[llvm] r320699 - [InlineCost] Tracking Values through PHI Nodes
Haicheng Wu via llvm-commits
llvm-commits at lists.llvm.org
Thu Dec 14 06:36:18 PST 2017
Author: haicheng
Date: Thu Dec 14 06:36:18 2017
New Revision: 320699
URL: http://llvm.org/viewvc/llvm-project?rev=320699&view=rev
Log:
[InlineCost] Tracking Values through PHI Nodes
This patch fix this FIXME in visitPHI()
FIXME: We should potentially be tracking values through phi nodes,
especially when they collapse to a single value due to deleted CFG edges
during inlining.
Differential Revision: https://reviews.llvm.org/D38594
Added:
llvm/trunk/test/Transforms/Inline/AArch64/phi.ll
Modified:
llvm/trunk/lib/Analysis/InlineCost.cpp
Modified: llvm/trunk/lib/Analysis/InlineCost.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InlineCost.cpp?rev=320699&r1=320698&r2=320699&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InlineCost.cpp (original)
+++ llvm/trunk/lib/Analysis/InlineCost.cpp Thu Dec 14 06:36:18 2017
@@ -21,6 +21,7 @@
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ProfileSummaryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.h"
@@ -163,12 +164,20 @@ class CallAnalyzer : public InstVisitor<
/// Keep track of values which map to a pointer base and constant offset.
DenseMap<Value *, std::pair<Value *, APInt>> ConstantOffsetPtrs;
+ /// Keep track of dead blocks due to the constant arguments.
+ SetVector<BasicBlock *> DeadBlocks;
+
+ /// The mapping of the blocks to their known unique successors due to the
+ /// constant arguments.
+ DenseMap<BasicBlock *, BasicBlock *> KnownSuccessors;
+
// Custom simplification helper routines.
bool isAllocaDerivedArg(Value *V);
bool lookupSROAArgAndCost(Value *V, Value *&Arg,
DenseMap<Value *, int>::iterator &CostIt);
void disableSROA(DenseMap<Value *, int>::iterator CostIt);
void disableSROA(Value *V);
+ void findDeadBlocks(BasicBlock *CurrBB, BasicBlock *NextBB);
void accumulateSROACost(DenseMap<Value *, int>::iterator CostIt,
int InstructionCost);
bool isGEPFree(GetElementPtrInst &GEP);
@@ -420,15 +429,94 @@ bool CallAnalyzer::visitAlloca(AllocaIns
}
bool CallAnalyzer::visitPHI(PHINode &I) {
- // FIXME: We should potentially be tracking values through phi nodes,
- // especially when they collapse to a single value due to deleted CFG edges
- // during inlining.
-
// FIXME: We need to propagate SROA *disabling* through phi nodes, even
// though we don't want to propagate it's bonuses. The idea is to disable
// SROA if it *might* be used in an inappropriate manner.
// Phi nodes are always zero-cost.
+
+ APInt ZeroOffset = APInt::getNullValue(DL.getPointerSizeInBits());
+ bool CheckSROA = I.getType()->isPointerTy();
+
+ // Track the constant or pointer with constant offset we've seen so far.
+ Constant *FirstC = nullptr;
+ std::pair<Value *, APInt> FirstBaseAndOffset = {nullptr, ZeroOffset};
+ Value *FirstV = nullptr;
+
+ for (unsigned i = 0, e = I.getNumIncomingValues(); i != e; ++i) {
+ BasicBlock *Pred = I.getIncomingBlock(i);
+ // If the incoming block is dead, skip the incoming block.
+ if (DeadBlocks.count(Pred))
+ continue;
+ // If the parent block of phi is not the known successor of the incoming
+ // block, skip the incoming block.
+ BasicBlock *KnownSuccessor = KnownSuccessors[Pred];
+ if (KnownSuccessor && KnownSuccessor != I.getParent())
+ continue;
+
+ Value *V = I.getIncomingValue(i);
+ // If the incoming value is this phi itself, skip the incoming value.
+ if (&I == V)
+ continue;
+
+ Constant *C = dyn_cast<Constant>(V);
+ if (!C)
+ C = SimplifiedValues.lookup(V);
+
+ std::pair<Value *, APInt> BaseAndOffset = {nullptr, ZeroOffset};
+ if (!C && CheckSROA)
+ BaseAndOffset = ConstantOffsetPtrs.lookup(V);
+
+ if (!C && !BaseAndOffset.first)
+ // The incoming value is neither a constant nor a pointer with constant
+ // offset, exit early.
+ return true;
+
+ if (FirstC) {
+ if (FirstC == C)
+ // If we've seen a constant incoming value before and it is the same
+ // constant we see this time, continue checking the next incoming value.
