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Oct 29, 2025
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[DFAJumpThreading] Add MaxOuterUseBlocks threshold #163428

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2c9d30d
[DFAJumpThreading] Add MaxOuterUseBlocks threshold
XChy Oct 14, 2025
c23bc4b
Add test coverage
XChy Oct 28, 2025
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Improve tests
XChy Oct 29, 2025
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38 changes: 33 additions & 5 deletions llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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");
Expand All @@ -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;
Expand Down
326 changes: 326 additions & 0 deletions llvm/test/Transforms/DFAJumpThreading/max-outer-uses.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,326 @@
; 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

declare void @use(i32)

define void @max_outer_uses_by_switch(i32 %cond, ptr %p) {
; CHECK-LABEL: define void @max_outer_uses_by_switch(
; CHECK-SAME: i32 [[COND:%.*]], ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[SWITCH_BB:.*]]
; CHECK: [[SWITCH_BB]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[DETERMINE:%.*]], %[[SUB_SWITCH_BB:.*]] ], [ 2, %[[CASE2:.*]] ]
; 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: [[CASE1]]:
; CHECK-NEXT: br label %[[SUB_SWITCH_BB]]
; CHECK: [[CASE3]]:
; CHECK-NEXT: br label %[[SUB_SWITCH_BB]]
; CHECK: [[SUB_SWITCH_BB]]:
; CHECK-NEXT: [[DETERMINE]] = phi i32 [ 1, %[[CASE1]] ], [ 3, %[[CASE3]] ]
; CHECK-NEXT: [[DEF:%.*]] = load i32, ptr [[P]], align 4
; 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: call void @use(i32 [[DEF]])
; CHECK-NEXT: ret void
; CHECK: [[OUTER2]]:
; CHECK-NEXT: call void @use(i32 [[DEF]])
; CHECK-NEXT: ret void
; CHECK: [[OUTER3]]:
; CHECK-NEXT: call void @use(i32 [[DEF]])
; CHECK-NEXT: ret void
; CHECK: [[OUTER4]]:
; CHECK-NEXT: call void @use(i32 [[DEF]])
; CHECK-NEXT: ret void
; CHECK: [[DEFAULT_DEST]]:
; CHECK-NEXT: ret void
;
entry:
br label %switch_bb

switch_bb:
%phi = phi i32 [ 0, %entry ], [ %determine, %sub_switch_bb ], [ 2, %case2 ]
switch i32 %phi, label %default_dest [
i32 0, label %case1
i32 1, label %case2
i32 2, label %case3
]

case1:
br label %sub_switch_bb

case3:
br label %sub_switch_bb

sub_switch_bb:
%determine = phi i32 [ 1, %case1 ], [ 3, %case3 ]
%def = load i32, ptr %p
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:
call void @use(i32 %def)
ret void

