[llvm] [SimplifyCFG] Simplify uncond br with icmp & select (PR #165580)
Kunqiu Chen via llvm-commits
llvm-commits at lists.llvm.org
Sat Nov 8 04:04:24 PST 2025
https://github.com/Camsyn updated https://github.com/llvm/llvm-project/pull/165580
>From ffc7b77221848d94e50c8dad30bbb7bcd8c500d5 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Wed, 29 Oct 2025 23:23:40 +0800
Subject: [PATCH 01/10] [SimplifyCFG] Simplify uncond br with icmp & select
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 177 +++++++++++++++++++++-
1 file changed, 175 insertions(+), 2 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 4fac5d36ddb3f..f21a32fdb240d 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -301,7 +301,9 @@ class SimplifyCFGOpt {
bool tryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI,
IRBuilder<> &Builder);
-
+ bool tryToSimplifyUncondBranchWithICmpSelectInIt(ICmpInst *ICI,
+ SelectInst *Select,
+ IRBuilder<> &Builder);
bool hoistCommonCodeFromSuccessors(Instruction *TI, bool AllInstsEqOnly);
bool hoistSuccIdenticalTerminatorToSwitchOrIf(
Instruction *TI, Instruction *I1,
@@ -5116,6 +5118,172 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
return true;
}
+/// Similar to tryToSimplifyUncondBranchWithICmpInIt, but handle a more generic
+/// case. This is called when we find an icmp instruction (a seteq/setne with a
+/// constant) and its following select instruction as the only TWO instruction
+/// in a block that ends with an uncond branch. We are looking for a very
+/// specific pattern that occurs when "
+/// if (A == 1) return C1;
+/// if (A == 2) return C2;
+/// if (A < 3) return C3;
+/// return C4;
+/// " gets simplified. In this case, we merge the first two "branches of icmp"
+/// into a switch, but then the default value goes to an uncond block with a lt
+/// icmp and select in it, as InstCombine can not simplify "A < 3" as "A == 2".
+/// After SimplifyCFG and other subsequent optimizations (e.g., SCCP), we might
+/// get something like:
+///
+/// case1:
+/// switch i8 %A, label %DEFAULT [ i8 0, label %end i8 1, label %case2 ]
+/// case2:
+/// br label %end
+/// DEFAULT:
+/// %tmp = icmp eq i8 %A, 2
+/// %val = select i1 %tmp, i8 C3, i8 C4
+/// br label %end
+/// end:
+/// _ = phi i8 [ C1, %case1 ], [ C2, %case2 ], [ %val, %DEFAULT ]
+///
+/// We prefer to split the edge to 'end' so that there are TWO entries of V3/V4
+/// to the PHI, merging the icmp & select into the switch, as follows:
+///
+/// case1:
+/// switch i8 %A, label %DEFAULT [
+/// i8 0, label %end
+/// i8 1, label %case2
+/// i8 2, label %case3
+/// ]
+/// case2:
+/// br label %end
+/// case3:
+/// br label %end
+/// DEFAULT:
+/// br label %end
+/// end:
+/// _ = phi i8 [ C1, %case1 ], [ C2, %case2 ], [ C3, %case2 ], [ C4, %DEFAULT]
+bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
+ ICmpInst *ICI, SelectInst *Select, IRBuilder<> &Builder) {
+ BasicBlock *BB = ICI->getParent();
+
+ // If the block has any PHIs in it or the icmp/select has multiple uses, it is too
+ // complex.
+ if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse() || !Select->hasOneUse())
+ return false;
+
+ // The pattern we're looking for is where our only predecessor is a switch on
+ // 'V' and this block is the default case for the switch. In this case we can
+ // fold the compared value into the switch to simplify things.
+ BasicBlock *Pred = BB->getSinglePredecessor();
+ if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
+ return false;
+
+ Value *IcmpCond;
+ ConstantInt *NewCaseVal;
+ CmpPredicate Predicate;
+
+ // Match icmp X, C
+ if (!match(ICI, m_ICmp(Predicate, m_Value(IcmpCond), m_ConstantInt(NewCaseVal))))
+ return false;
+
+ Value *SelectCond, *SelectTrueVal, *SelectFalseVal;
+ // Match select Cond, TrueVal, FalseVal
+ if (!match(Select, m_Select(m_Value(SelectCond), m_Value(SelectTrueVal),
+ m_Value(SelectFalseVal))))
+ return false;
+
+ // Check if the select condition is the same as the icmp condition.