+ continue;
+ // Otherwise early exit because we either see a different constant or saw
+ // a constant before but we have a pointer with constant offset this time.
+ return true;
+ }
+
+ if (FirstV) {
+ // The same logic as above, but check pointer with constant offset here.
+ if (FirstBaseAndOffset == BaseAndOffset)
+ continue;
+ return true;
+ }
+
+ if (C) {
+ // This is the 1st time we've seen a constant, record it.
+ FirstC = C;
+ continue;
+ }
+
+ // The remaining case is that this is the 1st time we've seen a pointer with
+ // constant offset, record it.
+ FirstV = V;
+ FirstBaseAndOffset = BaseAndOffset;
+ }
+
+ // Check if we can map phi to a constant.
+ if (FirstC) {
+ SimplifiedValues[&I] = FirstC;
+ return true;
+ }
+
+ // Check if we can map phi to a pointer with constant offset.
+ if (FirstBaseAndOffset.first) {
+ ConstantOffsetPtrs[&I] = FirstBaseAndOffset;
+
+ Value *SROAArg;
+ DenseMap<Value *, int>::iterator CostIt;
+ if (lookupSROAArgAndCost(FirstV, SROAArg, CostIt))
+ SROAArgValues[&I] = SROAArg;
+ }
+
return true;
}
@@ -1512,6 +1600,44 @@ ConstantInt *CallAnalyzer::stripAndCompu
return cast<ConstantInt>(ConstantInt::get(IntPtrTy, Offset));
}
+/// \brief Find dead blocks due to deleted CFG edges during inlining.
+///
+/// If we know the successor of the current block, \p CurrBB, has to be \p
+/// NextBB, the other successors of \p CurrBB are dead if these successors have
+/// no live incoming CFG edges. If one block is found to be dead, we can
+/// continue growing the dead block list by checking the successors of the dead
+/// blocks to see if all their incoming edges are dead or not.
+void CallAnalyzer::findDeadBlocks(BasicBlock *CurrBB, BasicBlock *NextBB) {
+ auto IsEdgeDead = [&](BasicBlock *Pred, BasicBlock *Succ) {
+ // A CFG edge is dead if the predecessor is dead or the predessor has a
+ // known successor which is not the one under exam.
+ return (DeadBlocks.count(Pred) ||
+ (KnownSuccessors[Pred] && KnownSuccessors[Pred] != Succ));
+ };
+
+ auto IsNewlyDead = [&](BasicBlock *BB) {
+ // If all the edges to a block are dead, the block is also dead.
+ return (!DeadBlocks.count(BB) &&
+ llvm::all_of(predecessors(BB),
+ [&](BasicBlock *P) { return IsEdgeDead(P, BB); }));
+ };
+
+ for (BasicBlock *Succ : successors(CurrBB)) {
+ if (Succ == NextBB || !IsNewlyDead(Succ))
+ continue;
+ SmallVector<BasicBlock *, 4> NewDead;
+ NewDead.push_back(Succ);
+ while (!NewDead.empty()) {
+ BasicBlock *Dead = NewDead.pop_back_val();
+ if (DeadBlocks.insert(Dead))
+ // Continue growing the dead block lists.
+ for (BasicBlock *S : successors(Dead))
+ if (IsNewlyDead(S))
+ NewDead.push_back(S);
+ }
+ }
+}
+
/// \brief Analyze a call site for potential inlining.
