[RFC PATCH 00/11] GCC/X64: use hidden visibility for LTO PIE code

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

The GCC toolchain uses PIE mode when building code for X64, because it
is the most efficient in size: it uses relative references where
possible, but still uses 64-bit quantities for absolute symbol
references, which is optimal for executables that need to be converted
to PE/COFF using GenFw.

Enabling PIE mode has a couple of side effects though, primarily caused
by the fact that the primary application area of GCC is to build programs
for userland. GCC will assume that ELF symbols should be preemptible (which
makes sense for PIC but not for PIE, but this simply seems to be the result
of code being shared between the two modes), and it will attempt to keep
absolute references close to each other so that dynamic relocations that
trigger CoW for text pages have the smallest possible footprint.

These side effects can be mititgated by overriding the visibility of all
symbol definitions *and* symbol references, using a special #pragma. This
will inform the compiler that symbol preemption and dynamic relocations
are not a concern, and that all symbol references can be emitted as direct
relative references rather than relative references to a GOT entry containing
the absolute address. Unsurprisingly, this leads to better and smaller code.

Unfortunately, we have not been able to set this override when LTO is in
effect, because the LTO code generator infers from the hidden visibility
of all symbols that none of the code is reachable, and discards it all,
leading to corrupt, empty binaries.

We can work around this by overriding the visibility for symbols that are
module entry points. So implement this for all occcurrences of the symbol
'_ModuleEntryPoint', and enable 'hidden' visibility in LTO builds as well.

Note that all the changes in this series resolve to no-ops if USING_LTO
is not #defined.

Code can be found here:
https://github.com/ardbiesheuvel/edk2/tree/x64-lto-visibility

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Zenith432 <zenith432@users.sourceforge.net>
Cc: "Shi, Steven" <steven.shi@intel.com>

Ard Biesheuvel (11):
  MdePkg/ProcessorBind.h: define macro to decorate module entry points
  DuetPkg: annotate module entry points with EFI_ENTRYPOINT
  EdkCompatibilityPkg: annotate module entry points with EFI_ENTRYPOINT
  EmbeddedPkg: annotate module entry points with EFI_ENTRYPOINT
  EmulatorPkg: annotate module entry points with EFI_ENTRYPOINT
  IntelFrameWorkPkg: annotate module entry points with EFI_ENTRYPOINT
  MdeModulePkg: annotate module entry points with EFI_ENTRYPOINT
  MdePkg: annotate module entry points with EFI_ENTRYPOINT
  Nt32Pkg: annotate module entry points with EFI_ENTRYPOINT
  UefiCpuPkg: annotate module entry points with EFI_ENTRYPOINT
  MdePkg/ProcessorBind.h X64: drop non-LTO limitation on visiblity
    override

 DuetPkg/DxeIpl/DxeInit.c                         |  1 +
 DuetPkg/EfiLdr/EfiLoader.c                       |  1 +
 .../EntryPoints/EdkIIGlueDxeDriverEntryPoint.c   |  1 +
 .../EntryPoints/EdkIIGluePeimEntryPoint.c        |  1 +
 .../EntryPoints/EdkIIGlueSmmDriverEntryPoint.c   |  1 +
 .../Library/EdkIIGlueDxeSmmDriverEntryPoint.h    |  1 +
 .../Include/Library/EdkIIGluePeimEntryPoint.h    |  1 +
 .../Library/EdkIIGlueUefiDriverEntryPoint.h      |  1 +
 EmbeddedPkg/TemplateSec/TemplateSec.c            |  1 +
 EmulatorPkg/Sec/Sec.c                            |  1 +
 .../DxeSmmDriverEntryPoint/DriverEntryPoint.c    |  1 +
 MdeModulePkg/Universal/CapsulePei/X64/X64Entry.c |  1 +
 MdePkg/Include/Base.h                            |  7 +++++++
 MdePkg/Include/Library/DxeCoreEntryPoint.h       |  1 +
 MdePkg/Include/Library/PeiCoreEntryPoint.h       |  1 +
 MdePkg/Include/Library/PeimEntryPoint.h          |  1 +
 .../Include/Library/UefiApplicationEntryPoint.h  |  1 +
 MdePkg/Include/Library/UefiDriverEntryPoint.h    |  1 +
 MdePkg/Include/X64/ProcessorBind.h               | 16 +++++++++++-----
 .../DxeCoreEntryPoint/DxeCoreEntryPoint.c        |  1 +
 .../PeiCoreEntryPoint/PeiCoreEntryPoint.c        |  1 +
 MdePkg/Library/PeimEntryPoint/PeimEntryPoint.c   |  1 +
 .../ApplicationEntryPoint.c                      |  1 +
 .../UefiDriverEntryPoint/DriverEntryPoint.c      |  1 +
 Nt32Pkg/Sec/SecMain.c                            |  1 +
 .../PlatformSecLibNull/PlatformSecLibNull.c      |  1 +
 26 files changed, 42 insertions(+), 5 deletions(-)

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2.17.1