+ if (SelectCond != ICI)
+ return false;
+
+ SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
+ if (SI->getCondition() != IcmpCond)
+ return false;
+
+ // If BB is reachable on a non-default case, then we simply know the value of
+ // V in this block. Substitute it and constant fold the icmp instruction
+ // away.
+ if (SI->getDefaultDest() != BB) {
+ ConstantInt *VVal = SI->findCaseDest(BB);
+ assert(VVal && "Should have a unique destination value");
+ ICI->setOperand(0, VVal);
+
+ if (Value *V = simplifyInstruction(ICI, {DL, ICI})) {
+ ICI->replaceAllUsesWith(V);
+ ICI->eraseFromParent();
+ }
+ // BB is now empty, so it is likely to simplify away.
+ return requestResimplify();
+ }
+
+ // Ok, the block is reachable from the default dest. If the constant we're
+ // comparing exists in one of the other edges, then we can constant fold ICI
+ // and zap it.
+ if (SI->findCaseValue(NewCaseVal) != SI->case_default()) {
+ Value *V;
+ if (Predicate == ICmpInst::ICMP_EQ)
+ V = ConstantInt::getFalse(BB->getContext());
+ else
+ V = ConstantInt::getTrue(BB->getContext());
+
+ ICI->replaceAllUsesWith(V);
+ ICI->eraseFromParent();
+ // BB is now empty, so it is likely to simplify away.
+ return requestResimplify();
+ }
+
+ // The use of the select has to be in the 'end' block, by the only PHI node in
+ // the block.
+ BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
+ PHINode *PHIUse = dyn_cast<PHINode>(Select->user_back());
+ if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
+ isa<PHINode>(++BasicBlock::iterator(PHIUse)))
+ return false;
+
+ // If the icmp is a SETEQ, then the default dest gets SelectFalseVal, the new edge gets
+ // SelectTrueVal in the PHI.
+ Value *DefaultCst = SelectFalseVal;
+ Value *NewCst = SelectTrueVal;
+
+ if (ICI->getPredicate() == ICmpInst::ICMP_NE)
+ std::swap(DefaultCst, NewCst);
+
+ // Replace Select (which is used by the PHI for the default value) with
+ // SelectFalseVal or SelectTrueVal depending on if ICI is EQ or NE.
+ Select->replaceAllUsesWith(DefaultCst);
+ Select->eraseFromParent();
+ ICI->eraseFromParent();
+
+ SmallVector<DominatorTree::UpdateType, 2> Updates;
+
+ // Okay, the switch goes to this block on a default value. Add an edge from
+ // the switch to the merge point on the compared value.
+ BasicBlock *NewBB =
+ BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB);
+ {
+ SwitchInstProfUpdateWrapper SIW(*SI);
+ auto W0 = SIW.getSuccessorWeight(0);
+ SwitchInstProfUpdateWrapper::CaseWeightOpt NewW;
+ if (W0) {
+ NewW = ((uint64_t(*W0) + 1) >> 1);
+ SIW.setSuccessorWeight(0, *NewW);
+ }
+ SIW.addCase(NewCaseVal, NewBB, NewW);
+ if (DTU)
+ Updates.push_back({DominatorTree::Insert, Pred, NewBB});
+ }
+
+ // NewBB branches to the phi block, add the uncond branch and the phi entry.
+ Builder.SetInsertPoint(NewBB);
+ Builder.SetCurrentDebugLocation(SI->getDebugLoc());
+ Builder.CreateBr(SuccBlock);
+ PHIUse->addIncoming(NewCst, NewBB);
+ if (DTU) {
+ Updates.push_back({DominatorTree::Insert, NewBB, SuccBlock});
+ DTU->applyUpdates(Updates);
+ }
+ return true;
+}
+
/// The specified branch is a conditional branch.
/// Check to see if it is branching on an or/and chain of icmp instructions, and
/// fold it into a switch instruction if so.
@@ -8167,13 +8335,18 @@ bool SimplifyCFGOpt::simplifyUncondBranch(BranchInst *BI,
// If the only instruction in the block is a seteq/setne comparison against a
// constant, try to simplify the block.