///
/// Returns true if inlining this call is viable, and false if it is not
@@ -1649,7 +1775,10 @@ bool CallAnalyzer::analyzeCall(CallSite
Value *Cond = BI->getCondition();
if (ConstantInt *SimpleCond =
dyn_cast_or_null<ConstantInt>(SimplifiedValues.lookup(Cond))) {
- BBWorklist.insert(BI->getSuccessor(SimpleCond->isZero() ? 1 : 0));
+ BasicBlock *NextBB = BI->getSuccessor(SimpleCond->isZero() ? 1 : 0);
+ BBWorklist.insert(NextBB);
+ KnownSuccessors[BB] = NextBB;
+ findDeadBlocks(BB, NextBB);
continue;
}
}
@@ -1657,7 +1786,10 @@ bool CallAnalyzer::analyzeCall(CallSite
Value *Cond = SI->getCondition();
if (ConstantInt *SimpleCond =
dyn_cast_or_null<ConstantInt>(SimplifiedValues.lookup(Cond))) {
- BBWorklist.insert(SI->findCaseValue(SimpleCond)->getCaseSuccessor());
+ BasicBlock *NextBB = SI->findCaseValue(SimpleCond)->getCaseSuccessor();
+ BBWorklist.insert(NextBB);
+ KnownSuccessors[BB] = NextBB;
+ findDeadBlocks(BB, NextBB);
continue;
}
}
Added: llvm/trunk/test/Transforms/Inline/AArch64/phi.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/Inline/AArch64/phi.ll?rev=320699&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/Inline/AArch64/phi.ll (added)
+++ llvm/trunk/test/Transforms/Inline/AArch64/phi.ll Thu Dec 14 06:36:18 2017
@@ -0,0 +1,504 @@
+; RUN: opt -inline -mtriple=aarch64--linux-gnu -S -o - < %s -inline-threshold=0 | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64--linux-gnu"
+
+declare void @pad()
+ at glbl = external global i32
+
+define i1 @outer1() {
+; CHECK-LABEL: @outer1(
+; CHECK-NOT: call i1 @inner1
+ %C = call i1 @inner1()
+ ret i1 %C
+}
+
+define i1 @inner1() {
+entry:
+ br label %if_true
+
+if_true:
+ %phi = phi i1 [0, %entry], [%phi, %if_true] ; Simplified to 0
+ br i1 %phi, label %if_true, label %exit
+
+exit:
+ store i32 0, i32* @glbl
+ store i32 1, i32* @glbl
+ store i32 2, i32* @glbl
+ store i32 3, i32* @glbl
+ store i32 4, i32* @glbl
+ ret i1 %phi
+}
+
+
+define i1 @outer2(i1 %val) {
+; CHECK-LABEL: @outer2(
+; CHECK: call i1 @inner2
+ %C = call i1 @inner2(i1 %val)
+ ret i1 %C
+}
+
+define i1 @inner2(i1 %val) {
+entry:
+ br label %if_true
+
+if_true:
+ %phi = phi i1 [%val, %entry], [%phi, %if_true] ; Cannot be simplified to a constant
+ br i1 %phi, label %if_true, label %exit
+
+exit:
+ call void @pad()
+ ret i1 %phi
+}
+
+
+define i1 @outer3(i1 %cond) {
+; CHECK-LABEL: @outer3(
+; CHECK-NOT: call i1 @inner3
+ %C = call i1 @inner3(i1 %cond)
+ ret i1 %C
+}
+
+define i1 @inner3(i1 %cond) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32 [0, %entry], [0, %if_true] ; Simplified to 0
+ %cmp = icmp eq i32 %phi, 0
+ store i32 0, i32* @glbl
+ store i32 1, i32* @glbl
+ store i32 2, i32* @glbl
+ store i32 3, i32* @glbl
+ store i32 4, i32* @glbl
+ ret i1 %cmp
+}
+
+
+define i1 @outer4(i1 %cond) {
+; CHECK-LABEL: @outer4(
+; CHECK-NOT: call i1 @inner4
+ %C = call i1 @inner4(i1 %cond, i32 0)
+ ret i1 %C
+}
+
+define i1 @inner4(i1 %cond, i32 %val) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32 [0, %entry], [%val, %if_true] ; Simplified to 0
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ ret i1 %cmp
+}
+
+
+define i1 @outer5_1(i1 %cond) {
+; CHECK-LABEL: @outer5_1(
+; CHECK-NOT: call i1 @inner5
+ %C = call i1 @inner5(i1 %cond, i32 0, i32 0)
+ ret i1 %C
+}
+
+
+define i1 @outer5_2(i1 %cond) {
+; CHECK-LABEL: @outer5_2(
+; CHECK: call i1 @inner5
+ %C = call i1 @inner5(i1 %cond, i32 0, i32 1)
+ ret i1 %C
+}
+
+define i1 @inner5(i1 %cond, i32 %val1, i32 %val2) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32 [%val1, %entry], [%val2, %if_true] ; Can be simplified to a constant if %val1 and %val2 are the same constants
