[gmp | mpir] Add --with-pic

[lrineau]

Describe the pull request

• What does your PR fix?

  Fixes [MPIR:x64-linux] build failure (static linking) #13827.

  Adding -fPIC is normally handled by the toolchain. However, GMP and MPIR need their own specific define -DPIC.

  See also the other attempt at [vcpkg-tool] Add -DPIC to pic flags on Linux #25158.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  all

• Does your PR follow the maintainer guide?

  Yes, I think so.

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes.

[github-actions]

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• ports/gmp/vcpkg.json

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[BillyONeal]

I added a line to the description describing why this isn't the typical "the toolchain should handle that" case :).

[BillyONeal]

Hmmm... usually it's dynamic libs that need PIC, not static. Are you sure you didn't flip the sense here?

[lrineau]

For non-Windows systems, dynamic libraries require position independent code (aka PIC). On Linux, vcpkg default toolchain, on x86_64, uses static libraries by default:

vcpkg/triplets/x64-linux.cmake

Lines 1 to 5 in 8e1f46d

	set(VCPKG_TARGET_ARCHITECTURE x64)
	set(VCPKG_CRT_LINKAGE dynamic)
	set(VCPKG_LIBRARY_LINKAGE static)
	set(VCPKG_CMAKE_SYSTEM_NAME Linux)

But toolchain adds -fPIC as well:

vcpkg/scripts/toolchains/linux.cmake

Lines 51 to 52 in 8e1f46d

	string(APPEND CMAKE_C_FLAGS_INIT " -fPIC ${VCPKG_C_FLAGS} ")
	string(APPEND CMAKE_CXX_FLAGS_INIT " -fPIC ${VCPKG_CXX_FLAGS} ")

That means that static libraries will be created, but with position independent code inside. That allows to link those static libraries to shared libraries.

The issue #13827 is a bug with GMP or MPIR, because GMP and MPIR have assembler code, to speed-up critical algorithms. That is why GMP and MPIR needs the macro PIC defined (with -DPIC, or with the option --with-pic to ./configure).

[lrineau]

PIC is already the default for shared libraries. We can add the option --with-pic:

• only to static builds, like in MPIR port file,
• or to any kind of build, like in GMP port file,

and that will have the same behavior: both static and dynamic builds will have PIC code.

[BillyONeal]

Makes sense 👍. Should we add a comment or something indicating that?

[dg0yt]

I was told that it is a concious decision that vcpkg always uses PIC, even for static triplets, to allow mixing of static and shared libs. It doesn't need to be documented per port. If there is a need to add a comment, it should be in a central location.

[BillyONeal]

@dg0yt I don't mean that overall consistency decision, I mean something like:

# Add --with-pic even in static for consistency with vcpkg's toolchain.
# Note that this is necessary for GMP's `-DPIC` rather than `-fPIC`.

[BillyONeal]

(To be clear, I'm not saying "please do this", I'm saying "have you considered doing this to stop people's eyebrows from being raised" @lrineau )