- if (ICmpInst *ICI = dyn_cast<ICmpInst>(I))
+ if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
if (ICI->isEquality() && isa<ConstantInt>(ICI->getOperand(1))) {
++I;
if (I->isTerminator() &&
tryToSimplifyUncondBranchWithICmpInIt(ICI, Builder))
return true;
+ if (isa<SelectInst>(I) && I->getNextNode()->isTerminator() &&
+ tryToSimplifyUncondBranchWithICmpSelectInIt(ICI, cast<SelectInst>(I),
+ Builder))
+ return true;
}
+ }
// See if we can merge an empty landing pad block with another which is
// equivalent.
>From 7a37f17ed38be33664be73e00f295df037ad3c5c Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Wed, 29 Oct 2025 23:59:04 +0800
Subject: [PATCH 02/10] Make formatter happy
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index f21a32fdb240d..bbef4c83152dd 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5165,8 +5165,8 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
ICmpInst *ICI, SelectInst *Select, IRBuilder<> &Builder) {
BasicBlock *BB = ICI->getParent();
- // If the block has any PHIs in it or the icmp/select has multiple uses, it is too
- // complex.
+ // If the block has any PHIs in it or the icmp/select has multiple uses, it is
+ // too complex.
if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse() || !Select->hasOneUse())
return false;
@@ -5176,13 +5176,14 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
BasicBlock *Pred = BB->getSinglePredecessor();
if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
return false;
-
+
Value *IcmpCond;
ConstantInt *NewCaseVal;
CmpPredicate Predicate;
// Match icmp X, C
- if (!match(ICI, m_ICmp(Predicate, m_Value(IcmpCond), m_ConstantInt(NewCaseVal))))
+ if (!match(ICI,
+ m_ICmp(Predicate, m_Value(IcmpCond), m_ConstantInt(NewCaseVal))))
return false;
Value *SelectCond, *SelectTrueVal, *SelectFalseVal;
@@ -5239,8 +5240,8 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
isa<PHINode>(++BasicBlock::iterator(PHIUse)))
return false;
- // If the icmp is a SETEQ, then the default dest gets SelectFalseVal, the new edge gets
- // SelectTrueVal in the PHI.
+ // If the icmp is a SETEQ, then the default dest gets SelectFalseVal, the new
+ // edge gets SelectTrueVal in the PHI.
Value *DefaultCst = SelectFalseVal;
Value *NewCst = SelectTrueVal;
>From 8dbdccd70dd852254eb8787b23690b5cb2dbb31a Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Thu, 30 Oct 2025 22:25:08 +0800
Subject: [PATCH 03/10] Regenerate some tests
---
.../Transforms/SimplifyCFG/ARM/switch-to-lookup-table.ll | 4 ++--
.../Transforms/SimplifyCFG/switch-transformations-no-lut.ll | 5 ++---
2 files changed, 4 insertions(+), 5 deletions(-)
diff --git a/llvm/test/Transforms/SimplifyCFG/ARM/switch-to-lookup-table.ll b/llvm/test/Transforms/SimplifyCFG/ARM/switch-to-lookup-table.ll
index 6def8f4eeb089..a51b816846cdc 100644
--- a/llvm/test/Transforms/SimplifyCFG/ARM/switch-to-lookup-table.ll
+++ b/llvm/test/Transforms/SimplifyCFG/ARM/switch-to-lookup-table.ll
@@ -15,8 +15,8 @@
; DISABLE-NOT: @{{.*}} = private unnamed_addr constant [3 x ptr] [ptr @c1, ptr @c2, ptr @c3]
; ENABLE: @{{.*}} = private unnamed_addr constant [3 x ptr] [ptr @g1, ptr @g2, ptr @g3]
; DISABLE-NOT: @{{.*}} = private unnamed_addr constant [3 x ptr] [ptr @g1, ptr @g2, ptr @g3]
-; ENABLE: @{{.*}} = private unnamed_addr constant [3 x ptr] [ptr @f1, ptr @f2, ptr @f3]
-; DISABLE-NOT: @{{.*}} = private unnamed_addr constant [3 x ptr] [ptr @f1, ptr @f2, ptr @f3]
+; ENABLE: @{{.*}} = private unnamed_addr constant [4 x ptr] [ptr @f1, ptr @f2, ptr @f3, ptr @f4]
+; DISABLE-NOT: @{{.*}} = private unnamed_addr constant [4 x ptr] [ptr @f1, ptr @f2, ptr @f3, ptr @f4]
target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "armv7a--none-eabi"
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