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ store i32 0, i32* @glbl
+ ret i1 %cmp
+}
+
+
+define i1 @outer6(i1 %cond, i32 %val) {
+; CHECK-LABEL: @outer6(
+; CHECK-NOT: call i1 @inner6
+ %C = call i1 @inner6(i1 true, i32 %val, i32 0)
+ ret i1 %C
+}
+
+define i1 @inner6(i1 %cond, i32 %val1, i32 %val2) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32 [%val1, %entry], [%val2, %if_true] ; Simplified to 0
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ store i32 0, i32* @glbl
+ store i32 1, i32* @glbl
+ ret i1 %cmp
+}
+
+
+define i1 @outer7(i1 %cond, i32 %val) {
+; CHECK-LABEL: @outer7(
+; CHECK-NOT: call i1 @inner7
+ %C = call i1 @inner7(i1 false, i32 0, i32 %val)
+ ret i1 %C
+}
+
+define i1 @inner7(i1 %cond, i32 %val1, i32 %val2) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32 [%val1, %entry], [%val2, %if_true] ; Simplified to 0
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ store i32 0, i32* @glbl
+ store i32 1, i32* @glbl
+ ret i1 %cmp
+}
+
+
+define i1 @outer8_1() {
+; CHECK-LABEL: @outer8_1(
+; CHECK-NOT: call i1 @inner8
+ %C = call i1 @inner8(i32 0)
+ ret i1 %C
+}
+
+
+
+define i1 @outer8_2() {
+; CHECK-LABEL: @outer8_2(
+; CHECK-NOT: call i1 @inner8
+ %C = call i1 @inner8(i32 3)
+ ret i1 %C
+}
+
+define i1 @inner8(i32 %cond) {
+entry:
+ switch i32 %cond, label %default [ i32 0, label %zero
+ i32 1, label %one
+ i32 2, label %two ]
+
+zero:
+ br label %exit
+
+one:
+ br label %exit
+
+two:
+ br label %exit
+
+default:
+ br label %exit
+
+exit:
+ %phi = phi i32 [0, %zero], [1, %one], [2, %two], [-1, %default] ; Can be simplified to a constant if the switch condition is known
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ ret i1 %cmp
+}
+
+
+define i1 @outer9(i1 %cond) {
+; CHECK-LABEL: @outer9(
+; CHECK-NOT: call i1 @inner9
+ %C = call i1 @inner9(i32 0, i1 %cond)
+ ret i1 %C
+}
+
+define i1 @inner9(i32 %cond1, i1 %cond2) {
+entry:
+ switch i32 %cond1, label %exit [ i32 0, label %zero
+ i32 1, label %one
+ i32 2, label %two ]
+
+zero:
+ br label %exit
+
+one:
+ br label %exit
+
+two:
+ br i1 %cond2, label %two_true, label %two_false
+
+two_true:
+ br label %exit
+
+two_false:
+ br label %exit
+
+exit:
+ %phi = phi i32 [0, %zero], [1, %one], [2, %two_true], [2, %two_false], [-1, %entry] ; Simplified to 0
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ store i32 0, i32* @glbl
+ ret i1 %cmp
+}
+
+
+define i32 @outer10(i1 %cond) {
+; CHECK-LABEL: @outer10(
+; CHECK-NOT: call i32 @inner10
+ %A = alloca i32
+ %C = call i32 @inner10(i1 %cond, i32* %A)
+ ret i32 %C
+}
+
+define i32 @inner10(i1 %cond, i32* %A) {
+entry:
+ br label %if_true
+
+if_true:
+ %phi = phi i32* [%A, %entry], [%phi, %if_true] ; Simplified to %A
+ %load = load i32, i32* %phi
+ br i1 %cond, label %if_true, label %exit
+
+exit:
+ call void @pad()
+ ret i32 %load
+}
+
+
+define i32 @outer11(i1 %cond, i32* %ptr) {
+; CHECK-LABEL: @outer11(
+; CHECK: call i32 @inner11
+ %C = call i32 @inner11(i1 %cond, i32* %ptr)
+ ret i32 %C
+}
+
+define i32 @inner11(i1 %cond, i32* %ptr) {
+entry:
+ br label %if_true
+
+if_true:
+ %phi = phi i32* [%ptr, %entry], [%phi, %if_true] ; Cannot be simplified
+ %load = load i32, i32* %phi
+ br i1 %cond, label %if_true, label %exit
+
+exit:
+ call void @pad()
+ ret i32 %load
+}
+
+
+define i32 @outer12(i1 %cond) {
+; CHECK-LABEL: @outer12(
+; CHECK-NOT: call i32 @inner12
+ %A = alloca i32
+ %C = call i32 @inner12(i1 %cond, i32* %A)
+ ret i32 %C
+}
+
+define i32 @inner12(i1 %cond, i32* %ptr) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32* [%ptr, %entry], [%ptr, %if_true] ; Simplified to %A
