When code size is the priority (Oz, MinSize attribute), help llvm

turning a code like this:

if (foo)
   free(foo)

into that:
free(foo)

Move a call to free from basic block FB into FB's predecessor, P,
when the path from P to FB is taken only if the argument of free is
not equal to NULL.

Some restrictions apply on P and FB to be sure that this code motion
is profitable. Namely:
1. FB must have only one predecessor P.
2. FB must contain only the call to free plus an unconditional
   branch to S.
3. P's successors are FB and S.

Because of 1., we will not increase the code size when moving the call
to free from FB to P.
Because of 2., FB will be empty after the move.
Because of 2. and 3., P's branch instruction becomes useless, so as FB
(simplifycfg will do the job).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171762 91177308-0d34-0410-b5e6-96231b3b80d8

4 files changed, 119 insertions, 0 deletions

diff --git a/include/llvm/Support/PatternMatch.h b/include/llvm/Support/PatternMatch.h
index 386380c6bc..9fbe4349b3 100644
--- a/include/llvm/Support/PatternMatch.h
+++ b/include/llvm/Support/PatternMatch.h
@@ -781,6 +781,25 @@ inline fneg_match<LHS> m_FNeg(const LHS &L) { return L; }
 // Matchers for control flow.
 //
 
+struct br_match {
+  BasicBlock *&Succ;
+  br_match(BasicBlock *&Succ)
+    : Succ(Succ) {
+  }
+
+  template<typename OpTy>
+  bool match(OpTy *V) {
+    if (BranchInst *BI = dyn_cast<BranchInst>(V))
+      if (BI->isUnconditional()) {
+        Succ = BI->getSuccessor(0);
+        return true;
+      }
+    return false;
+  }
+};
+
+inline br_match m_UnconditionalBr(BasicBlock *&Succ) { return br_match(Succ); }
+
 template<typename Cond_t>
 struct brc_match {
   Cond_t Cond;
diff --git a/lib/Transforms/InstCombine/InstCombine.h b/lib/Transforms/InstCombine/InstCombine.h
index a207bce0d5..38c636462c 100644
--- a/lib/Transforms/InstCombine/InstCombine.h
+++ b/lib/Transforms/InstCombine/InstCombine.h
@@ -76,6 +76,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner
   TargetLibraryInfo *TLI;
   bool MadeIRChange;
   LibCallSimplifier *Simplifier;
+  bool MinimizeSize;
 public:
   /// Worklist - All of the instructions that need to be simplified.
   InstCombineWorklist Worklist;
@@ -87,6 +88,7 @@ public:
       
   static char ID; // Pass identification, replacement for typeid
   InstCombiner() : FunctionPass(ID), TD(0), Builder(0) {
+    MinimizeSize = false;
     initializeInstCombinerPass(*PassRegistry::getPassRegistry());
   }
 
diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 5e4274c7f7..6f24cdd738 100644
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1475,6 +1475,62 @@ Instruction *InstCombiner::visitAllocSite(Instruction &MI) {
   return 0;
 }
 
