[CMake] Deprecate GCC_INSTALL_PREFIX

[MaskRay]

Part of https://reviews.llvm.org/D158218

GCC_INSTALL_PREFIX is a rarely-used legacy option inherited from
pre-CMake build system and has configuration file replacement nowadays.
Many clang/test/Driver tests specify --gcc-toolchain= to prevent
failures when GCC_INSTALL_PREFIX is specified: some contributors add
them to fix tests and some just do cargo culting. This is not healthy
for contributors adding cross compilation support for this rarely used
option.

DEFAULT_SYSROOT should in spirit be deprecated as well, but a relative
path doesn't have good replacement, so don't deprecate it for now.

Link: https://discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link: https://discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833

---

With GCC_INSTALL_PREFIX=/usr, clang a.c behaves like
clang --gcc-toolchain=/usr a.c.

Here is a simplified version of GCC installation detection code.

if (OPT_gcc_install_dir_EQ)
  return OPT_gcc_install_dir_EQ;

if (OPT_gcc_triple)
  candidate_gcc_triples = {OPT_gcc_triple};
else
  candidate_gcc_triples = collectCandidateTriples();
if (OPT_gcc_toolchain)
  prefixes = {OPT_gcc_toolchain};
else
  prefixes = {OPT_sysroot/usr, OPT_sysroot};
for (prefix : prefixes)
  if "$prefix/lib/gcc" exists // also tries $prefix/lib/gcc-cross
    for (triple : candidate_gcc_triples)
      if "$prefix/lib/gcc/$triple" exists
        return "$prefix/lib/gcc/$triple/$version"; // pick the largest version

--gcc-toolchain= specifies a directory where
lib/gcc{,-cross}/$triple/$version can be found. If you actually want
to use a specific version of GCC, specify something like
--gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/11 in a configuration
file. You can also specify --gcc-triple=.

On Debian and its derivatives where the target triple omits the vendor part, the following ways are roughly equivalent, except that --gcc-install-dir= specifies a version as well:

clang --gcc-toolchain=/usr a.c
clang --gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/11 a.c
clang --gcc-triple=x86_64-linux-gnu a.c

[llvmbot]

@llvm/pr-subscribers-clang

Author: Fangrui Song (MaskRay)

Changes

Part of https://reviews.llvm.org/D158218

GCC_INSTALL_PREFIX is a rarely-used legacy option inherited from
pre-CMake build system and has configuration file replacement nowadays.
Many clang/test/Driver tests specify --gcc-toolchain= or
--gcc-toolchain="" to prevent failures when GCC_INSTALL_PREFIX is
specified: some contributors add them to fix tests and some just do
cargo culting. This is not healthy for contributors adding cross
compilation support for this rarely used option.

DEFAULT_SYSROOT should in spirit be deprecated as well, but a relative
path doesn't have good replacement, so don't deprecate it for now.

Link: https://discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link: https://discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833

---

Full diff: https://github.com/llvm/llvm-project/pull/77537.diff

1 Files Affected:

• (modified) clang/CMakeLists.txt (+6)

diff --git a/clang/CMakeLists.txt b/clang/CMakeLists.txt
index 9f814478c45503..7076907da2528a 100644
--- a/clang/CMakeLists.txt
+++ b/clang/CMakeLists.txt
@@ -193,6 +193,12 @@ set(C_INCLUDE_DIRS "" CACHE STRING
 set(GCC_INSTALL_PREFIX "" CACHE PATH "Directory where gcc is installed." )
 set(DEFAULT_SYSROOT "" CACHE STRING
   "Default <path> to all compiler invocations for --sysroot=<path>." )
+if(GCC_INSTALL_PREFIX)
+  message(WARNING "GCC_INSTALL_PREFIX is deprecated and will be removed. Use "
+    "configuration files (https://clang.llvm.org/docs/UsersManual.html#configuration-files)"
+    "to specify the default --gcc-install-dir= or --gcc-triple=. --gcc-toolchain= is discouraged. "
+    "The option is often unneeded or better replaced with --gcc-install-dir=/--gcc-triple.")
+endif()
 
 set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 

[tstellar]

I'm in favor of removing this. It doesn't make a lot of sense now that we have better config file support.

[zmodem]

Seems okay to me, but it would be good to include an update to the release notes, which ideally shows an example of what replacing the cmake flag with a config file would look like.

[MaskRay]

Seems okay to me, but it would be good to include an update to the release notes, which ideally shows an example of what replacing the cmake flag with a config file would look like.

Thanks for the feedback. I have added pseudocode how GCC installation detection works in the description and changed release note/CMake warning to mention this pull request. The Debian example is too long, so I decide not to add too much code in the release node:)

https://github.com/llvm/llvm-project/compare/36acf7d2bdcdcfae5a60259a9a6f4869419d721e..8bd31c3cdb6ce41194b063c9ac13e5e2fbfbcc02

[trcrsired]

Are you going to fix gcc search for clang? i build a gcc 14 manually and clang cannot find it on linux.

It actually works for x86_64-w64-mingw32-gcc cross compiler on linux.

it cannot find any installation i have if gcc is set up in the $PATH.

/usr/local/bin/gcc
$HOME/toolchains/native/bin/gcc

I mean, it should at least try to find gcc libs in $PATH automatically on linux.