Add a new function attribute 'cold' to functions.

Other than recognizing the attribute, the patch does little else.
It changes the branch probability analyzer so that edges into
blocks postdominated by a cold function are given low weight.

Added analysis and code generation tests.  Added documentation for the
new attribute.

llvm-svn: 182638

Author: Diego Novillo

Diff for: llvm/docs/LangRef.rst

@@ -812,6 +812,11 @@ example:
     This attribute indicates that the inliner should attempt to inline
     this function into callers whenever possible, ignoring any active
     inlining size threshold for this caller.
+``cold``
+    This attribute indicates that this function is rarely called. When
+    computing edge weights, basic blocks post-dominated by a cold
+    function call are also considered to be cold; and, thus, given low
+    weight.
 ``nonlazybind``
     This attribute suppresses lazy symbol binding for the function. This
     may make calls to the function faster, at the cost of extra program

Diff for: llvm/include/llvm-c/Core.h

@@ -166,6 +166,7 @@ typedef enum {
        and the path forward agreed upon.
     LLVMAddressSafety = 1ULL << 32,
     LLVMStackProtectStrongAttribute = 1ULL<<33
+    LLVMCold = 1ULL << 34
     */
 } LLVMAttribute;
 

Diff for: llvm/include/llvm/Analysis/BranchProbabilityInfo.h

@@ -131,11 +131,15 @@ class BranchProbabilityInfo : public FunctionPass {
   /// \brief Track the set of blocks directly succeeded by a returning block.
   SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
 
+  /// \brief Track the set of blocks that always lead to a cold call.
+  SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
+
   /// \brief Get sum of the block successors' weights.
   uint32_t getSumForBlock(const BasicBlock *BB) const;
 
   bool calcUnreachableHeuristics(BasicBlock *BB);
   bool calcMetadataWeights(BasicBlock *BB);
+  bool calcColdCallHeuristics(BasicBlock *BB);
   bool calcPointerHeuristics(BasicBlock *BB);
   bool calcLoopBranchHeuristics(BasicBlock *BB);
   bool calcZeroHeuristics(BasicBlock *BB);

Diff for: llvm/include/llvm/IR/Attributes.h

@@ -68,6 +68,7 @@ class Attribute {
                            ///< 0 means unaligned (different from align(1))
     AlwaysInline,          ///< inline=always
     ByVal,                 ///< Pass structure by value
+    Cold,                  ///< Marks function as being in a cold path.
     InlineHint,            ///< Source said inlining was desirable
     InReg,                 ///< Force argument to be passed in register
     MinSize,               ///< Function must be optimized for size first

Diff for: llvm/lib/Analysis/BranchProbabilityInfo.cpp

@@ -69,6 +69,20 @@ static const uint32_t UR_TAKEN_WEIGHT = 1;
 /// easily subsume it.
 static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
 
+/// \brief Weight for a branch taken going into a cold block.
+///
+/// This is the weight for a branch taken toward a block marked
+/// cold.  A block is marked cold if it's postdominated by a
+/// block containing a call to a cold function.  Cold functions
+/// are those marked with attribute 'cold'.
+static const uint32_t CC_TAKEN_WEIGHT = 4;
+
+/// \brief Weight for a branch not-taken into a cold block.
+///
+/// This is the weight for a branch not taken toward a block marked
+/// cold.
+static const uint32_t CC_NONTAKEN_WEIGHT = 64;
+
 static const uint32_t PH_TAKEN_WEIGHT = 20;
 static const uint32_t PH_NONTAKEN_WEIGHT = 12;
 
@@ -193,6 +207,69 @@ bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
   return true;
 }
 