index 25267dcc6dbcb..48be76c19e48f 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
@@ -410,13 +410,12 @@ define i1 @single_value_with_mask(i32 %x) {
; OPTNOLUT-NEXT: i32 21, label %[[END]]
; OPTNOLUT-NEXT: i32 48, label %[[END]]
; OPTNOLUT-NEXT: i32 16, label %[[END]]
+; OPTNOLUT-NEXT: i32 80, label %[[END]]
; OPTNOLUT-NEXT: ]
; OPTNOLUT: [[DEFAULT]]:
-; OPTNOLUT-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
-; OPTNOLUT-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i1 false, i1 true
; OPTNOLUT-NEXT: br label %[[END]]
; OPTNOLUT: [[END]]:
-; OPTNOLUT-NEXT: [[RES:%.*]] = phi i1 [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ [[SEL]], %[[DEFAULT]] ]
+; OPTNOLUT-NEXT: [[RES:%.*]] = phi i1 [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ true, %[[DEFAULT]] ], [ false, %[[ENTRY]] ]
; OPTNOLUT-NEXT: ret i1 [[RES]]
;
; TTINOLUT-LABEL: define i1 @single_value_with_mask(
>From 14dc5c207aa328fd4b82393390e7200afd171619 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Thu, 30 Oct 2025 22:30:10 +0800
Subject: [PATCH 04/10] Make formatter happy
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index bbef4c83152dd..927d17cfc6d2c 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5122,7 +5122,7 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
/// case. This is called when we find an icmp instruction (a seteq/setne with a
/// constant) and its following select instruction as the only TWO instruction
/// in a block that ends with an uncond branch. We are looking for a very
-/// specific pattern that occurs when "
+/// specific pattern that occurs when "
/// if (A == 1) return C1;
/// if (A == 2) return C2;
/// if (A < 3) return C3;
>From 3ecd2868a5f581b9d58276effd8e20932d5b4834 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Wed, 5 Nov 2025 00:55:36 +0800
Subject: [PATCH 05/10] Refactor to follow the review comments
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 144 +++++-----------------
1 file changed, 29 insertions(+), 115 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 927d17cfc6d2c..a9756b09a104e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5013,109 +5013,9 @@ bool SimplifyCFGOpt::simplifyIndirectBrOnSelect(IndirectBrInst *IBI,
/// the PHI, merging the third icmp into the switch.
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder) {
- BasicBlock *BB = ICI->getParent();
-
- // If the block has any PHIs in it or the icmp has multiple uses, it is too
- // complex.
- if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse())
- return false;
-
- Value *V = ICI->getOperand(0);
- ConstantInt *Cst = cast<ConstantInt>(ICI->getOperand(1));
-
- // The pattern we're looking for is where our only predecessor is a switch on
- // 'V' and this block is the default case for the switch. In this case we can
- // fold the compared value into the switch to simplify things.
- BasicBlock *Pred = BB->getSinglePredecessor();
- if (!Pred || !isa<SwitchInst>(Pred->getTerminator()))
- return false;
-
- SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
- if (SI->getCondition() != V)
- return false;
-
- // If BB is reachable on a non-default case, then we simply know the value of
- // V in this block. Substitute it and constant fold the icmp instruction
- // away.
- if (SI->getDefaultDest() != BB) {
- ConstantInt *VVal = SI->findCaseDest(BB);
- assert(VVal && "Should have a unique destination value");
- ICI->setOperand(0, VVal);
-
- if (Value *V = simplifyInstruction(ICI, {DL, ICI})) {
- ICI->replaceAllUsesWith(V);
- ICI->eraseFromParent();
- }
- // BB is now empty, so it is likely to simplify away.
- return requestResimplify();
- }
-
- // Ok, the block is reachable from the default dest. If the constant we're
- // comparing exists in one of the other edges, then we can constant fold ICI
- // and zap it.
