Description

This PR reduces the application's SOD (size on disk) by making __LINKEDIT Export Info section smaller in the stripped Mach-O binaries.

The feature is controlled by _ExportSymbolsExplicitly MSBuild property and can be disabled by specifying: -p:_ExportSymbolsExplicitly=true

Fixes #18332

Initial problem

It has been noticed that during stripping, the strip tool does not resize the export info section after it removes the symbols. Instead it only zeroes out the entries (achieved by calling prune_trie function):

• https://github.com/apple-oss-distributions/cctools/blob/cctools-986/misc/strip.c
• https://github.com/apple-oss-distributions/ld64/blob/ld64-711/src/other/PruneTrie.cpp

Thanks @lambdageek for helping to track this down.

Proposed approach

As Xamarin build process already collects all the required symbols needed for the application to run and preserves them during the strip phase, we can use the same file to instruct clang toolchain to export only those symbols via the command line options: -exported_symbols_list <file> (source). This will make the export info section only include what is necessary for the runtime - and at the same time eliminate the problem of the strip tool which does not resize stripped symbols.

Investigation setup

The issue is observable by building and inspecting the test application: https://github.com/xamarin/xamarin-macios/blob/main/tests/dotnet/MySingleView/MySingleView.csproj and targeting iOS platform in Release mode.

Results:

Even though zeroes are compressed well, the SOD is still affected and unused section takes around 1.5% of the most simplistic app size.
Much bigger impact has been noted when trying out a MAUI iOS template app with NativeAOT where the __LINKEDIT Export Info zeroes take up to 20MB of the SOD, but also with the regular macOS applications: dotnet/runtime#86707

Repros (outdated - see note)

NOTE I will leave the detailed analysis as part of this PR description for reference. However it should be noted that the used make targets listed bellow are outdated and are used during investigation.

Bellow I listed 3 scenarios for reproducing the current app sizes (with and without stripping) and the app size that we can get by using changes of this PR:

1. Repro current state of MySingleView.app with stripped binary
2. Repro current state of MySingleView.app with unstripped binary
3. Repro the new approach (-exported_symbols_list <file> + strip)

The repro steps assume that this PR has been checkout and that tools are invoked from the test sample directory: cd tests/dotnet/MySingleView

Repro current state of MySingleView.app with stripped binary

1. Build the app (you can ignore the need to run the sample, I just did it to make sure the changes do not break anything)

make run-device

2. Print the load commands - load_cmds_strip.list

otool -l bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > load_cmds_strip.list

• We are interested in the export info section:

cmd LC_DYLD_INFO_ONLY
...
export_off 5942960
export_size 207712

3. Create a hex dump of the export info section - hex_dump_strip.list

xxd -s 5942960 -l 207712 bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > hex_dump_strip.list

• NOTE: Notice around ~200kb of zeroes from ~ 0x005ab490 to ~ 0x005dda00

4. Verify exported symbols are correct - dyld_info_strip.list

dyld_info -exports bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > dyld_info_strip.list

Repro current state of MySingleView.app with unstripped binary

1. Build the app (the make target preserves the symbols)

make run-device-no-strip

2. Print the load commands - load_cmds_nostrip.list

otool -l bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > load_cmds_nostrip.list

• We are interested in the export info section:

cmd LC_DYLD_INFO_ONLY
...
export_off 5942960
export_size 207712

3. Create a hex dump of the export info section - hex_dump_nostrip.list

xxd -s 5942960 -l 207712 bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > hex_dump_nostrip.list

• Notice that the range: ~ 0x005ab490 to ~ 0x005dda00 now includes exported symbol entries

4. Verify exported symbols are correct - dyld_info_nostrip.list

dyld_info -exports bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > dyld_info_nostrip.list

Repro the new approach

1. Build the app (the make target uses the new approach)

make run-device-export-syms

2. Print the load commands - load_cmds_export.list

otool -l bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > load_cmds_export.list

• We are interested in the export info section notice the reduced size of the section:

cmd LC_DYLD_INFO_ONLY
...
export_off 5942432
export_size 1048

3. Create a hex dump of the export info section - hex_dump_export.list

xxd -s 5942432 -l 1048 bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > hex_dump_export.list

4. Verify exported symbols are correct - dyld_info_export.list

dyld_info -exports bin/Release/net7.0-ios/ios-arm64/MySingleView.app/MySingleView > dyld_info_export.list

Additional benefits

• With this approach we could also switch the way strip tool is invoked - to always strip all debug and local symbols via strip -S -x instead of passing the file with symbols to preserve. This would remove the warning that we are currently getting (which is being ignored):

/Applications/Xcode_14.3.0.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip: warning: removing global symbols from a final linked no longer supported.  Use -exported_symbols_list at link time when building...

Other references:

• https://github.com/qyang-nj/llios/blob/main/exported_symbol/README.md