK-SAME: i32 [[COND:%.*]], i1 [[C1:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: br label %[[SWITCH_BB:.*]]
+; CHECK: [[SWITCH_BB]]:
+; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ poison, %[[CASE1_SUCC:.*]] ]
+; CHECK-NEXT: switch i32 [[PHI]], label %[[DEFAULT_DEST:.*]] [
+; CHECK-NEXT: i32 0, label %[[CASE1:.*]]
+; CHECK-NEXT: i32 1, label %[[CASE2:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[SWITCH_BB_JT2:.*]]:
+; CHECK-NEXT: [[PHI_JT2:%.*]] = phi i32 [ 2, %[[CASE2]] ]
+; CHECK-NEXT: br label %[[DEFAULT_DEST]]
+; CHECK: [[SWITCH_BB_JT1:.*]]:
+; CHECK-NEXT: [[PHI_JT1:%.*]] = phi i32 [ [[ONE_JT1:%.*]], %[[CASE1_SUCC_JT1:.*]] ]
+; CHECK-NEXT: br label %[[CASE2]]
+; CHECK: [[CASE1]]:
+; CHECK-NEXT: br label %[[CASE1_SUCC_JT1]]
+; CHECK: [[CASE1_SUCC]]:
+; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB]] [
+; CHECK-NEXT: i32 0, label %[[OUTER1:.*]]
+; CHECK-NEXT: i32 1, label %[[OUTER2:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[CASE1_SUCC_JT1]]:
+; CHECK-NEXT: [[ONE_JT1]] = phi i32 [ 1, %[[CASE1]] ]
+; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB_JT1]] [
+; CHECK-NEXT: i32 0, label %[[OUTER1]]
+; CHECK-NEXT: i32 1, label %[[OUTER2]]
+; CHECK-NEXT: ]
+; CHECK: [[CASE2]]:
+; CHECK-NEXT: br label %[[SWITCH_BB_JT2]]
+; CHECK: [[OUTER1]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER2]]:
+; CHECK-NEXT: ret void
+; CHECK: [[DEFAULT_DEST]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %switch_bb
+
+switch_bb:
+ %phi = phi i32 [ 0, %entry ], [ %one, %case1_succ ], [ 2, %case2 ]
+ switch i32 %phi, label %default_dest [
+ i32 0, label %case1
+ i32 1, label %case2
+ ]
+
+case1:
+ br label %case1_succ
+
+case1_succ:
+ %one = phi i32 [ 1, %case1 ]
+ switch i32 %cond, label %switch_bb [
+ i32 0, label %outer1
+ i32 1, label %outer2
+ ]
+
+case2:
+ br label %switch_bb
+
+outer1:
+ ret void
+
+outer2:
+ ret void
+
+default_dest:
+ ret void
+}
+
+
+define void @max_outer_uses_multi_preds(i32 %cond) {
+; CHECK-LABEL: define void @max_outer_uses_multi_preds(
+; CHECK-SAME: i32 [[COND:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: br label %[[SWITCH_BB:.*]]
+; CHECK: [[SWITCH_BB]]:
+; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ poison, %[[CASE1_SUCC:.*]] ]
+; CHECK-NEXT: switch i32 [[PHI]], label %[[DEFAULT_DEST:.*]] [
+; CHECK-NEXT: i32 0, label %[[CASE1:.*]]
+; CHECK-NEXT: i32 1, label %[[CASE2:.*]]
+; CHECK-NEXT: i32 2, label %[[CASE3:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[SWITCH_BB_JT2:.*]]:
+; CHECK-NEXT: [[PHI_JT2:%.*]] = phi i32 [ 2, %[[CASE2]] ]
+; CHECK-NEXT: br label %[[CASE3]]
+; CHECK: [[SWITCH_BB_JT1:.*]]:
+; CHECK-NEXT: [[PHI_JT1:%.*]] = phi i32 [ [[ONE_JT1:%.*]], %[[CASE1_SUCC_JT1:.*]] ]
+; CHECK-NEXT: br label %[[CASE2]]
+; CHECK: [[CASE1]]:
+; CHECK-NEXT: br label %[[CASE1_SUCC_JT1]]
+; CHECK: [[CASE1_SUCC]]:
+; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB]] [
+; CHECK-NEXT: i32 0, label %[[OUTER1:.*]]
+; CHECK-NEXT: i32 1, label %[[OUTER2:.*]]
+; CHECK-NEXT: i32 2, label %[[OUTER3:.*]]
+; CHECK-NEXT: i32 3, label %[[OUTER4:.*]]
+; CHECK-NEXT: ]
+; CHECK: [[CASE1_SUCC_JT1]]:
+; CHECK-NEXT: [[ONE_JT1]] = phi i32 [ 1, %[[CASE1]] ]
+; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB_JT1]] [
+; CHECK-NEXT: i32 0, label %[[OUTER1]]
+; CHECK-NEXT: i32 1, label %[[OUTER2]]
+; CHECK-NEXT: i32 2, label %[[OUTER3]]
+; CHECK-NEXT: i32 3, label %[[OUTER4]]
+; CHECK-NEXT: ]
+; CHECK: [[CASE2]]:
+; CHECK-NEXT: br label %[[SWITCH_BB_JT2]]
+; CHECK: [[CASE3]]:
+; CHECK-NEXT: switch i32 [[COND]], label %[[OUTER4]] [
+; CHECK-NEXT: i32 0, label %[[OUTER1]]
+; CHECK-NEXT: i32 1, label %[[OUTER2]]
+; CHECK-NEXT: i32 2, label %[[OUTER3]]
+; CHECK-NEXT: ]
+; CHECK: [[OUTER1]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER2]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER3]]:
+; CHECK-NEXT: ret void
+; CHECK: [[OUTER4]]:
+; CHECK-NEXT: ret void
+; CHECK: [[DEFAULT_DEST]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %switch_bb
+
+switch_bb:
+ %phi = phi i32 [ 0, %entry ], [ %one, %case1_succ ], [ 2, %case2 ]
+ switch i32 %phi, label %default_dest [
+ i32 0, label %case1
+ i32 1, label %case2
+ i32 2, label %case3
+ ]
+
+case1:
+ br label %case1_succ
+
+case1_succ:
+ %one = phi i32 [ 1, %case1 ]
+ switch i32 %cond, label %switch_bb [
+ i32 0, label %outer1
+ i32 1, label %outer2
+ i32 2, label %outer3
+ i32 3, label %outer4
+ ]
+
+case2:
+ br label %switch_bb
+
+case3:
+ switch i32 %cond, label %outer4 [
+ i32 0, label %outer1
+ i32 1, label %outer2
+ i32 2, label %outer3
+ ]
+
+outer1:
+ ret void
+
+outer2:
+ ret void
+
+outer3:
+ ret void
+
+outer4:
+ ret void
+
+default_dest:
+ ret void
+}
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