When testing different linkers, it's sometimes useful to run the tests
with `SWIFT_DRIVER_TEST_OPTIONS=" -use-ld=<linker>"`. If we do this,
it will break a handful of tests because they expect the compiler driver
to choose an appropriate linker automatically.
To avoid having these fail, detect when someone has done this, and
set a new feature, `linker_overridden`, then mark the tests in question
with `UNSUPPORTED: linker_overridden`.
rdar://123504095
We changed to `llvm.compiler.used` because of the behaviour of `gold`,
which refuses to coalesce sections that have different `SHF_GNU_RETAIN`
flags, which causes problems with metadata.
Originally I thought we were going to have to generate two sections
with distinct names and have the runtime look for both of them, but
it turns out that the runtime only wants to see sections that have
`SHF_GNU_RETAIN` in any case. It's really the reflection code that
is interested in being able to see non-retained sections. The upshot
is that we don't need to use `llvm.compiler.used`; it's just fine if
we have duplicate sections, as long as the reflection code looks for
them when it's inspecting an ELF image.
This also means we no longer need to pass `-z nostart-stop-gc` to the
linker if we're using `lld`.
rdar://123504095
Also, make the analogous change to apple/swift-driver#1372, which gets the
sanitizer tests working on Android again, and remove the lld_lto feature in the
tests, which is now unused.
Fixes#60406
Ran the linux x86_64 compiler validation suite on this pull with `-use-ld=lld -Xlinker --gc-sections`, then without stripping as in the linked issue, and only saw the 20 unrelated failures just like @drodriguez had.
This commit adds LTO support for handling linker options and LLVM BC
emission. Even for ELF, swift-autolink-extract is unnecessary because
linker options are embeded in LLVM BC content when LTO.
This commit adds -lto flag for driver to enable LTO at LLVM level.
When -lto=llvm given, compiler emits LLVM bitcode file instead of object
file and perform thin LTO using libLTO.dylib plugin.
When -lto=llvm-full given, perform full LTO instead of thin LTO.