- if (SI->findCaseValue(Cst) != SI->case_default()) {
- Value *V;
- if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
- V = ConstantInt::getFalse(BB->getContext());
- else
- V = ConstantInt::getTrue(BB->getContext());
-
- ICI->replaceAllUsesWith(V);
- ICI->eraseFromParent();
- // BB is now empty, so it is likely to simplify away.
- return requestResimplify();
- }
-
- // The use of the icmp has to be in the 'end' block, by the only PHI node in
- // the block.
- BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
- PHINode *PHIUse = dyn_cast<PHINode>(ICI->user_back());
- if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
- isa<PHINode>(++BasicBlock::iterator(PHIUse)))
- return false;
-
- // If the icmp is a SETEQ, then the default dest gets false, the new edge gets
- // true in the PHI.
- Constant *DefaultCst = ConstantInt::getTrue(BB->getContext());
- Constant *NewCst = ConstantInt::getFalse(BB->getContext());
-
- if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
- std::swap(DefaultCst, NewCst);
-
- // Replace ICI (which is used by the PHI for the default value) with true or
- // false depending on if it is EQ or NE.
- ICI->replaceAllUsesWith(DefaultCst);
- ICI->eraseFromParent();
-
- SmallVector<DominatorTree::UpdateType, 2> Updates;
-
- // Okay, the switch goes to this block on a default value. Add an edge from
- // the switch to the merge point on the compared value.
- BasicBlock *NewBB =
- BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB);
- {
- SwitchInstProfUpdateWrapper SIW(*SI);
- auto W0 = SIW.getSuccessorWeight(0);
- SwitchInstProfUpdateWrapper::CaseWeightOpt NewW;
- if (W0) {
- NewW = ((uint64_t(*W0) + 1) >> 1);
- SIW.setSuccessorWeight(0, *NewW);
- }
- SIW.addCase(Cst, NewBB, NewW);
- if (DTU)
- Updates.push_back({DominatorTree::Insert, Pred, NewBB});
- }
-
- // NewBB branches to the phi block, add the uncond branch and the phi entry.
- Builder.SetInsertPoint(NewBB);
- Builder.SetCurrentDebugLocation(SI->getDebugLoc());
- Builder.CreateBr(SuccBlock);
- PHIUse->addIncoming(NewCst, NewBB);
- if (DTU) {
- Updates.push_back({DominatorTree::Insert, NewBB, SuccBlock});
- DTU->applyUpdates(Updates);
- }
- return true;
+ // Select == nullptr means we assume that there is a hidden no-op select
+ // instruction of _ = `select %icmp, true, false` just after `%icmp = ...`
+ return tryToSimplifyUncondBranchWithICmpSelectInIt(ICI, nullptr, Builder);
}
/// Similar to tryToSimplifyUncondBranchWithICmpInIt, but handle a more generic
@@ -5167,7 +5067,8 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
// If the block has any PHIs in it or the icmp/select has multiple uses, it is
// too complex.
- if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse() || !Select->hasOneUse())
+ if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse() ||
+ (Select && !Select->hasOneUse()))
return false;
// The pattern we're looking for is where our only predecessor is a switch on
@@ -5187,14 +5088,23 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
return false;
Value *SelectCond, *SelectTrueVal, *SelectFalseVal;
- // Match select Cond, TrueVal, FalseVal
- if (!match(Select, m_Select(m_Value(SelectCond), m_Value(SelectTrueVal),
- m_Value(SelectFalseVal))))
- return false;
-
- // Check if the select condition is the same as the icmp condition.
- if (SelectCond != ICI)
- return false;
+ Instruction *User;
+ if (!Select) {
+ // If Select == nullptr, we can assume that there is a hidden no-op select
+ // just after icmp
+ SelectCond = ICI;
+ SelectTrueVal = Builder.getTrue();
+ SelectFalseVal = Builder.getFalse();
+ User = ICI->user_back();
+ } else {
+ SelectCond = Select->getCondition();
+ // Check if the select condition is the same as the icmp condition.
+ if (SelectCond != ICI)
+ return false;
+ SelectTrueVal = Select->getTrueValue();
+ SelectFalseVal = Select->getFalseValue();
+ User = Select->user_back();
+ }
SwitchInst *SI = cast<SwitchInst>(Pred->getTerminator());
if (SI->getCondition() != IcmpCond)
@@ -5235,7 +5145,7 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
// The use of the select has to be in the 'end' block, by the only PHI node in
// the block.
BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0);
- PHINode *PHIUse = dyn_cast<PHINode>(Select->user_back());
+ PHINode *PHIUse = dyn_cast<PHINode>(User);
if (PHIUse == nullptr || PHIUse != &SuccBlock->front() ||
isa<PHINode>(++BasicBlock::iterator(PHIUse)))
return false;
@@ -5250,8 +5160,12 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
// Replace Select (which is used by the PHI for the default value) with
// SelectFalseVal or SelectTrueVal depending on if ICI is EQ or NE.
- Select->replaceAllUsesWith(DefaultCst);
- Select->eraseFromParent();
+ if (!Select) {
+ Select->replaceAllUsesWith(DefaultCst);
+ Select->eraseFromParent();
+ } else {
+ ICI->replaceAllUsesWith(DefaultCst);
+ }
ICI->eraseFromParent();
SmallVector<DominatorTree::UpdateType, 2> Updates;
>From aaa7e2260f60e64d91f2b7834b870820615c9ad3 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Wed, 5 Nov 2025 15:40:28 +0800
Subject: [PATCH 06/10] Fix CI failures
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a9756b09a104e..3b8ce6a37f73e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5160,7 +5160,7 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
// Replace Select (which is used by the PHI for the default value) with
// SelectFalseVal or SelectTrueVal depending on if ICI is EQ or NE.
- if (!Select) {
+ if (Select) {
Select->replaceAllUsesWith(DefaultCst);
Select->eraseFromParent();
} else {
>From b8d06c556b895ed6fa684beaafe06fb093065a16 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Sat, 8 Nov 2025 19:55:24 +0800
Subject: [PATCH 07/10] Add new tests
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 3 +-
.../Transforms/SimplifyCFG/switch_create.ll | 140 ++++++++++++++++++
2 files changed, 142 insertions(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 3b8ce6a37f73e..26b0099558ed0 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5014,7 +5014,7 @@ bool SimplifyCFGOpt::simplifyIndirectBrOnSelect(IndirectBrInst *IBI,
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder) {
// Select == nullptr means we assume that there is a hidden no-op select
- // instruction of _ = `select %icmp, true, false` just after `%icmp = ...`
+ // instruction of `_ = select %icmp, true, false` just after `%icmp = icmp ...`
return tryToSimplifyUncondBranchWithICmpSelectInIt(ICI, nullptr, Builder);
}
@@ -5067,6 +5067,7 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpSelectInIt(
// If the block has any PHIs in it or the icmp/select has multiple uses, it is
// too complex.
+ /// TODO: support multi-phis in succ BB of select's BB.
if (isa<PHINode>(BB->begin()) || !ICI->hasOneUse() ||
(Select && !Select->hasOneUse()))
return false;
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_create.ll b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
index ef5aee68e268e..d2f2c6851f2c8 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_create.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
@@ -1314,6 +1314,146 @@ if.end:
ret void
}
+define i32 @switch_with_icmp_select_after_it(i32 %x) {
+; CHECK-LABEL: @switch_with_icmp_select_after_it(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: switch i32 [[X:%.*]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT: i32 18, label [[END:%.*]]
+; CHECK-NEXT: i32 21, label [[END]]
+; CHECK-NEXT: i32 48, label [[END]]
+; CHECK-NEXT: i32 16, label [[END]]
+; CHECK-NEXT: ]
+; CHECK: default:
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 2, i32 3
+; CHECK-NEXT: br label [[END]]
+; CHECK: end:
+; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
+; CHECK-NEXT: ret i32 [[RES]]
+;
+entry:
+ switch i32 %x, label %default [
+ i32 18, label %end
+ i32 21, label %end
+ i32 48, label %end
+ i32 16, label %end
+ ]
+default:
+ %cmp = icmp eq i32 %x, 80
+ ; Create a new switch case BB for case 80.
