[docs] Bump minimum GCC version to 7.4

[MaskRay]

Summary

GCC 7.3 cannot build 16.x releases.

In file included from /tmp/llvm-16/llvm/lib/Transforms/IPO/AttributorAttributes.cpp:14:0:
/tmp/llvm-16/llvm/include/llvm/Transforms/IPO/Attributor.h:1137:32: error: duplicate initialization of ‘llvm::AnalysisGetter::HasLegacyWrapper<Analysis, std::void_t<typename Analysis::Lega
cyWrapper> >’
 constexpr bool AnalysisGetter::HasLegacyWrapper<
                                ^~~~~~~~~~~~~~~~~
       Analysis, std::void_t<typename Analysis::LegacyWrapper>> = true;
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/llvm-16/llvm/include/llvm/Transforms/IPO/Attributor.h:1137:32: error: got 1 template parameters for ‘constexpr const bool llvm::AnalysisGetter::HasLegacyWrapper< <template-parameter-1
-1>, <template-parameter-1-2> >’
/tmp/llvm-16/llvm/include/llvm/Transforms/IPO/Attributor.h:1137:32: error:   but 2 required

The 17.x and main branches have more failures, e.g.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp: `error: cannot decompose class type ‘std::pair<llvm::Value*, const llvm::SCEV*>’: ...`

We probably should just give up 7.1 and say that GCC<=7.3 is unsupported.
There is evidence that GCC 7.4 works.
I have verified that GCC 7.5 is able to build check-{llvm,clang,clang-tools,lldb,lld,polly,mlir,bolt},
but not flang due to at least flang/Common/enum-class.h and a <charconv> in a unittest.

Link: https://discourse.llvm.org/t/require-gcc-7-5-as-gcc-7-3-cannot-build-llvm/72310

[aaron.ballman]

There's a mention on the RFC thread that Ubuntu 18.04 still ships with GCC 7.3. That's an LTS release but it EOLed just last month, so it's not clear how disruptive this would be. (To be clear, I'm not saying I'm opposed to the changes.)

[MaskRay]

In D156286#4534968, @aaron.ballman wrote:

There's a mention on the RFC thread that Ubuntu 18.04 still ships with GCC 7.3. That's an LTS release but it EOLed just last month, so it's not clear how disruptive this would be. (To be clear, I'm not saying I'm opposed to the changes.)

The entire 16.x releases LLVMAnalysis cannot be built with GCC 7.3, so this CMake change shouldn't be a larger disruption to any GCC <= 7.3 user.
And we should get the GCC bump patch into 17.x so that users will not get confused by the supportness.

The question is whether we bump to 7.4 or 7.5. I'd prefer 7.5, as there appears to be more users on 7.5 and can verify that 7.5 works reliably.

There is a user who prefers 7.4 but 7.5 works for them.
https://discourse.llvm.org/t/require-gcc-7-5-as-gcc-7-3-cannot-build-llvm-16-x-or-main/72310/29 says that Ubuntu 18.04 (End of Standard Support in June 2023) gets GCC 7.5 updates.

[aaron.ballman]

In D156286#4544631, @MaskRay wrote:

In D156286#4534968, @aaron.ballman wrote:

There's a mention on the RFC thread that Ubuntu 18.04 still ships with GCC 7.3. That's an LTS release but it EOLed just last month, so it's not clear how disruptive this would be. (To be clear, I'm not saying I'm opposed to the changes.)

The entire 16.x releases LLVMAnalysis cannot be built with GCC 7.3, so this CMake change shouldn't be a larger disruption to any GCC <= 7.3 user.
And we should get the GCC bump patch into 17.x so that users will not get confused by the supportness.

The question is whether we bump to 7.4 or 7.5. I'd prefer 7.5, as there appears to be more users on 7.5 and can verify that 7.5 works reliably.

There is a user who prefers 7.4 but 7.5 works for them.
https://discourse.llvm.org/t/require-gcc-7-5-as-gcc-7-3-cannot-build-llvm-16-x-or-main/72310/29 says that Ubuntu 18.04 (End of Standard Support in June 2023) gets GCC 7.5 updates.

Okay, it sounds like the changes won't be particularly disruptive then, so this is reasonable to me. Thank you!

llvm/docs/GettingStarted.rst

284–298

[aaron.ballman]

LG with the minor cleanup I found.

[jyknight]

Given that folks have successfully tested with GCC 7.4, and the lateness of the change in the release process for LLVM 17, I think it'd be better to require GCC 7.4 (the earliest that actually works), instead of increasing the requirement to 7.5.

[MaskRay]

7.5 => 7.4

[MaskRay]

reword description

[PiotrZSL]

llvm/docs/GettingStarted.rst

345

What about Glibc version ?
I were using GCC 9.1 + glibc 2.13 + some old kernel to compile Clang 15, after upgrading to Clang 16 I had to upgrade to kernel 3.11 and glibc 2.17 due to missing (I think) aligned_alloc.

