[PATCH] Add function entry count metadata.

[Justin Bogner]

Diego Novillo <dnovillo at google.com> writes:
> Hi dexonsmith, bogner,
>
> This adds three Function methods to handle function entry counts:
> setEntryCount(), hasEntryCount() and getEntryCount().
>
> Entry counts are stored under the MD_prof metadata node with the name
> "function_entry_count". They are unsigned 64 bit values set by profilers
> (instrumentation and sample profiler changes coming up).
>
> I had some internal debate about what to return when
> Function::getEntryCount() is called without the function having its
> metadata set. I don't want to return a special sentinel value, because
> those are typically problematic down the road.
>
> So, I added a Function::hasEntryCount() predicate. If
> Function::getEntryCount() is called with no metadata present, it will
> cause a compiler assertion.

How about returning an Optional<uint64_t>? It should be about the same
for the caller and it's harder to misuse, ie, instead of:

  if (Fn.hasEntryCount())
    doStuffWith(Fn.getEntryCount())

We'd get:

  if (auto C = Fn.getEntryCount())
    doStuffWith(*C)

> http://reviews.llvm.org/D9628
>
> Files:
>   include/llvm/IR/Function.h
>   include/llvm/IR/MDBuilder.h
>   lib/IR/MDBuilder.cpp
>   unittests/IR/MetadataTest.cpp
>
> Index: include/llvm/IR/Function.h
> ===================================================================
> --- include/llvm/IR/Function.h
> +++ include/llvm/IR/Function.h
> @@ -23,7 +23,11 @@
>  #include "llvm/IR/Attributes.h"
>  #include "llvm/IR/BasicBlock.h"
>  #include "llvm/IR/CallingConv.h"
> +#include "llvm/IR/Constants.h"
>  #include "llvm/IR/GlobalObject.h"
> +#include "llvm/IR/LLVMContext.h"
> +#include "llvm/IR/MDBuilder.h"
> +#include "llvm/IR/Metadata.h"
>  #include "llvm/Support/Compiler.h"
>  
>  namespace llvm {
> @@ -194,6 +198,29 @@
>                                   AttributeSet::FunctionIndex, Kind, Value));
>    }
>  
> +  /// @brief Set the entry count for this function.
> +  void setEntryCount(uint64_t Count) {
> +    MDBuilder MDB(getContext());
> +    setMetadata(LLVMContext::MD_prof, MDB.createFunctionEntryCount(Count));
> +  }
> +
> +  /// @brief Return true if the function has a known entry count.
> +  bool hasEntryCount() const {
> +    MDNode *MD = getMetadata(LLVMContext::MD_prof);
> +    if (MD && MD->getOperand(0))
> +      if (MDString* MDS = dyn_cast<MDString>(MD->getOperand(0)))
> +        return MDS->getString().equals("function_entry_count");
> +    return false;
> +  }
> +
> +  /// @brief Get the entry count for this function.
> +  uint64_t getEntryCount() const {
> +    assert(hasEntryCount() && "No entry count available for this function");
> +    MDNode *MD = getMetadata(LLVMContext::MD_prof);
> +    ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(1));
> +    return CI->getValue().getZExtValue();
> +  }
> +
>    /// @brief Return true if the function has the attribute.
>    bool hasFnAttribute(Attribute::AttrKind Kind) const {
>      return AttributeSets.hasAttribute(AttributeSet::FunctionIndex, Kind);
> Index: include/llvm/IR/MDBuilder.h
> ===================================================================
> --- include/llvm/IR/MDBuilder.h
> +++ include/llvm/IR/MDBuilder.h
> @@ -60,6 +60,9 @@
>    /// \brief Return metadata containing a number of branch weights.
>    MDNode *createBranchWeights(ArrayRef<uint32_t> Weights);
>  
> +  /// \brief Return metadata containing the entry count for a function.
> +  MDNode *createFunctionEntryCount(uint64_t Count);
> +
>    //===------------------------------------------------------------------===//
>    // Range metadata.
>    //===------------------------------------------------------------------===//
> Index: lib/IR/MDBuilder.cpp
> ===================================================================
> --- lib/IR/MDBuilder.cpp
> +++ lib/IR/MDBuilder.cpp
> @@ -53,6 +53,16 @@
>    return MDNode::get(Context, Vals);
>  }
>  
> +MDNode *MDBuilder::createFunctionEntryCount(uint64_t Count) {
> +  SmallVector<Metadata *, 2> Vals(2);
> +  Vals[0] = createString("function_entry_count");
> +
> +  Type *Int64Ty = Type::getInt64Ty(Context);
> +  Vals[1] = createConstant(ConstantInt::get(Int64Ty, Count));
> +
> +  return MDNode::get(Context, Vals);
> +}
> +
>  MDNode *MDBuilder::createRange(const APInt &Lo, const APInt &Hi) {
>    assert(Lo.getBitWidth() == Hi.getBitWidth() && "Mismatched bitwidths!");
>  
> Index: unittests/IR/MetadataTest.cpp
> ===================================================================
> --- unittests/IR/MetadataTest.cpp
> +++ unittests/IR/MetadataTest.cpp
> @@ -2272,4 +2272,12 @@
>    EXPECT_FALSE(verifyFunction(*F));
>  }
>  
> +TEST_F(FunctionAttachmentTest, EntryCount) {
> +  Function *F = getFunction("foo");
> +  EXPECT_FALSE(F->hasEntryCount());
> +  F->setEntryCount(12304);
> +  EXPECT_TRUE(F->hasEntryCount());
> +  EXPECT_EQ(12304u, F->getEntryCount());
> +}
> +
>  }
>
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