[LLVMdev] RFC: Indirect Call Target Profiling

Justin Bogner mail at justinbogner.com
Thu Oct 30 14:44:04 PDT 2014

betulb at codeaurora.org writes:
> Hi All,
> We've been working on adding indirect call target profiling support to the
> instrumented profiler for PGO purposes. I’d like to propose the following
> design.

This is an interesting idea! Do you have any data on performance
improvements we might be able to expect from this work?

> Goal: Our aim is to add instrumentation around indirect call sites, so
> that the run-time can track the callee addresses and their access
> frequencies. From the addresses we’d like to infer the callee names and
> use it in optimizations to improve the performance of applications which
> make heavy use of indirect calls. Spec is a candidate benchmark that gives
> us applications both written in C and C++ and makes use of indirect calls.
> Spec can prove the effectiveness of optimizations making use of this
> additional data.
> Design:
> To determine the function names from the profiled target addresses, we've
> extended the data variable that is built by build_data_var() in
> CodeGenPGO.cpp (abbr. PFDV: Per Function Data Variable) to save the
> function addresses. PFDV is communicated to the run-time during function
> registration and outputted in the raw profile data file. This data
> structure is also extended to contain the number of indirect call sites
> for each function.

Where are the function addresses stored? The layout of the data
variables has been designed to be very simple to write to a file
efficiently and fairly simple to read back in for conversion. Will this
change that?

> To help communicate the target addresses to run-time, we insert a call to
> a run-time routine before each indirect call site in clang. Something
> like:
> void instrument_indirect_call_site(uint8_t *TargetAddress, void *Data,
> uint32_t CounterIndex);
> This run-time function takes in the target address, the index/id of the
> indirect call site and the pointer to the profile data variable of the
> caller (i.e. PFDV). The runtime routine checks if the target address has
> been seen before for the indirect call site index/id or not. If not, then
> an entry is added into an internal data structure. If yes, the counter
> associated with the target address is incremented by 1. This counter
> records the number of times the target address is called.

This sounds like it will be a fairly high overhead. Also, how will we
manage the memory for the internal data structure? It's currently
possible to use instrumentation based profiling in environments where
malloc isn't available, and it would be unfortunate to lose this

> Raw profile data file stores the target addresses and the number of times
> any target address is taken per each call site index. llvm-profdata reads
> the function addresses from the raw profile data file, then compares them
> against the target addresses from the same file. Each match helps identify
> the function names for the recorded addresses.
> llvm-profdata processed files contain the target function names. In case
> no function matches the target address then the target address is
> converted to string and stored in that format in the “indexed” data files.
> On the PGO path, clang consumes the returned indirect target data and
> attaches the following metadata at the indirect call sites.
> !33 = metadata !{metadata !"indirect_call_targets", i64
> <total_exec_count>, metadata !"target_fn1”, i64 <target_fn1_count>,
> metadata !"target_fn2”, i64 <target_fn2_count>, ….}
> Only the top most called N function names are recorded at each indirect
> call site. “indirect_call_targets” is the string literal identifying the
> fields of this metadata. <total_exec_count> is a 64 bit value for the
> total number of times the indirect call is executed followed by the
> function names and execution counts of each target.
> We're working on collecting further data points on the overhead of this
> additional instrumentation on the original profiler. Looking forward to
> hearing your comments.
> Thanks,
> -Betul Buyukkurt
> Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> a Linux Foundation Collaborative Project
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