From 5c3117b0a98dd11717eaffd7fb583985d39544b2 Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanaka@apple.com>
Date: Tue, 23 Apr 2019 19:49:03 +0000
Subject: [PATCH] [ObjC][ARC] Check the basic block size before calling
 DominatorTree::dominate.

ARC contract pass has an optimization that replaces the uses of the
argument of an ObjC runtime function call with the call result.

For example:

; Before optimization
%1 = tail call i8* @foo1()
%2 = tail call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %1)
store i8* %1, i8** @g0, align 8

; After optimization
%1 = tail call i8* @foo1()
%2 = tail call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %1)
store i8* %2, i8** @g0, align 8 // %1 is replaced with %2

Before replacing the argument use, DominatorTree::dominate is called to
determine whether the user instruction is dominated by the ObjC runtime
function call instruction. The call to DominatorTree::dominate can be
expensive if the two instructions belong to the same basic block and the
size of the basic block is large. This patch checks the basic block size
and just bails out if the size exceeds the limit set by command line
option "arc-contract-max-bb-size".

rdar://problem/49477063

Differential Revision: https://reviews.llvm.org/D60900

llvm-svn: 359027
---
 llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp    | 35 +++++++++++++++++++++-
 .../Transforms/ObjCARC/contract-max-bb-size.ll     | 17 +++++++++++
 2 files changed, 51 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/Transforms/ObjCARC/contract-max-bb-size.ll

diff --git a/llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp b/llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp
index 906425d..36aa513 100644
--- a/llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp
+++ b/llvm/lib/Transforms/ObjCARC/ObjCARCContract.cpp
@@ -45,6 +45,10 @@ using namespace llvm::objcarc;
 STATISTIC(NumPeeps,       "Number of calls peephole-optimized");
 STATISTIC(NumStoreStrongs, "Number objc_storeStrong calls formed");
 
+static cl::opt<unsigned> MaxBBSize("arc-contract-max-bb-size", cl::Hidden,
+    cl::desc("Maximum basic block size to discover the dominance relation of "
+             "two instructions in the same basic block"), cl::init(65535));
+
 //===----------------------------------------------------------------------===//
 //                                Declarations
 //===----------------------------------------------------------------------===//
@@ -573,6 +577,24 @@ bool ObjCARCContract::runOnFunction(Function &F) {
   // reduces register pressure.
   SmallPtrSet<Instruction *, 4> DependingInstructions;
   SmallPtrSet<const BasicBlock *, 4> Visited;
+
+  // Cache the basic block size.
+  DenseMap<const BasicBlock *, unsigned> BBSizeMap;
+
+  // A lambda that lazily computes the size of a basic block and determines
+  // whether the size exceeds MaxBBSize.
+  auto IsLargeBB = [&](const BasicBlock *BB) {
+    unsigned BBSize;
+    auto I = BBSizeMap.find(BB);
+
+    if (I != BBSizeMap.end())
+      BBSize = I->second;
+    else
+      BBSize = BBSizeMap[BB] = BB->size();
+
+    return BBSize > MaxBBSize;
+  };
+
   for (inst_iterator I = inst_begin(&F), E = inst_end(&F); I != E;) {
     Instruction *Inst = &*I++;
 
@@ -590,7 +612,7 @@ bool ObjCARCContract::runOnFunction(Function &F) {
     // and such; to do the replacement, the argument must have type i8*.
 
     // Function for replacing uses of Arg dominated by Inst.
-    auto ReplaceArgUses = [Inst, this](Value *Arg) {
+    auto ReplaceArgUses = [Inst, IsLargeBB, this](Value *Arg) {
       // If we're compiling bugpointed code, don't get in trouble.
       if (!isa<Instruction>(Arg) && !isa<Argument>(Arg))
         return;
@@ -602,6 +624,17 @@ bool ObjCARCContract::runOnFunction(Function &F) {
         Use &U = *UI++;
         unsigned OperandNo = U.getOperandNo();
 
+        // Don't replace the uses if Inst and the user belong to the same basic
+        // block and the size of the basic block is large. We don't want to call
+        // DominatorTree::dominate in that case. We can remove this check if we
+        // can use OrderedBasicBlock to compute the dominance relation between
+        // two instructions, but that's not currently possible since it doesn't
+        // recompute the instruction ordering when new instructions are inserted
+        // to the basic block.
+        if (Inst->getParent() == cast<Instruction>(U.getUser())->getParent() &&
+            IsLargeBB(Inst->getParent()))
+          continue;
+
         // If the call's return value dominates a use of the call's argument
         // value, rewrite the use to use the return value. We check for
         // reachability here because an unreachable call is considered to
diff --git a/llvm/test/Transforms/ObjCARC/contract-max-bb-size.ll b/llvm/test/Transforms/ObjCARC/contract-max-bb-size.ll
new file mode 100644
index 0000000..c591738
--- /dev/null
+++ b/llvm/test/Transforms/ObjCARC/contract-max-bb-size.ll
@@ -0,0 +1,17 @@
+; RUN: opt -objc-arc-contract -S < %s | FileCheck -check-prefix=ENABLE %s
+; RUN: opt -objc-arc-contract -arc-contract-max-bb-size=3 -S < %s | FileCheck -check-prefix=DISABLE %s
+
+@g0 = common global i8* null, align 8
+
+; ENABLE: store i8* %2, i8** @g0
+; DISABLE: store i8* %1, i8** @g0
+
+define void @foo0() {
+  %1 = tail call i8* @foo1()
+  %2 = tail call i8* @llvm.objc.retainAutoreleasedReturnValue(i8* %1)
+  store i8* %1, i8** @g0, align 8
+  ret void
+}
+
+declare i8* @foo1()
+declare i8* @llvm.objc.retainAutoreleasedReturnValue(i8*)
-- 
2.7.4