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

["Shi, Steven" <steven.shi@intel.com>]

Hi Ard, Zenith,
Thank you both explained the complete knowledge about ELF GOT, LTO, PIC/PIE, machine code mode and GCC visibility #pragma. It is pretty good to read them all in one picture. And I believe copying these explain to a edk2 wiki page in GitHub could be very useful for other edk2 developers. 

>From code change impact view, I see to use the hidden visibility for LTO, which is to remove the !defined(USING_LTO) in X64/ProcessorBind.h actually, need to change other 20+ files overall the edk2. The cost looks not small. We might need more justification to accept such change.  Does the hidden visibility in LTO can improve the LTO build code size? Is there any other benefit?

Steven Shi
Intel\SSG\STO\UEFI Firmware

Tel: +86 021-61166522
iNet: 821-6522

> -----Original Message-----
> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
> Sent: Tuesday, June 12, 2018 11:23 PM
> To: edk2-devel@lists.01.org
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; Kinney, Michael D
> <michael.d.kinney@intel.com>; Gao, Liming <liming.gao@intel.com>; Ni,
> Ruiyu <ruiyu.ni@intel.com>; Wu, Hao A <hao.a.wu@intel.com>; Leif
> Lindholm <leif.lindholm@linaro.org>; Justen, Jordan L
> <jordan.l.justen@intel.com>; Andrew Fish <afish@apple.com>; Zeng, Star
> <star.zeng@intel.com>; Dong, Eric <eric.dong@intel.com>; Laszlo Ersek
> <lersek@redhat.com>; Zenith432 <zenith432@users.sourceforge.net>; Shi,
> Steven <steven.shi@intel.com>
> Subject: [RFC PATCH 00/11] GCC/X64: use hidden visibility for LTO PIE code
> 
> 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(-)
> 
> --
> 2.17.1