[llvm-dev] [RFC] Order File Instrumentation
Hans Wennborg via llvm-dev
llvm-dev at lists.llvm.org
Fri Jan 18 02:43:13 PST 2019
On Thu, Jan 17, 2019 at 7:24 PM Manman Ren via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Order file is used to teach ld64 how to order the functions in a binary. If we put all functions executed during startup together in the right order, we will greatly reduce the page faults during startup.
> To generate order file for iOS apps, we usually use dtrace, but some apps have various startup scenarios that we want to capture in the order file. dtrace approach is not easy to automate, it is hard to capture the different ways of starting an app without automation. Instrumented builds however can be deployed to phones and profile data can be automatically collected.
> For the Facebook app, by looking at the startup distribution, we are expecting a big win out of the order file instrumentation, from 100ms to 500ms+, in startup time.
> The basic idea of the pass is to use a circular buffer to log the execution ordering of the functions. We only log the function when it is first executed. Instead of logging the symbol name of the function, we log a pair of integers, with one integer specifying the module id, and the other specifying the function id within the module.
> clang has '-finstrument-function-entry-bare' which inserts a function call and is not as efficient.
Can you elaborate on why this existing functionality is not efficient
enough for you?
For Chrome on Windows, we use -finstrument-functions-after-inlining to
insert calls at function entry (after inlining) that calls a function
which captures the addresses in a buffer, and later symbolizes and
dumps them to an order file that we feed the linker. We use a similar
approach on for Chrome on Android, but I'm not as familiar with the
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