+ %sel = select i1 %cmp, i32 2, i32 3
+ br label %end
+end:
+ %res = phi i32 [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ %sel, %default ]
+ ret i32 %res
+}
+
+define i32 @switch_with_icmp_select_after_it2(i32 %x) {
+; CHECK-LABEL: @switch_with_icmp_select_after_it2(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: switch i32 [[X:%.*]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT: i32 18, label [[END:%.*]]
+; CHECK-NEXT: i32 21, label [[END]]
+; CHECK-NEXT: i32 48, label [[END]]
+; CHECK-NEXT: i32 16, label [[END]]
+; CHECK-NEXT: ]
+; CHECK: default:
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 1, i32 3
+; CHECK-NEXT: br label [[END]]
+; CHECK: end:
+; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
+; CHECK-NEXT: ret i32 [[RES]]
+;
+entry:
+ switch i32 %x, label %default [
+ i32 18, label %end
+ i32 21, label %end
+ i32 48, label %end
+ i32 16, label %end
+ ]
+default:
+ %cmp = icmp eq i32 %x, 80
+ ; Should not create new case BB
+ %sel = select i1 %cmp, i32 1, i32 3
+ br label %end
+end:
+ %res = phi i32 [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ %sel, %default ]
+ ret i32 %res
+}
+
+define i32 @switch_with_icmp_select_after_it3(i32 %x) {
+; CHECK-LABEL: @switch_with_icmp_select_after_it3(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: switch i32 [[X:%.*]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT: i32 18, label [[END:%.*]]
+; CHECK-NEXT: i32 21, label [[END]]
+; CHECK-NEXT: i32 48, label [[END]]
+; CHECK-NEXT: i32 16, label [[END]]
+; CHECK-NEXT: ]
+; CHECK: default:
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 3, i32 1
+; CHECK-NEXT: br label [[END]]
+; CHECK: end:
+; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
+; CHECK-NEXT: ret i32 [[RES]]
+;
+entry:
+ switch i32 %x, label %default [
+ i32 18, label %end
+ i32 21, label %end
+ i32 48, label %end
+ i32 16, label %end
+ ]
+default:
+ %cmp = icmp eq i32 %x, 80
+ ; Should not create new case BB
+ %sel = select i1 %cmp, i32 3, i32 1
+ br label %end
+end:
+ %res = phi i32 [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ %sel, %default ]
+ ret i32 %res
+}
+
+; TODO: support this case (multi-phis).
+define i32 @switch_with_icmp_select_after_it_multi_phis(i32 %x) {
+; CHECK-LABEL: @switch_with_icmp_select_after_it_multi_phis(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: switch i32 [[X:%.*]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT: i32 18, label [[END:%.*]]
+; CHECK-NEXT: i32 21, label [[END]]
+; CHECK-NEXT: i32 48, label [[END]]
+; CHECK-NEXT: i32 16, label [[END]]
+; CHECK-NEXT: ]
+; CHECK: default:
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 2, i32 3
+; CHECK-NEXT: br label [[END]]
+; CHECK: end:
+; CHECK-NEXT: [[RES1:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ 0, [[ENTRY]] ], [ 0, [[ENTRY]] ], [ 0, [[ENTRY]] ], [ 100, [[DEFAULT]] ]
+; CHECK-NEXT: [[RES2:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
+; CHECK-NEXT: [[RES:%.*]] = xor i32 [[RES1]], [[RES2]]
+; CHECK-NEXT: ret i32 [[RES]]
+;
+entry:
+ switch i32 %x, label %default [
+ i32 18, label %end
+ i32 21, label %end
+ i32 48, label %end
+ i32 16, label %end
+ ]
+default:
+ %cmp = icmp eq i32 %x, 80
+ %sel = select i1 %cmp, i32 2, i32 3
+ br label %end
+end:
+ %res1 = phi i32 [ 0, %entry ], [ 0, %entry ], [ 0, %entry ], [ 0, %entry ], [ 100, %default ]
+ %res2 = phi i32 [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ 1, %entry ], [ %sel, %default ]
+ %res = xor i32 %res1, %res2
+ ret i32 %res
+}
+
!0 = !{!"function_entry_count", i32 100}
!1 = !{!"branch_weights", i32 6, i32 10}
;.