[MaskRay]

llvm/docs/GettingStarted.rst

345

glibc support is worth its own discussion. glibc 2.13 was released in 2011. It's possible that no others has actively used this old version.

I think 2.19 has been tested and should work, but I am unclear about the support of older releases.

[aaron.ballman]

llvm/docs/GettingStarted.rst

284–298

I think you missed these requested changes when landing.

[MaskRay]

llvm/docs/GettingStarted.rst

284–298

Sorry:( Would you mind fixing it?

[aaron.ballman]

llvm/docs/GettingStarted.rst

284–298

No worries! I fixed it up in 80e80fa79bf66a74caf959bc420823e2b544dee9 -- can you squash those changes into your patch to move this to the 17.x branch? Or should we run consecutive cherry picks?

llvm/cmake/modules/CheckCompilerVersion.cmake

 # Check if the host compiler is new enough.
 # These versions are updated based on the following policy:
 #   llvm.org/docs/DeveloperPolicy.html#toolchain
 
 include(CheckCXXSourceCompiles)
 
-set(GCC_MIN 7.1)
-set(GCC_SOFT_ERROR 7.1)
+set(GCC_MIN 7.4)
+set(GCC_SOFT_ERROR 7.4)
 set(CLANG_MIN 5.0)
 set(CLANG_SOFT_ERROR 5.0)
 set(APPLECLANG_MIN 10.0)
 set(APPLECLANG_SOFT_ERROR 10.0)
 
 # https://en.wikipedia.org/wiki/Microsoft_Visual_C#Internal_version_numbering
 # _MSC_VER == 1927 MSVC++ 14.27 Visual Studio 2019 Version 16.7
 set(MSVC_MIN 19.27)

llvm/docs/GettingStarted.rst

 for the software package that LLVM depends on. The Version column provides
 "known to work" versions of the package. The Notes column describes how LLVM
 uses the package and provides other details.
 
 =========================================================== ============ ==========================================
 Package                                                     Version      Notes
 =========================================================== ============ ==========================================
 `CMake <http://cmake.org/>`__                               >=3.20.0     Makefile/workspace generator
-`GCC <http://gcc.gnu.org/>`_                                >=7.1.0      C/C++ compiler\ :sup:`1`
 `python <http://www.python.org/>`_                          >=3.6        Automated test suite\ :sup:`2`
 `zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`3`
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor\ :sup:`4`
 =========================================================== ============ ==========================================
 
 .. note::
 
    #. Only the C and C++ languages are needed so there's no need to build the
       other languages for LLVM's purposes. See `below` for specific version
       info.
    #. Only needed if you want to run the automated test suite in the
       ``llvm/test`` directory.
    #. Optional, adds compression / uncompression capabilities to selected LLVM
       tools.
    #. Optional, you can use any other build tool supported by CMake.
 
 Additionally, your compilation host is expected to have the usual plethora of
 Unix utilities. Specifically:
 
 * **ar** --- archive library builder
 * **bzip2** --- bzip2 command for distribution generation
 * **bunzip2** --- bunzip2 command for distribution checking
 * **chmod** --- change permissions on a file

 host C++ toolchain, both compiler and standard library, in order to build LLVM.
 
 LLVM is written using the subset of C++ documented in :doc:`coding
 standards<CodingStandards>`. To enforce this language version, we check the most
 popular host toolchains for specific minimum versions in our build systems:
 
 * Clang 5.0
 * Apple Clang 10.0
-* GCC 7.1
+* GCC 7.4
 * Visual Studio 2019 16.7
 
 Anything older than these toolchains *may* work, but will require forcing the
 build system with a special option and is not really a supported host platform.
 Also note that older versions of these compilers have often crashed or
 miscompiled LLVM.
 
 For less widely used host toolchains such as ICC or xlC, be aware that a very

 
 .. _toolchain testing PPA:
   https://launchpad.net/~ubuntu-toolchain-r/+archive/test
 .. _ask ubuntu stack exchange:
   https://askubuntu.com/questions/466651/how-do-i-use-the-latest-gcc-on-ubuntu/581497#58149
 .. _github gist:
   https://gist.github.com/application2000/73fd6f4bf1be6600a2cf9f56315a2d91
 
-Easy steps for installing GCC 7.1.0:
+Easy steps for installing a specific version of GCC:
 
 .. code-block:: console
 
-  % gcc_version=7.1.0
+  % gcc_version=7.4.0
   % wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2
   % wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2.sig
   % wget https://ftp.gnu.org/gnu/gnu-keyring.gpg
   % signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-${gcc_version}.tar.bz2.sig`
   % if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi
   % tar -xvjf gcc-${gcc_version}.tar.bz2
   % cd gcc-${gcc_version}
   % ./contrib/download_prerequisites