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[xray] Function coverage groups

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Add the ability to selectively instrument a subset of functions by dividing the functions into N logical groups and then selecting a group to cover. By selecting different groups over time you could cover the entire application incrementally with lower overhead than instrumenting the entire application at once.

Differential Revision: https://reviews.llvm.org/D87953
This commit is contained in:
Ian Levesque 2020-09-18 14:45:51 -04:00
parent ded74e922b
commit 919066e494
1 changed files with 35 additions and 5 deletions
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docs/XRay.rst
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@ -62,17 +62,18 @@ For example:
clang -fxray-instrument ...
By default, functions that have at least 200 instructions will get XRay
instrumentation points. You can tweak that number through the
By default, functions that have at least 200 instructions (or contain a loop) will
get XRay instrumentation points. You can tweak that number through the
``-fxray-instruction-threshold=`` flag:
::
clang -fxray-instrument -fxray-instruction-threshold=1 ...
You can also specifically instrument functions in your binary to either always
or never be instrumented using source-level attributes. You can do it using the
GCC-style attributes or C++11-style attributes.
The loop detection can be disabled with ``-fxray-ignore-loops`` to use only the
instruction threshold. You can also specifically instrument functions in your
binary to either always or never be instrumented using source-level attributes.
You can do it using the GCC-style attributes or C++11-style attributes.
.. code-block:: c++
@ -309,6 +310,35 @@ libraries, distributed with the LLVM distribution. These are:
instrumentation map in XRay-instrumented object files and binaries. The
``extract`` and ``stack`` subcommands uses this particular library.
Minimizing Binary Size
----------------------
XRay supports several different instrumentation points including ``function-entry``,
``function-exit``, ``custom``, and ``typed`` points. These can be enabled individually
using the ``-fxray-instrumentaton-bundle=`` flag. For example if you only wanted to
instrument function entry and custom points you could specify:
::
clang -fxray-instrument -fxray-instrumentation-bundle=function-entry,custom ...
This will omit the other sled types entirely, reducing the binary size. You can also
instrument just a sampled subset of functions using instrumentation groups.
For example, to instrument only a quarter of available functions invoke:
::
clang -fxray-instrument -fxray-function-groups=4
A subset will be chosen arbitrarily based on a hash of the function name. To sample a
different subset you can specify ``-fxray-selected-function-group=`` with a group number
in the range of 0 to ``xray-function-groups`` - 1. Together these options could be used
to produce multiple binaries with different instrumented subsets. If all you need is
runtime control over which functions are being traced at any given time it is better
to selectively patch and unpatch the individual functions you need using the XRay
Runtime Library's ``__xray_patch_function()`` method.
Future Work
===========