[PATCH] D25766: [ELF] - Implemented --section-ordering-file option.

Rui Ueyama via llvm-commits llvm-commits at lists.llvm.org
Fri Oct 21 11:17:29 PDT 2016

Thank you for the update. It's interesting.

I think clang is too small to use as a test for the feature. It can easily
fit in in-memory cache, so the accesses to the file won't actually go to
the disks. Even if it hits a disk, it's still probably too small. You want
to test it with a latency-sensitive (i.e. user-facing) huge apps, such as
Firefox or Chromium.

Do you have an SSD to test this feature? I think we are interested in
performance numbers on SSDs.

On Fri, Oct 21, 2016 at 9:12 AM, George Rimar <grimar at accesssoftek.com>

> grimar added a comment.
> Short update: I was able to use gcc to extract section ordering files. For
> start I generated order list for clang launch without any arguments.
> I used gcc from svn://gcc.gnu.org/svn/gcc/branches/google/gcc-4_9.
> For doing these next steps were performed:
> 1. Build llvm with:
> -ffunction-sections -fdata-sections -fprofile-generate=/home/umb/LLVM/gcda"
> -DCMAKE_C_FLAGS="-fPIC -ffunction-sections -fdata-sections
> -fprofile-generate=/home/umb/LLVM/gcda" -DCMAKE_C_COMPILER=/home/umb/gcc49google/bin/gcc
> -DCMAKE_CXX_COMPILER=/home/umb/gcc49google/bin/g++
> 2. Run binary in the way we are interested in. It was ./clang w/o
> arguments in my case (.gcda files should be produced after that).
> 3. Now rebuild llvm with -fprofile-use and use of .gcda files we obtained
> earlier. Use gold linker.
> -std=c++11 -ffunction-sections -fdata-sections -fprofile-dir=/home/umb/LLVM/gcda
> -fprofile-use -freorder-functions=callgraph -Wl,--plugin-opt,file=/home/umb/LLVM/order.txt"
> -DCMAKE_C_FLAGS="-B/usr/local/bin -fPIC -ffunction-sections
> -fdata-sections -fprofile-dir=/home/umb/LLVM/gcda -fprofile-use
> -freorder-functions=callgraph -Wl,--plugin-opt,file=/home/umb/LLVM/order.txt"
> -DCMAKE_C_COMPILER=/home/umb/gcc49google/bin/gcc
> -DCMAKE_CXX_COMPILER=/home/umb/gcc49google/bin/g++
> 4. Remove order.txt and relink clang, that will create order.txt
> containing sections order for its executable.
> I shared it here: https://drive.google.com/file/d/0B_
> OWr6ld9gUmd1hnV1k4Z25NQVk/view?usp=sharing
> It is 18 megabytes in size.
> Order list file contains next sections:
> .text.unlikely.*
> .text.exit.*
> .text.startup.*
> .text.hot.*
> BTW, both gold and ld recognize suzh sections by name and use names as
> hint for sorting (no matter was order file used or not).
> I did not check how it works, but that is what I saw in bfd/gold source
> code.
> Do we want to implement the same for compatibility ?
> Finally, now when I have such file I am going to write a tool or script to
> prepare it to be used as linker input ordering file (looks it contains
> excessive data, I'll check) and run some tests.
> First one will be estimate clang launch time with sections ordered and
> without. I am thinking about other possible ones, may be compile simple
> helloworld ?
> https://reviews.llvm.org/D25766
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