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Merge #19407: doc: explain why passing -mlinker-version is required when cross-compiling
a8d39b8840 doc: explain why passing -mlinker-version is required (fanquake)

Pull request description:

  I have been down a 🐇 hole. Closes #19359.

  When Clang is compiled, [a check is run](https://github.com/llvm/llvm-project/blob/release/8.x/clang/CMakeLists.txt#L353) to define `HOST_LINK_VERSION` as the output of `$CMAKE_LINKER -v`. Note the this is the version of the linker being used to compile Clang itself.. and this check is only run when compiling Clang for macOS.

  In the Clang driver, if `HOST_LINK_VERSION` has been defined, there is some additional runtime functionality. An `-mlinker-version` argument, with the value of `HOST_LINK_VERSION` [will be added to the linker arguments](89de0d8dfb/clang/lib/Driver/Driver.cpp (L382)), if `-mlinker-version` has not been passed in by the user.

  This is a bit weird, as by default, you are setting `-mlinker-version` to the version of the linker that was used to build the Clang binary, not the linker which will be used when compiling. The commit which introduced the functionality, 628fcf4e3b, described it as a "hack", that should be replaced. However, that was 10 years ago, and the behaviour is still here.

  In the Darwin driver, [a check is done](89de0d8dfb/clang/lib/Driver/ToolChains/Darwin.cpp (L208)) for the `-mlinker-version` argument. If there is no argument, the version will default to `0`. Given the above, this should never happen when using Clang for macOS. A series of comparisons are then performed, to check whether the linker version is modern enough to enable certain features, like [`-demangle`](89de0d8dfb/clang/lib/Driver/ToolChains/Darwin.cpp (L215)).

  ### What this means

  #### macOS

  A Clang compiled for macOS, i.e `clang+llvm-8.0.0-x86_64-apple-darwin`, will have `HOST_LINKER_VERSION` set to the version of the linker used to compile Clang itself.

  At runtime, `-mlinker-version=HOST_LINKER_VERSION` will be added to the linker args, if `-mlinker-version` wasn't passed in. In the Darwin driver, additional arguments, like `-demangle`, will be added to the linker arguments, because `HOST_LINKER_VERSION` was likely some very modern version of `lld` or `ld64`.

  #### Linux (cross compilation in depends)

  A Clang compiled for Linux, i.e `clang+llvm-8.0.0-x86_64-linux-gnu-ubuntu-14.04`, which we now use for macOS builds in depends, will behave differently. As it's built for Linux, `HOST_LINKER_VERSION` was not defined at compile time, and there will be no default behaviour of appending `-mlinker-version=HOST_LINKER_VERSION` to the linker args. Thus, unless you pass in `-mlinker-version` yourself, when the version checks are done in the Darwin driver, no modern linker features will be enabled, as the version will have defaulted to `0`.

  Therefore, it's important that we continue to pass `-mlinker-version="our LD64 version"` as part of our compilation flags, if we want to have "modern" linker features enabled for our macOS builds.

  #### Summary

  [Clang 8](https://releases.llvm.org/download.html#8.0.0). Building a macOS binary. Link line with path arguments trimmed.

  | | default behaviour | `-mlinker-version=100` (`-demangle threshold`) | `-mlinker-version=530` |
  | - | --------------- | --------------------- | ---------------------- |
  | macOS Clang | `-demangle -lto_library ../libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -macosx_version_min 10.15.0 -o a.out ../test-b8b9b3.o -lc++ -lSystem ../libclang_rt.osx.a` | `-demangle -dynamic -arch x86_64 -macosx_version_min 10.15.0 -o a.out ../test-a66966.o -lc++ -lSystem ../libclang_rt.osx.a` | same as default |
  | Linux Clang | `-dynamic -arch x86_64 -macosx_version_min 10.12.0 -o a.out ../test-bfce57.o -lc++ -lSystem` | `-demangle -dynamic -arch x86_64 -macosx_version_min 10.12.0 -o a.out ../test-a846a3.o -lc++ -lSystem` | `-demangle -lto_library ../libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -macosx_version_min 10.12.0 -o a.out ../test-de0280.o -lc++ -lSystem` |

  Note: Most links here are pointing to the 8.x branch of LLVM/Clang, as we are using that version in depends.

  Note: To add a little more confusion, you wont see `-mlinker-version X` in your compile flags, you'll see [`-target-linker-version X`](431daedee4/clang/lib/Driver/ToolChains/Clang.cpp (L4777)).