+/// \brief Move the call to free before a NULL test.
+///
+/// Check if this free is accessed after its argument has been test
+/// against NULL (property 0).
+/// If yes, it is legal to move this call in its predecessor block.
+///
+/// The move is performed only if the block containing the call to free
+/// will be removed, i.e.:
+/// 1. it has only one predecessor P, and P has two successors
+/// 2. it contains the call and an unconditional branch
+/// 3. its successor is the same as its predecessor's successor
+///
+/// The profitability is out-of concern here and this function should
+/// be called only if the caller knows this transformation would be
+/// profitable (e.g., for code size).
+static Instruction *
+tryToMoveFreeBeforeNullTest(CallInst &FI) {
+  Value *Op = FI.getArgOperand(0);
+  BasicBlock *FreeInstrBB = FI.getParent();
+  BasicBlock *PredBB = FreeInstrBB->getSinglePredecessor();
+
+  // Validate part of constraint #1: Only one predecessor
+  // FIXME: We can extend the number of predecessor, but in that case, we
+  //        would duplicate the call to free in each predecessor and it may
+  //        not be profitable even for code size.
+  if (!PredBB)
+    return 0;
+
+  // Validate constraint #2: Does this block contains only the call to
+  //                         free and an unconditional branch?
+  // FIXME: We could check if we can speculate everything in the
+  //        predecessor block
+  if (FreeInstrBB->size() != 2)
+    return 0;
+  BasicBlock *SuccBB;
+  if (!match(FreeInstrBB->getTerminator(), m_UnconditionalBr(SuccBB)))
+    return 0;
+
+  // Validate the rest of constraint #1 by matching on the pred branch.
+  TerminatorInst *TI = PredBB->getTerminator();
+  BasicBlock *TrueBB, *FalseBB;
+  ICmpInst::Predicate Pred;
+  if (!match(TI, m_Br(m_ICmp(Pred, m_Specific(Op), m_Zero()), TrueBB, FalseBB)))
+    return 0;
+  if (Pred != ICmpInst::ICMP_EQ && Pred != ICmpInst::ICMP_NE)
+    return 0;
+
+  // Validate constraint #3: Ensure the null case just falls through.
+  if (SuccBB != (Pred == ICmpInst::ICMP_EQ ? TrueBB : FalseBB))
+    return 0;
+  assert(FreeInstrBB == (Pred == ICmpInst::ICMP_EQ ? FalseBB : TrueBB) &&
+         "Broken CFG: missing edge from predecessor to successor");
+
+  FI.moveBefore(TI);
+  return &FI;
+}
 
 
 Instruction *InstCombiner::visitFree(CallInst &FI) {
@@ -1493,6 +1549,16 @@ Instruction *InstCombiner::visitFree(CallInst &FI) {
   if (isa<ConstantPointerNull>(Op))
     return EraseInstFromFunction(FI);
 
+  // If we optimize for code size, try to move the call to free before the null
+  // test so that simplify cfg can remove the empty block and dead code
+  // elimination the branch. I.e., helps to turn something like:
+  // if (foo) free(foo);
+  // into
+  // free(foo);
+  if (MinimizeSize)
+    if (Instruction *I = tryToMoveFreeBeforeNullTest(FI))
+      return I;
+
   return 0;
 }
 
@@ -2393,6 +2459,9 @@ public:
 bool InstCombiner::runOnFunction(Function &F) {
   TD = getAnalysisIfAvailable<DataLayout>();
   TLI = &getAnalysis<TargetLibraryInfo>();
+  // Minimizing size?
+  MinimizeSize = F.getAttributes().hasAttribute(AttributeSet::FunctionIndex,
+                                                Attribute::MinSize);
 
   /// Builder - This is an IRBuilder that automatically inserts new
   /// instructions into the worklist when they are created.
diff --git a/test/Transforms/InstCombine/malloc-free-delete.ll b/test/Transforms/InstCombine/malloc-free-delete.ll
index 4e3217dc2d..cd12b29b11 100644
--- a/test/Transforms/InstCombine/malloc-free-delete.ll
+++ b/test/Transforms/InstCombine/malloc-free-delete.ll
@@ -91,3 +91,32 @@ define void @test5(i8* %ptr, i8** %esc) {
   store volatile i8 4, i8* %g
   ret void
 }
+
+;; When a basic block contains only a call to free and this block is accessed
+;; through a test of the argument of free against null, move the call in the
+;; predecessor block.
+;; Using simplifycfg will remove the empty basic block and the branch operation
+;; Then, performing a dead elimination will remove the comparison.
+;; This is what happens with -O1 and upper.
+; CHECK: @test6
+define void @test6(i8* %foo) minsize {
+; CHECK:  %tobool = icmp eq i8* %foo, null
+;; Call to free moved
+; CHECK-NEXT: tail call void @free(i8* %foo)
+; CHECK-NEXT: br i1 %tobool, label %if.end, label %if.then
+; CHECK: if.then:
+;; Block is now empty and may be simplified by simplifycfg
+; CHECK-NEXT:   br label %if.end
+; CHECK: if.end:
+; CHECK-NEXT:  ret void
+entry:
+  %tobool = icmp eq i8* %foo, null
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @free(i8* %foo)
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  ret void
+}