+ %load = load i32, i32* %phi
+ call void @pad()
+ ret i32 %load
+}
+
+
+define i32 @outer13(i1 %cond) {
+; CHECK-LABEL: @outer13(
+; CHECK-NOT: call i32 @inner13
+ %A = alloca i32
+ %C = call i32 @inner13(i1 %cond, i32* %A)
+ ret i32 %C
+}
+
+define i32 @inner13(i1 %cond, i32* %ptr) {
+entry:
+ %gep1 = getelementptr inbounds i32, i32* %ptr, i32 2
+ %gep2 = getelementptr inbounds i32, i32* %ptr, i32 1
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ %gep3 = getelementptr inbounds i32, i32* %gep2, i32 1
+ br label %exit
+
+exit:
+ %phi = phi i32* [%gep1, %entry], [%gep3, %if_true] ; Simplifeid to %gep1
+ %load = load i32, i32* %phi
+ call void @pad()
+ ret i32 %load
+}
+
+
+define i32 @outer14(i1 %cond) {
+; CHECK-LABEL: @outer14(
+; CHECK: call i32 @inner14
+ %A1 = alloca i32
+ %A2 = alloca i32
+ %C = call i32 @inner14(i1 %cond, i32* %A1, i32* %A2)
+ ret i32 %C
+}
+
+define i32 @inner14(i1 %cond, i32* %ptr1, i32* %ptr2) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32* [%ptr1, %entry], [%ptr2, %if_true] ; Cannot be simplified
+ %load = load i32, i32* %phi
+ call void @pad()
+ store i32 0, i32* @glbl
+ ret i32 %load
+}
+
+
+define i32 @outer15(i1 %cond, i32* %ptr) {
+; CHECK-LABEL: @outer15(
+; CHECK-NOT: call i32 @inner15
+ %A = alloca i32
+ %C = call i32 @inner15(i1 true, i32* %ptr, i32* %A)
+ ret i32 %C
+}
+
+define i32 @inner15(i1 %cond, i32* %ptr1, i32* %ptr2) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32* [%ptr1, %entry], [%ptr2, %if_true] ; Simplified to %A
+ %load = load i32, i32* %phi
+ call void @pad()
+ store i32 0, i32* @glbl
+ store i32 1, i32* @glbl
+ ret i32 %load
+}
+
+
+define i32 @outer16(i1 %cond, i32* %ptr) {
+; CHECK-LABEL: @outer16(
+; CHECK-NOT: call i32 @inner16
+ %A = alloca i32
+ %C = call i32 @inner16(i1 false, i32* %A, i32* %ptr)
+ ret i32 %C
+}
+
+define i32 @inner16(i1 %cond, i32* %ptr1, i32* %ptr2) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32* [%ptr1, %entry], [%ptr2, %if_true] ; Simplified to %A
+ %load = load i32, i32* %phi
+ call void @pad()
+ store i32 0, i32* @glbl
+ store i32 1, i32* @glbl
+ ret i32 %load
+}
+
+
+define i1 @outer17(i1 %cond) {
+; CHECK-LABEL: @outer17(
+; CHECK: call i1 @inner17
+ %A = alloca i32
+ %C = call i1 @inner17(i1 %cond, i32* %A)
+ ret i1 %C
+}
+
+define i1 @inner17(i1 %cond, i32* %ptr) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32* [null, %entry], [%ptr, %if_true] ; Cannot be mapped to a constant
+ %cmp = icmp eq i32* %phi, null
+ call void @pad()
+ ret i1 %cmp
+}
+
+
+define i1 @outer18(i1 %cond) {
+; CHECK-LABEL: @outer18(
+; CHECK-NOT: call i1 @inner18
+ %C = call i1 @inner18(i1 %cond, i1 true)
+ ret i1 %C
+}
+
+define i1 @inner18(i1 %cond1, i1 %cond2) {
+entry:
+ br i1 %cond1, label %block1, label %block2
+
+block1:
+ br i1 %cond2, label %block3, label %block4
+
+block2:
+ br i1 %cond2, label %block5, label %block4
+
+block3:
+ %phi = phi i32 [0, %block1], [1, %block4], [0, %block5] ; Simplified to 0
+ %cmp = icmp eq i32 %phi, 0
+ call void @pad()
+ ret i1 %cmp
+
+block4: ; Unreachable block
+ br label %block3
+
+block5:
+ br label %block3
+}
+
+
+define i1 @outer19(i1 %cond) {
+; CHECK-LABEL: @outer19(
+; CHECK: call i1 @inner19
+ %A = alloca i32
+ %C = call i1 @inner19(i1 %cond, i32* %A)
+ ret i1 %C
+}
+
+define i1 @inner19(i1 %cond, i32* %ptr) {
+entry:
+ br i1 %cond, label %if_true, label %exit
+
+if_true:
+ br label %exit
+
+exit:
+ %phi = phi i32* [%ptr, %entry], [null, %if_true] ; Cannot be mapped to a constant
+ %cmp = icmp eq i32* %phi, null
+ call void @pad()
+ ret i1 %cmp
+}
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