ACKs for top commit:
  laanwj:
    ACK a8d39b8840

Tree-SHA512: 92f93079a5e59a0d561e74336b5cb03e3bf5a34437f5850283b9128c7624494b8285ec16290b1fa8103fe87f8789a53ce44b17902b8c1db5fde24d74b76fb168
2020-07-03 17:38:27 +08:00
.github doc: Add redirect for GUI issues and pull requests 2020-06-08 10:06:02 -04:00
.tx tx: Bump transifex slug to 020x 2020-03-16 10:52:55 +01:00
build-aux/m4 Merge #18297: build: Use pkg-config in BITCOIN_QT_CONFIGURE for all hosts including Windows 2020-06-13 15:41:39 +08:00
build_msvc build: improve builtin_clz* detection 2020-06-29 11:31:17 +08:00
ci Merge #18288: build: Add MemorySanitizer (MSan) in Travis to detect use of uninitialized memory 2020-07-02 10:26:50 -04:00
contrib Merge #19344: docs: update testgen usage example 2020-06-29 14:21:51 +08:00
depends Merge #19407: doc: explain why passing -mlinker-version is required when cross-compiling 2020-07-03 17:38:27 +08:00
doc Merge #19408: doc: Windows WSL build recommendation to temporarily disable Win32 PE support 2020-07-01 17:24:41 +02:00
share Merge #18616: refactor: Cleanup clientversion.cpp 2020-05-13 20:14:51 +02:00
src Merge #19215: psbt: Include and allow both non_witness_utxo and witness_utxo for segwit inputs 2020-07-03 09:23:22 +12:00
test Merge #18649: tests: Add std::locale::global to list of locale dependent functions in lint-locale-dependence.sh 2020-07-03 10:42:18 +08:00
.appveyor.yml Merge #18640: appveyor: Remove clcache 2020-04-15 16:19:52 -04:00
.cirrus.yml ci: Upgrade most ci configs to focal 2020-06-19 10:44:11 -04:00
.fuzzbuzz.yml ci: Add fuzzbuzz integration 2020-04-14 16:38:26 +00:00
.gitattributes Separate protocol versioning from clientversion 2014-10-29 00:24:40 -04:00
.gitignore This PR adds initial support for type hints checking in python scripts. 2020-06-02 08:03:02 +02:00
.python-version .python-version: Specify full version 3.5.6 2019-03-02 12:06:26 -05:00
.style.yapf test: .style.yapf: Set column_limit=160 2019-03-04 18:28:13 -05:00
.travis.yml Merge #18288: build: Add MemorySanitizer (MSan) in Travis to detect use of uninitialized memory 2020-07-02 10:26:50 -04:00
autogen.sh scripted-diff: Bump copyright of files changed in 2019 2019-12-30 10:42:20 +13:00
configure.ac Merge #18307: build: Require pkg-config for all of the hosts 2020-07-03 16:15:52 +08:00
CONTRIBUTING.md doc: Mention repo split in the READMEs 2020-06-08 10:06:14 -04:00
COPYING doc: Update license year range to 2020 2019-12-26 23:11:21 +01:00
INSTALL.md Update INSTALL landing redirection notice for build instructions. 2016-10-06 12:27:23 +13:00
libbitcoinconsensus.pc.in build: remove libcrypto as internal dependency in libbitcoinconsensus.pc 2019-11-19 15:03:44 +01:00
Makefile.am tests: run test-security-check.py in CI 2020-06-16 19:52:30 +08:00
README.md doc: Mention repo split in the READMEs 2020-06-08 10:06:14 -04:00
SECURITY.md doc: Remove explicit mention of version from SECURITY.md 2019-06-14 06:39:17 -04:00

Bitcoin Core integration/staging tree

https://bitcoincore.org

What is Bitcoin?

Bitcoin is an experimental digital currency that enables instant payments to anyone, anywhere in the world. Bitcoin uses peer-to-peer technology to operate with no central authority: managing transactions and issuing money are carried out collectively by the network. Bitcoin Core is the name of open source software which enables the use of this currency.

For more information, as well as an immediately usable, binary version of the Bitcoin Core software, see https://bitcoincore.org/en/download/, or read the original whitepaper.

License

Bitcoin Core is released under the terms of the MIT license. See COPYING for more information or see https://opensource.org/licenses/MIT.

Development Process

The master branch is regularly built (see doc/build-*.md for instructions) and tested, but it is not guaranteed to be completely stable. Tags are created regularly from release branches to indicate new official, stable release versions of Bitcoin Core.

The https://github.com/bitcoin-core/gui repository is used exclusively for the development of the GUI. Its master branch is identical in all monotree repositories. Release branches and tags do not exist, so please do not fork that repository unless it is for development reasons.

The contribution workflow is described in CONTRIBUTING.md and useful hints for developers can be found in doc/developer-notes.md.

Testing

Testing and code review is the bottleneck for development; we get more pull requests than we can review and test on short notice. Please be patient and help out by testing other people's pull requests, and remember this is a security-critical project where any mistake might cost people lots of money.

Automated Testing

Developers are strongly encouraged to write unit tests for new code, and to submit new unit tests for old code. Unit tests can be compiled and run (assuming they weren't disabled in configure) with: make check. Further details on running and extending unit tests can be found in /src/test/README.md.

There are also regression and integration tests, written in Python, that are run automatically on the build server. These tests can be run (if the test dependencies are installed) with: test/functional/test_runner.py

The Travis CI system makes sure that every pull request is built for Windows, Linux, and macOS, and that unit/sanity tests are run automatically.

Manual Quality Assurance (QA) Testing

Changes should be tested by somebody other than the developer who wrote the code. This is especially important for large or high-risk changes. It is useful to add a test plan to the pull request description if testing the changes is not straightforward.

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Changes to translations as well as new translations can be submitted to Bitcoin Core's Transifex page.

Translations are periodically pulled from Transifex and merged into the git repository. See the translation process for details on how this works.

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