+/// \brief Calculate edge weights for edges leading to cold blocks.
+///
+/// A cold block is one post-dominated by  a block with a call to a
+/// cold function.  Those edges are unlikely to be taken, so we give
+/// them relatively low weight.
+///
+/// Return true if we could compute the weights for cold edges.
+/// Return false, otherwise.
+bool BranchProbabilityInfo::calcColdCallHeuristics(BasicBlock *BB) {
+  TerminatorInst *TI = BB->getTerminator();
+  if (TI->getNumSuccessors() == 0)
+    return false;
+
+  // Determine which successors are post-dominated by a cold block.
+  SmallVector<unsigned, 4> ColdEdges;
+  ColdEdges.reserve(TI->getNumSuccessors());
+  SmallVector<unsigned, 4> NormalEdges;
+  NormalEdges.reserve(TI->getNumSuccessors());
+  for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
+    if (PostDominatedByColdCall.count(*I))
+      ColdEdges.push_back(I.getSuccessorIndex());
+    else
+      NormalEdges.push_back(I.getSuccessorIndex());
+
+  // If all successors are in the set of blocks post-dominated by cold calls,
+  // this block is in the set post-dominated by cold calls.
+  if (ColdEdges.size() == TI->getNumSuccessors())
+    PostDominatedByColdCall.insert(BB);
+  else {
+    // Otherwise, if the block itself contains a cold function, add it to the
+    // set of blocks postdominated by a cold call.
+    assert(!PostDominatedByColdCall.count(BB));
+    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+      if (CallInst *CI = dyn_cast<CallInst>(I))
+        if (CI->hasFnAttr(Attribute::Cold)) {
+          PostDominatedByColdCall.insert(BB);
+          break;
+        }
+  }
+
+  // Skip probabilities if this block has a single successor.
+  if (TI->getNumSuccessors() == 1 || ColdEdges.empty())
+    return false;
+
+  uint32_t ColdWeight =
+      std::max(CC_TAKEN_WEIGHT / (unsigned) ColdEdges.size(), MIN_WEIGHT);
+  for (SmallVector<unsigned, 4>::iterator I = ColdEdges.begin(),
+                                          E = ColdEdges.end();
+       I != E; ++I)
+    setEdgeWeight(BB, *I, ColdWeight);
+
+  if (NormalEdges.empty())
+    return true;
+  uint32_t NormalWeight = std::max(
+      CC_NONTAKEN_WEIGHT / (unsigned) NormalEdges.size(), NORMAL_WEIGHT);
+  for (SmallVector<unsigned, 4>::iterator I = NormalEdges.begin(),
+                                          E = NormalEdges.end();
+       I != E; ++I)
+    setEdgeWeight(BB, *I, NormalWeight);
+
+  return true;
+}
+
 // Calculate Edge Weights using "Pointer Heuristics". Predict a comparsion
 // between two pointer or pointer and NULL will fail.
 bool BranchProbabilityInfo::calcPointerHeuristics(BasicBlock *BB) {
@@ -397,6 +474,7 @@ bool BranchProbabilityInfo::runOnFunction(Function &F) {
   LastF = &F; // Store the last function we ran on for printing.
   LI = &getAnalysis<LoopInfo>();
   assert(PostDominatedByUnreachable.empty());
+  assert(PostDominatedByColdCall.empty());
 
   // Walk the basic blocks in post-order so that we can build up state about
   // the successors of a block iteratively.
@@ -408,6 +486,8 @@ bool BranchProbabilityInfo::runOnFunction(Function &F) {
       continue;
     if (calcMetadataWeights(*I))
       continue;
+    if (calcColdCallHeuristics(*I))
+      continue;
     if (calcLoopBranchHeuristics(*I))
       continue;
     if (calcPointerHeuristics(*I))
@@ -420,6 +500,7 @@ bool BranchProbabilityInfo::runOnFunction(Function &F) {
   }
 
   PostDominatedByUnreachable.clear();
+  PostDominatedByColdCall.clear();
   return false;
 }
 

Diff for: llvm/lib/AsmParser/LLLexer.cpp

@@ -564,6 +564,7 @@ lltok::Kind LLLexer::LexIdentifier() {
 
   KEYWORD(alwaysinline);
   KEYWORD(byval);
+  KEYWORD(cold);
   KEYWORD(inlinehint);
   KEYWORD(inreg);
   KEYWORD(minsize);

Diff for: llvm/lib/AsmParser/LLParser.cpp

@@ -909,6 +909,7 @@ bool LLParser::ParseFnAttributeValuePairs(AttrBuilder &B,
       continue;
     }
     case lltok::kw_alwaysinline:      B.addAttribute(Attribute::AlwaysInline); break;
+    case lltok::kw_cold:              B.addAttribute(Attribute::Cold); break;
     case lltok::kw_inlinehint:        B.addAttribute(Attribute::InlineHint); break;
     case lltok::kw_minsize:           B.addAttribute(Attribute::MinSize); break;
     case lltok::kw_naked:             B.addAttribute(Attribute::Naked); break;
@@ -1222,6 +1223,7 @@ bool LLParser::ParseOptionalReturnAttrs(AttrBuilder &B) {
 
     case lltok::kw_alignstack:
     case lltok::kw_alwaysinline:
+    case lltok::kw_cold:
     case lltok::kw_inlinehint:
     case lltok::kw_minsize:
     case lltok::kw_naked:

Diff for: llvm/lib/AsmParser/LLToken.h

@@ -96,6 +96,7 @@ namespace lltok {
     kw_alwaysinline,
     kw_sanitize_address,
     kw_byval,
+    kw_cold,
     kw_inlinehint,
     kw_inreg,
     kw_minsize,

Diff for: llvm/lib/IR/Attributes.cpp

@@ -217,6 +217,8 @@ std::string Attribute::getAsString(bool InAttrGrp) const {
     return "uwtable";
   if (hasAttribute(Attribute::ZExt))
     return "zeroext";
+  if (hasAttribute(Attribute::Cold))
+    return "cold";
 
