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[SR-13631] Add CMake options + build-script flag to turn off linker optimizations #56066

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typesanitizer opened this issue Sep 30, 2020 · 4 comments
Closed

[SR-13631] Add CMake options + build-script flag to turn off linker optimizations #56066

typesanitizer opened this issue Sep 30, 2020 · 4 comments
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good first issue Good for newcomers improvement

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@typesanitizer
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Previous ID SR-13631
Radar rdar://problem/69768708
Original Reporter @typesanitizer
Type Improvement
Status Closed
Resolution Done
Additional Detail from JIRA
Votes 0
Component/s Project Infrastructure
Labels Improvement, StarterBug
Assignee mininny (JIRA)
Priority Medium

md5: 2fe271a48791ae85b5f8c0537041d016

Issue Description:

1. Add a build-script flag to turn off dead stripping (no-op on non-Darwin platform).
2. Pass it down to CMake and use it to turn off dead-stripping in the code below.
3. Suggest this flag in the getting started guide; debugging with a RelWithDebInfoAssert build should improve after turning this on because debugging-only functions like the various dump() methods should not be getting optimized out.

  if(NOT CMAKE_BUILD_TYPE STREQUAL Debug)
    if(CMAKE_SYSTEM_NAME MATCHES Darwin)
      # See rdar://48283130: This gives 6MB+ size reductions for swift and
      # SourceKitService, and much larger size reductions for sil-opt etc.
      target_link_options(${target} PRIVATE
        "SHELL:-Xlinker -dead_strip")
    endif()
  endif()

Strictly speaking, part 1 is optional as build-script already has a --extra-cmake-options flag.
@typesanitizer
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@swift-ci create

@swift-ci
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Comment by Minhyuk Kim (JIRA)

I've been trying to implement this, but I'm struggling because I'm not seeing much of a difference on the console output after making the changes to the build process. Can you give me any guidance on how to debug the changes I make on CMake through Logs or something? 🙂

@typesanitizer
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For CMake itself, you can use print debugging using MESSAGE (https://cmake.org/cmake/help/latest/command/message.html).

Once CMake has finished running, if you look at the generated build.ninja (under $BUILD_DIR/swift-macosx-x86_64), you will see some stuff like:

build bin/swift-frontend: # other stuff
  FLAGS = ...
  LINK_FLAGS = ...

In the LINK_FLAGS, you should see -Xlinker -dead_strip for non-debug builds; this should go away when your new flag is passed.

@swift-ci
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Comment by Minhyuk Kim (JIRA)

Thanks for the guide theindigamer (JIRA User)! 🙂

I created a PR here!

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@typesanitizer @swift-ci