[llvm-dev] [InstrProfiling] Lightweight Instrumentation

[Ellis Hoag via llvm-dev]

*RFC: Lightweight Instrumentation*

Hi all,

Our team at Facebook would like to propose a lightweight variant of IR
instrumentation PGO for use in the mobile space. IRPGO is a proven
technology in LLVM that can boost performance for server workloads.
However, the larger binary resulting from instrumentation significantly
limits its use for mobile applications. In this proposal, we introduce a
few changes to IRPGO to reduce the instrumented binary size, making it
suitable for PGO on mobile devices.

This proposal is driven by the same need behind the earlier MIP (machine IR
profile) prototype <https://reviews.llvm.org/D104060>. But unlike MIP where
there is significant divergence from IRPGO, this proposed lightweight
instrumentation fits into the existing IRPGO framework with a few
extensions to achieve a smaller instrumented binary.

We’d like to share the new design and results from our prototype and get
feedback.

Best,
Ellis, Kyungwoo, and Wenlei
Motivation
In the mobile space, profile guided optimization can also have an outsized
impact on performance just like PGO for server workloads, but conventional
instrumentation comes with a large binary size and code size increase as
high as 50%, which limits its use for mobile application for two reasons:

   - Mobile applications are very sensitive to total binary size as larger
   binaries take longer to download and use more space on devices. There could
   be a hard size limit for over-the-air (OTA) updates for this reason.
   - When code (.text) size increases, it takes longer for applications to
   start up and could also degrade runtime performance due to more page faults
   on devices with limited RAM.

Reducing the size overhead from instrumentation would make IRPGO usable for
mobile applications so we could send instrumented binaries through OTA
updates in production environments, collect representative production
profiles, and apply PGO.
OverviewThe size overhead from IRPGO mainly comes from two things: 1)
metadata for mapping raw counts back to IR/CFG, which has to stay with the
binary. 2) the increased .text size due to insertion of instrumented code
and less effective optimization after instrumentation. Two extensions are
proposed to reduce the size overhead from each of the above:

   - We allow the use of debug info / dwarf as alternative metadata for
   mapping counts to IR, aka profile correlation. Debug info is extractable
   from the binary, therefore such metadata doesn’t need to be shipped to
   mobile devices. Debug info has been used extensively for sampling based PGO
   in LLVM, so it has reasonable quality to support profile correlation.
   - We add the flexibility to allow coarse grained instrumentation that:
   1) only insert probes at function entry instead of each block (or blocks
   decided by MST placement); 2) optional coverage mode using one byte
   booleans in addition to today’s counting mode using 8 byte counters.

The extensions offer a spectrum of trade-off choices from the most accurate
PGO to something very lightweight that can be used in mobile space. With
debug info extracted and using function entry coverage mode, the size
increase can be reduced from close to 50% down to below 5% (measured with
clang self-build PGO).
Extractable MetadataWith today’s IRPGO, the instrumentation runtime dumps
out a profraw profile at the end of training. The runtime creates a header
and appends data from the __llvm_prf_data, __llvm_prf_cnts, and
__llvm_prf_names sections to create a profraw profile. The __llvm_prf_data
section contains references to each function’s profile data (in
__llvm_prf_cnts) and name (in __llvm_prf_names) so they are needed to
correlate profile data to the functions they instrument.

Some kind of metadata to correlate counts back to IR (specifically CFG
blocks) is unavoidable. One way to reduce binary size is to make such
metadata extractable so they don’t have to be shipped to mobile devices. We
could make __llvm_prf_data and __llvm_prf_names extractable, but the cost
will be non-trivial and it will be a breaking change. On the other hand,
debug info is extractable from binary and it already does a very good job
of maintaining mapping between address and source location / symbols.
Sample PGO depends entirely on debug info for profile correlation. So we
picked debug info as the alternative for extractable metadata.