>From 79f0c928fec8e4d81cf4e9ad169479991538c5c8 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Sat, 8 Nov 2025 19:57:37 +0800
Subject: [PATCH 08/10] Regenerate the tests after patch
---
.../Transforms/SimplifyCFG/switch_create.ll | 26 ++++++-------------
1 file changed, 8 insertions(+), 18 deletions(-)
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_create.ll b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
index d2f2c6851f2c8..64016f3a4b97c 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_create.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
@@ -1322,13 +1322,14 @@ define i32 @switch_with_icmp_select_after_it(i32 %x) {
; CHECK-NEXT: i32 21, label [[END]]
; CHECK-NEXT: i32 48, label [[END]]
; CHECK-NEXT: i32 16, label [[END]]
+; CHECK-NEXT: i32 80, label [[SWITCH_EDGE:%.*]]
; CHECK-NEXT: ]
+; CHECK: switch.edge:
+; CHECK-NEXT: br label [[END]]
; CHECK: default:
-; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
-; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 2, i32 3
; CHECK-NEXT: br label [[END]]
; CHECK: end:
-; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
+; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 3, [[DEFAULT]] ], [ 2, [[SWITCH_EDGE]] ]
; CHECK-NEXT: ret i32 [[RES]]
;
entry:
@@ -1356,13 +1357,12 @@ define i32 @switch_with_icmp_select_after_it2(i32 %x) {
; CHECK-NEXT: i32 21, label [[END]]
; CHECK-NEXT: i32 48, label [[END]]
; CHECK-NEXT: i32 16, label [[END]]
+; CHECK-NEXT: i32 80, label [[END]]
; CHECK-NEXT: ]
; CHECK: default:
-; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
-; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 1, i32 3
; CHECK-NEXT: br label [[END]]
; CHECK: end:
-; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
+; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 3, [[DEFAULT]] ], [ 1, [[ENTRY]] ]
; CHECK-NEXT: ret i32 [[RES]]
;
entry:
@@ -1385,19 +1385,9 @@ end:
define i32 @switch_with_icmp_select_after_it3(i32 %x) {
; CHECK-LABEL: @switch_with_icmp_select_after_it3(
; CHECK-NEXT: entry:
-; CHECK-NEXT: switch i32 [[X:%.*]], label [[DEFAULT:%.*]] [
-; CHECK-NEXT: i32 18, label [[END:%.*]]
-; CHECK-NEXT: i32 21, label [[END]]
-; CHECK-NEXT: i32 48, label [[END]]
-; CHECK-NEXT: i32 16, label [[END]]
-; CHECK-NEXT: ]
-; CHECK: default:
-; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X:%.*]], 80
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i32 3, i32 1
-; CHECK-NEXT: br label [[END]]
-; CHECK: end:
-; CHECK-NEXT: [[RES:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ 1, [[ENTRY]] ], [ [[SEL]], [[DEFAULT]] ]
-; CHECK-NEXT: ret i32 [[RES]]
+; CHECK-NEXT: ret i32 [[SEL]]
;
entry:
switch i32 %x, label %default [
>From 74f07b8edee3326453e12ab535830a4360ad9ea1 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Sat, 8 Nov 2025 19:59:43 +0800
Subject: [PATCH 09/10] Fix comments
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 26b0099558ed0..48deef4f538be 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5020,7 +5020,7 @@ bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
/// Similar to tryToSimplifyUncondBranchWithICmpInIt, but handle a more generic
/// case. This is called when we find an icmp instruction (a seteq/setne with a
-/// constant) and its following select instruction as the only TWO instruction
+/// constant) and its following select instruction as the only TWO instructions
/// in a block that ends with an uncond branch. We are looking for a very
/// specific pattern that occurs when "
/// if (A == 1) return C1;
>From d4f28b0dce5ee3ced3db76447740823af74f77e0 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Sat, 8 Nov 2025 20:04:07 +0800
Subject: [PATCH 10/10] Make formatter happy
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 48deef4f538be..b71c5b7f5b2f6 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5014,7 +5014,7 @@ bool SimplifyCFGOpt::simplifyIndirectBrOnSelect(IndirectBrInst *IBI,
bool SimplifyCFGOpt::tryToSimplifyUncondBranchWithICmpInIt(
ICmpInst *ICI, IRBuilder<> &Builder) {
// Select == nullptr means we assume that there is a hidden no-op select
- // instruction of `_ = select %icmp, true, false` just after `%icmp = icmp ...`
+ // instruction of `_ = select %icmp, true, false` after `%icmp = icmp ...`
return tryToSimplifyUncondBranchWithICmpSelectInIt(ICI, nullptr, Builder);
}
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