   // FIXME: These should be output like this:
   //
@@ -396,6 +398,7 @@ uint64_t AttributeImpl::getAttrMask(Attribute::AttrKind Val) {
   case Attribute::SanitizeMemory:  return 1ULL << 37;
   case Attribute::NoBuiltin:       return 1ULL << 38;
   case Attribute::Returned:        return 1ULL << 39;
+  case Attribute::Cold:            return 1ULL << 40;
   }
   llvm_unreachable("Unsupported attribute type");
 }

Diff for: llvm/lib/IR/Verifier.cpp

@@ -692,7 +692,8 @@ void Verifier::VerifyAttributeTypes(AttributeSet Attrs, unsigned Idx,
         I->getKindAsEnum() == Attribute::SanitizeMemory ||
         I->getKindAsEnum() == Attribute::MinSize ||
         I->getKindAsEnum() == Attribute::NoDuplicate ||
-        I->getKindAsEnum() == Attribute::NoBuiltin) {
+        I->getKindAsEnum() == Attribute::NoBuiltin ||
+        I->getKindAsEnum() == Attribute::Cold) {
       if (!isFunction)
           CheckFailed("Attribute '" + I->getKindAsString() +
                       "' only applies to functions!", V);

Diff for: llvm/test/Analysis/BranchProbabilityInfo/basic.ll

@@ -115,3 +115,61 @@ return:
 }
 
 !2 = metadata !{metadata !"branch_weights", i32 7, i32 6, i32 4, i32 4, i32 64}
+
+declare void @coldfunc() cold
+
+define i32 @test5(i32 %a, i32 %b, i1 %flag) {
+; CHECK: Printing analysis {{.*}} for function 'test5'
+entry:
+  br i1 %flag, label %then, label %else
+; CHECK: edge entry -> then probability is 4 / 68
+; CHECK: edge entry -> else probability is 64 / 68
+
+then:
+  call void @coldfunc()
+  br label %exit
+; CHECK: edge then -> exit probability is 16 / 16 = 100%
+
+else:
+  br label %exit
+; CHECK: edge else -> exit probability is 16 / 16 = 100%
+
+exit:
+  %result = phi i32 [ %a, %then ], [ %b, %else ]
+  ret i32 %result
+}
+
+declare i32 @regular_function(i32 %i)
+
+define i32 @test_cold_call_sites(i32* %a) {
+; Test that edges to blocks post-dominated by cold call sites
+; are marked as not expected to be taken.
+; TODO(dnovillo) The calls to regular_function should not be merged, but
+; they are currently being merged. Convert this into a code generation test
+; after that is fixed.
+
+; CHECK: Printing analysis {{.*}} for function 'test_cold_call_sites'
+; CHECK: edge entry -> then probability is 4 / 68 = 5.88235%
+; CHECK: edge entry -> else probability is 64 / 68 = 94.1176% [HOT edge]
+
+entry:
+  %gep1 = getelementptr i32* %a, i32 1
+  %val1 = load i32* %gep1
+  %cond1 = icmp ugt i32 %val1, 1
+  br i1 %cond1, label %then, label %else
+
+then:
+  ; This function is not declared cold, but this call site is.
+  %val4 = call i32 @regular_function(i32 %val1) cold
+  br label %exit
+
+else:
+  %gep2 = getelementptr i32* %a, i32 2
+  %val2 = load i32* %gep2
+  %val3 = call i32 @regular_function(i32 %val2)
+  br label %exit
+
+exit:
+  %ret = phi i32 [ %val4, %then ], [ %val3, %else ]
+  ret i32 %ret
+}

Diff for: llvm/test/CodeGen/X86/block-placement.ll

@@ -1089,3 +1089,35 @@ while.end:
   store double %rra.0, double* %arrayidx34, align 8
   br label %for.cond
 }
+
+declare void @cold_function() cold
+
+define i32 @test_cold_calls(i32* %a) {
+; Test that edges to blocks post-dominated by cold calls are
+; marked as not expected to be taken.  They should be laid out
+; at the bottom.
+; CHECK: test_cold_calls:
+; CHECK: %entry
+; CHECK: %else
+; CHECK: %exit
+; CHECK: %then
+
+entry:
+  %gep1 = getelementptr i32* %a, i32 1
+  %val1 = load i32* %gep1
+  %cond1 = icmp ugt i32 %val1, 1
+  br i1 %cond1, label %then, label %else
+
+then:
+  call void @cold_function()
+  br label %exit
+
+else:
+  %gep2 = getelementptr i32* %a, i32 2
+  %val2 = load i32* %gep2
+  br label %exit
+
+exit:
+  %ret = phi i32 [ %val1, %then ], [ %val2, %else ]
+  ret i32 %ret
+}

Diff for: llvm/test/Feature/cold.ll

@@ -0,0 +1,9 @@
+; RUN: llvm-as < %s | llvm-dis | FileCheck %s
+
+; CHECK: @fun() #0
+define void @fun() #0 {
+  ret void
+}
+
+; CHECK: attributes #0 = { cold }
+attributes #0 = { cold }