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A type profile is information which summarizes the type of some value at some program point in profile point in the program being executed by the JVM.  The summary is designed to allow the optimizing compiler to guess at future types at the same point in the program.  In many cases, these profiles are necessary for attaining best code quality.

A program point profile point is a specific instance of a bytecode.  Not all bytecodes perform profiling.

The type recorded type at a profile point is the concrete class of an object operand to the objectbytecode.  (I.e., it is the _klass object  object header field, of type Klass*.)

There is also an indication if nulls have been seen at the program pointprofile point.

Here are the bytecodes with their profiled operands:

Profile Point BytecodeValueType Profiled Operand
invokevirtual, invokeinterfacereceiver
checkcast, instanceoftested object
aastoreelement value

Primitive values are not currently profiled, but see bug 8015418.  Non-receiver arguments and return values are not currently profiled, but see bug 6919064.

(FIXME: Move the following to a new page, MethodData.)
The interpreter and tier one compiled code collect type profiles. The profiles are crucial to later optimizing compilation. Tier one currently emulates the interpreter with respect to profiling. 

A profile is a metadata structure of type MethodData.  Each method has zero or one of them.  The structure is laid out as a heterogeneous array which is sequenced in parallel with the bytecodes themselves.  Only only a minority of bytecodes capture profile data, the overall profile block is often larger than the bytecodes themselves.  Each element in the profile array captures information for one instance of a bytecode in the method.  (These are the program points referred to above.)

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Type profile structure

For background on profiles, see MethodData.

Within a MethodData block, each original instance of a type-profiled bytecode instruction (invokevirtual, checkcast, etc.) gets a ReceiverTypeData

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 record. Each record has a

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few rows (TypeProfileWidth, default 2)

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, each of which contains a count and an exact object type. The record also has an overall count, which may differ from the sum of the individual counts.

Failure modes

Type profiles have two failure modes: First, a method might be compiled before its profile exists and is "mature", so that no stable conclusions can be drawn about operands in that method. Second, a method might be used from many different contexts with independent operand types (as with ArrayList.contains), so that the profile becomes "polluted" by many independent types. A polluted profile contains the first few (2) encountered types with low counts, and a high total count that signals that the ReceiverTypeData structure was too small. 

Polluted profiles stem from the fact that a method (containing generically reusable code) has only one profile structure, but the method is reused from a variety of contexts, providing a variety of operand types.

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