outer2:
call void @use(i32 %def)
ret void

outer3:
call void @use(i32 %def)
ret void

outer4:
call void @use(i32 %def)
ret void

default_dest:
ret void
}

define void @less_outer_uses_by_switch(i32 %cond, ptr %p) {
; CHECK-LABEL: define void @less_outer_uses_by_switch(
; CHECK-SAME: i32 [[COND:%.*]], ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[SWITCH_BB:.*]]
; CHECK: [[SWITCH_BB]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ poison, %[[SUB_SWITCH_BB:.*]] ]
; 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_JT3:.*]]:
; CHECK-NEXT: [[PHI_JT3:%.*]] = phi i32 [ [[DETERMINE_JT3:%.*]], %[[SUB_SWITCH_BB_JT3:.*]] ]
; CHECK-NEXT: br label %[[DEFAULT_DEST]]
; CHECK: [[SWITCH_BB_JT1:.*]]:
; CHECK-NEXT: [[PHI_JT1:%.*]] = phi i32 [ [[DETERMINE_JT1:%.*]], %[[SUB_SWITCH_BB_JT1:.*]] ]
; CHECK-NEXT: br label %[[CASE2]]
; CHECK: [[CASE1]]:
; CHECK-NEXT: br label %[[SUB_SWITCH_BB_JT1]]
; CHECK: [[CASE3]]:
; CHECK-NEXT: br label %[[SUB_SWITCH_BB_JT3]]
; CHECK: [[SUB_SWITCH_BB]]:
; CHECK-NEXT: [[DEF:%.*]] = load i32, ptr [[P]], align 4
; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB]] [
; CHECK-NEXT: i32 0, label %[[OUTER1:.*]]
; CHECK-NEXT: ]
; CHECK: [[SUB_SWITCH_BB_JT3]]:
; CHECK-NEXT: [[DETERMINE_JT3]] = phi i32 [ 3, %[[CASE3]] ]
; CHECK-NEXT: [[DEF_JT3:%.*]] = load i32, ptr [[P]], align 4
; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB_JT3]] [
; CHECK-NEXT: i32 0, label %[[OUTER1]]
; CHECK-NEXT: ]
; CHECK: [[SUB_SWITCH_BB_JT1]]:
; CHECK-NEXT: [[DETERMINE_JT1]] = phi i32 [ 1, %[[CASE1]] ]
; CHECK-NEXT: [[DEF_JT1:%.*]] = load i32, ptr [[P]], align 4
; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB_JT1]] [
; CHECK-NEXT: i32 0, label %[[OUTER1]]
; CHECK-NEXT: ]
; CHECK: [[CASE2]]:
; CHECK-NEXT: br label %[[SWITCH_BB_JT2]]
; CHECK: [[OUTER1]]:
; CHECK-NEXT: [[DEF1:%.*]] = phi i32 [ [[DEF_JT3]], %[[SUB_SWITCH_BB_JT3]] ], [ [[DEF_JT1]], %[[SUB_SWITCH_BB_JT1]] ], [ [[DEF]], %[[SUB_SWITCH_BB]] ]
; CHECK-NEXT: call void @use(i32 [[DEF1]])
; CHECK-NEXT: ret void
; CHECK: [[DEFAULT_DEST]]:
; CHECK-NEXT: ret void
;
entry:
br label %switch_bb

switch_bb:
%phi = phi i32 [ 0, %entry ], [ %determine, %sub_switch_bb ], [ 2, %case2 ]
switch i32 %phi, label %default_dest [
i32 0, label %case1
i32 1, label %case2
i32 2, label %case3
]

case1:
br label %sub_switch_bb

case3:
br label %sub_switch_bb

sub_switch_bb:
%determine = phi i32 [ 1, %case1 ], [ 3, %case3 ]
%def = load i32, ptr %p
switch i32 %cond, label %switch_bb [
i32 0, label %outer1
]

