Implement function-sections for Haskell code, Trac #8405
This adds a flag -split-sections that does similar things to
-split-objs, but using sections in single object files instead of
relying on the Satanic Splitter and other abominations. This is very
similar to the GCC flags -ffunction-sections and -fdata-sections.
The --gc-sections linker flag, which allows unused sections to actually
be removed, is added to all link commands (if the linker supports it) so
that space savings from having base compiled with sections can be
Supported both in LLVM and the native code-gen, in theory for all
architectures, but really tested on x86 only.
In the GHC build, a new SplitSections variable enables -split-sections
for relevant parts of the build.
Test Plan: validate with both settings of SplitSections
Reviewers: dterei, Phyx, austin, simonmar, thomie, bgamari
Reviewed By: simonmar, thomie, bgamari
Subscribers: hsyl20, erikd, kgardas, thomie
Differential Revision: https://phabricator.haskell.org/D1242
GHC Trac Issues: Trac #8405