The -platform_version flag behavior is a breaking change

[llvmbot]

Bugzilla Link	44813
Resolution	FIXED
Resolved on	Feb 27, 2020 05:01
Version	unspecified
OS	Windows NT
Blocks	#43900
Reporter	LLVM Bugzilla Contributor
CC	@hyp,@zmodem,@zygoloid

Extended Description

One of our build configurations is a Linux-to-Mac cross-compile that uses cctools-port, which is currently at ld64 version 450.

Using the clang 10.0 release candidates, these builds fail, because of unknown option: -platform_version, coming from https://reviews.llvm.org/D71579.

I see that there is a way to get the old behavior by adding -mlinker-version, but something doesn't feel right about having to do this. Generally the tools are pretty good about preserving compatibility, unless there is a really strong reason not to.

I'd like to request that either:

• The default be reconsidered, or
• To make an explicit statement that the current behavior should stay, acknowledging that it is a build-breaking change for some codebases, and update the release notes to mention this change and the workaround.

[zmodem]

arphaman: Do you have any thoughts here?

[llvmbot]

As a comparison point, I'll note that all other uses of Version in darwin::Linker::AddLinkArgs use >= tests to enable new behavior only when a sufficient linker version is known, e.g.:

// Newer linkers support -demangle. Pass it if supported and not disabled by
// the user.
if (Version[0] >= 100 && !Args.hasArg(options::OPT_Z_Xlinker__no_demangle))
CmdArgs.push_back("-demangle");

in other words nothing else turns on by default when the version is unknown. So the -platform_version behavior is inconsistent with the rest.

[llvmbot]

Since I've heard neither support nor objection, I went ahead and proposed a patch in https://reviews.llvm.org/D74784; maybe that will evoke some feedback.

[zmodem]

That was committed to master as 5122e82 and pushed to 10.x as 38ee10d.