case2:
br label %switch_bb

outer1:
call void @use(i32 %def)
ret void

default_dest:
ret void
}


define void @max_outer_uses_multi_preds(i32 %cond, ptr %p) {
; CHECK-LABEL: define void @max_outer_uses_multi_preds(
; CHECK-SAME: i32 [[COND:%.*]], ptr [[P:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[SWITCH_BB:.*]]
; CHECK: [[SWITCH_BB]]:
; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ poison, %[[SUB_SWITCH_BB:.*]] ]
; 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: i32 3, label %[[CASE4:.*]]
; CHECK-NEXT: ]
; CHECK: [[SWITCH_BB_JT2:.*]]:
; CHECK-NEXT: [[PHI_JT2:%.*]] = phi i32 [ 2, %[[CASE2]] ]
; CHECK-NEXT: br label %[[CASE3]]
; CHECK: [[SWITCH_BB_JT3:.*]]:
; CHECK-NEXT: [[PHI_JT3:%.*]] = phi i32 [ [[DETERMINE_JT3:%.*]], %[[SUB_SWITCH_BB_JT3:.*]] ]
; CHECK-NEXT: br label %[[CASE4]]
; CHECK: [[SWITCH_BB_JT1:.*]]:
; CHECK-NEXT: [[PHI_JT1:%.*]] = phi i32 [ [[DETERMINE_JT1:%.*]], %[[SUB_SWITCH_BB_JT1:.*]] ]
; CHECK-NEXT: br label %[[CASE2]]
; CHECK: [[CASE1]]:
; CHECK-NEXT: br label %[[SUB_SWITCH_BB_JT1]]
; CHECK: [[CASE3]]:
; CHECK-NEXT: br label %[[SUB_SWITCH_BB_JT3]]
; CHECK: [[SUB_SWITCH_BB]]:
; CHECK-NEXT: [[DEF:%.*]] = load i32, ptr [[P]], align 4
; 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: [[SUB_SWITCH_BB_JT3]]:
; CHECK-NEXT: [[DETERMINE_JT3]] = phi i32 [ 3, %[[CASE3]] ]
; CHECK-NEXT: [[DEF_JT3:%.*]] = load i32, ptr [[P]], align 4
; CHECK-NEXT: switch i32 [[COND]], label %[[SWITCH_BB_JT3]] [
; 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: [[SUB_SWITCH_BB_JT1]]:
; CHECK-NEXT: [[DETERMINE_JT1]] = phi i32 [ 1, %[[CASE1]] ]
; CHECK-NEXT: [[DEF_JT1:%.*]] = load i32, ptr [[P]], align 4
; 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: [[CASE4]]:
; CHECK-NEXT: [[DEF1:%.*]] = load i32, ptr [[P]], align 4
; 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: [[CASE2]]:
; CHECK-NEXT: br label %[[SWITCH_BB_JT2]]
; CHECK: [[OUTER1]]:
; CHECK-NEXT: [[PHI1:%.*]] = phi i32 [ [[DEF]], %[[SUB_SWITCH_BB]] ], [ [[DEF1]], %[[CASE4]] ], [ [[DEF_JT1]], %[[SUB_SWITCH_BB_JT1]] ], [ [[DEF_JT3]], %[[SUB_SWITCH_BB_JT3]] ]
; CHECK-NEXT: call void @use(i32 [[PHI1]])
; CHECK-NEXT: ret void
; CHECK: [[OUTER2]]:
; CHECK-NEXT: [[PHI2:%.*]] = phi i32 [ [[DEF]], %[[SUB_SWITCH_BB]] ], [ [[DEF1]], %[[CASE4]] ], [ [[DEF_JT1]], %[[SUB_SWITCH_BB_JT1]] ], [ [[DEF_JT3]], %[[SUB_SWITCH_BB_JT3]] ]
; CHECK-NEXT: call void @use(i32 [[PHI2]])
; CHECK-NEXT: ret void
; CHECK: [[OUTER3]]:
; CHECK-NEXT: [[PHI3:%.*]] = phi i32 [ [[DEF]], %[[SUB_SWITCH_BB]] ], [ [[DEF1]], %[[CASE4]] ], [ [[DEF_JT1]], %[[SUB_SWITCH_BB_JT1]] ], [ [[DEF_JT3]], %[[SUB_SWITCH_BB_JT3]] ]
; CHECK-NEXT: call void @use(i32 [[PHI3]])
; CHECK-NEXT: ret void
; CHECK: [[OUTER4]]:
; CHECK-NEXT: [[PHI4:%.*]] = phi i32 [ [[DEF]], %[[SUB_SWITCH_BB]] ], [ [[DEF1]], %[[CASE4]] ], [ [[DEF_JT1]], %[[SUB_SWITCH_BB_JT1]] ], [ [[DEF_JT3]], %[[SUB_SWITCH_BB_JT3]] ]
; CHECK-NEXT: call void @use(i32 [[PHI4]])
; CHECK-NEXT: ret void
; CHECK: [[DEFAULT_DEST]]:
; CHECK-NEXT: ret void
;
entry:
br label %switch_bb

switch_bb:
%phi = phi i32 [ 0, %entry ], [ %determine, %sub_switch_bb ], [ 2, %case2 ]
switch i32 %phi, label %default_dest [
i32 0, label %case1
i32 1, label %case2
i32 2, label %case3
i32 3, label %case4
]

case1:
br label %sub_switch_bb

case3:
br label %sub_switch_bb

sub_switch_bb:
%determine = phi i32 [ 1, %case1 ], [ 3, %case3 ]
%def = load i32, ptr %p
switch i32 %cond, label %switch_bb [
i32 0, label %outer1
i32 1, label %outer2
i32 2, label %outer3
i32 3, label %outer4
]

case4:
%def1 = load i32, ptr %p
switch i32 %cond, label %outer4 [
i32 0, label %outer1
i32 1, label %outer2
i32 2, label %outer3
]

case2:
br label %switch_bb

outer1:
%phi1 = phi i32 [ %def, %sub_switch_bb ], [ %def1, %case4 ]
call void @use(i32 %phi1)
ret void

outer2:
%phi2 = phi i32 [ %def, %sub_switch_bb ], [ %def1, %case4 ]
call void @use(i32 %phi2)
ret void

outer3:
%phi3 = phi i32 [ %def, %sub_switch_bb ], [ %def1, %case4 ]
call void @use(i32 %phi3)
ret void

outer4:
%phi4 = phi i32 [ %def, %sub_switch_bb ], [ %def1, %case4 ]
call void @use(i32 %phi4)
ret void

default_dest:
ret void
}