In our proposed instrumentation, we create a special global struct, e.g.,
__profc__Z3foov, to hold counters for a particular function. The
__llvm_prf_cnts data section holds all of these structs and serves as
placeholder for raw profile counters. In our final instrumented binary, we
only have probe instructions and raw profile data without any
instrumentation metadata, i.e., there are no __llvm_prf_names or
__llvm_prf_data sections but we still have a __llvm_prf_cnts section. At
runtime, we dump the __llvm_prf_cnts section to a file without any
processing after profiling. To differentiate from IRPGO, the output from
runtime is called proflite and we can add another VARIANT_MASK_ flag to the
Version field of the profile header. At llvm-profdata post-processing time,
we use debug info to correlate our raw profile data as follows. First we
identify an instrumented function and look for its special global struct
that holds counters (__profc__Z3foov) in the debug info. The debug info can
tell us the address of that symbol in the binary and we can compute its
offset from the __llvm_prof_cnts section. Then we can use that offset to
read the function entry and block counters from the proflite file. Finally
we populate profdata output for each function following the existing
format.

Value profile is not going to be supported with extractable metadata right
now, though we believe it can also be added following a similar scheme.

To improve debug info quality for profile correlation,
-fdebug-info-for-profiling from AutoFDO can be used. Additionally, we could
also use pseudo-probe from CSSPGO
<https://groups.google.com/g/llvm-dev/c/1p1rdYbL93s/m/hBrJaOWVAwAJ> as the
alternative metadata which is also fully extractable.

We propose a new flag
-fprofile-generate-correlate=[profdata|debug-info|pseudo-probe] to choose
what metadata to use for profile correlation. Either we correlate with
today’s IRPGO metadata and keep them in their own sections (__llvm_prf_data and
__llvm_prf_names), with debug info, or with pseudo-probe.
Coarse-grained InstrumentationIn addition to reducing metadata size (
__llvm_prf_names and __llvm_prf_data), we can also tune down .text size and
__llvm_prf_cnts size. We do this by 1) only instrumenting function entries
instead of each block and 2) lowering precision by tracking single byte
coverage data rather than 8 byte counters. This is a trade-off between
profile quality and binary size.

Function profile vs block profile and counting mode vs coverage mode can
all be selected independently using our proposed flag
-fprofile-generate-mode=[func-cov|block-cov|func-cnt|block-cnt], and they
can work with both extractable metadata as well as IRPGO‘s correlation
method. func-cov and block-cov use single byte booleans for coverage data
while func-cnt and block-cnt use 8 byte counters. block-cnt represents
today’s IRPGO which is the default.

When using a profile generated from modes other than block-cnt, additional
profile inference is needed before the counts can be consumed by
optimizations. Such inference is done during profile loading and so it’s
transparent to optimizations.

   - For block coverage mode, we will use coverage info to seed block count
   inference, and leverage static branch probability at the same time to
   produce a CFG profile that honors zero count blocks and converts live block
   coverage data into synthetic counts.
   - For function count mode, we will derived a CFG profile entirely from
   static branch probability, then scale the CFG profile based on function
   entry count.
   - Function coverage mode is handled similar to function count mode. For
   covered/live functions, we will derived a CFG profile entirely from static
   branch probability first, then scale that CFG profile by a constant.

Experiments showed that even with coarse-grained function entry profiles,
mobile application can still benefit from PGO. But the smaller binary make
it possible for mobile to use PGO.
WorkflowSince these are extensions that share the same underlying PGO
framework, the workflow for lightweight PGO is very similar to existing
IRPGO.

The diagram below has the PGO workflow today (shown in red) in comparison
with the workflow for lightweight instrumentation (shown in green). We
first create an instrumentation build that produces a raw profile at
runtime. Then we use the llvm-profdata tool to convert that raw profile to
a profile that the compiler can consume in the PGO build. The main
difference for lightweight instrumentation is that we create an
instrumentation build with debug info and we use that debug info to create
our final profile.

[image: image.png]Prototype & Results
We have a proof of concept
<https://github.com/ellishg/llvm-project/commits/instr-correlate-debug-info>
using dwarf as the extractable metadata and single byte function coverage
instrumentation. We measured code size by building Clang with and without
instrumentation using -Oz and no value profiling. Our lightweight
instrumented Clang binary is only +4 MB (+3.48%) larger than a
non-instrumented binary. We compare this with today’s PGO instrumentation
Clang binary which is +54 MB (+46.96%) larger. If we used debug info to
correlate normal instrumentation (without value profiling) instead of just
function coverage then we would expect to see an overhead of +43.2 MB
(+37.5%). We don’t have performance data on clang experiments using the
prototype since not all components are implemented. However, an alternative
implementation earlier (similar to MIP) delivered good performance boost
for mobile applications.

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