LLVM 20 transition

[ziyao233]

TODO

• Fix build of LLVM 20
  • Rework pacman.conf and remove -fno-plt on riscv64 and loongarch64, where LLVM doesn't support the option and emits a warning starting from LLVM 20
• Rust compatibility
  • Update to LLVM 20 rust-lang/rust#135763
  • Is it necessary to wait for Rust 1.87.0? (will stablize on May 15th)
  • Rust 1.86.0 seems to work well with LLVM 20.1.4
  • Upgrade to 1.86.0: [rust] 1.86.0-1: new upstream version #3901, [rust] 1.86.0-2: Fix installation of license files #3905
  • Rebuild a temporary LLVM-19 library-only package to get old rustc working
  • Rebuild Rust against LLVM 20 with the help of LLVM 19 (the temporary library package)
  • Rebuild Rust against LLVM 20 in a clean root
• Graphics stack
  • spirv-llvm-translator: [spirv-llvm-translator] 20.1.2-1: new upstream version #3906
  • libclc: [libclc] 20.1.4-1: new upstream version #3907
  • mesa: [mesa] 25.0.4-2: Rebuild against LLVM 20.1.4 #3900

2025.05.11: The transition has been completed!

Workarounds/Bugs to recheck after upgrading

• linux: Workaround for stack frame size of AMDGPU
  • linux: https://github.com/eweOS/packages/blob/linux/0001-amdgpu-dml2-Increase-max-stack-size.patch
  • linux-lts: https://github.com/eweOS/packages/blob/linux-lts/0001-amdgpu-dml2-Increase-max-stack-size.patch
• [ ] vala: ICE on riscv64 caused by stability problems of one of our builder.
• lapack: ICE on loongarch64: has been fixed in LLVM 20, congratulations!

[ziyao233]

Noted some warnings when building 20.1.2,

CMake Warning at CMakeLists.txt:161 (message):                                                                                                                 
  Using LLVM_ENABLE_PROJECTS=compiler-rt is deprecated now, and will become a                                                                                  
  fatal error in the LLVM 21 release.  Please use                                                                                                              
  -DLLVM_ENABLE_RUNTIMES=compiler-rt or see the instructions at                                                                                                
  https://compiler-rt.llvm.org/ for building the runtimes.    

we'd better to fix it in #3600 as well.

[ziyao233]

Noted some warnings when building 20.1.2,

CMake Warning at CMakeLists.txt:161 (message):                                                                                                                 
  Using LLVM_ENABLE_PROJECTS=compiler-rt is deprecated now, and will become a                                                                                  
  fatal error in the LLVM 21 release.  Please use                                                                                                              
  -DLLVM_ENABLE_RUNTIMES=compiler-rt or see the instructions at                                                                                                
  https://compiler-rt.llvm.org/ for building the runtimes.    

we'd better to fix it in #3600 as well.

Building compiler-rt as a project instead of runtime seems to mess the buildsystem up, leading to visibility of symbols in compiler-rt are incorrectly set to default. This is the root cause #3839.

The change is temporarily reverted1. We should finish the transition first.

[ziyao233]

    * [ ]  Rework pacman.conf and remove `-fno-plt` on riscv64 and loongarch64, where LLVM doesn't support the option and [emits a warning starting from LLVM 20](https://github.com/llvm/llvm-project/pull/124081)* [ ]  Rust compatibility

Our builders have upgraded to LLVM 20, so we must workaround this when reworking pacman.conf or meson may misdetect the environment,

Running compile:
Working directory:  /root/packages/src/pacman-v7.0.0/build/meson-private/tmprojqrsat
Code:
#include <sys/types.h>
        void bar(void) {
            (void) sizeof(uid_t);
        }
-----------
Command line: `cc /root/packages/src/pacman-v7.0.0/build/meson-private/tmprojqrsat/testfile.c -o /root/packages/src/pacman-v7.0.0/build/meson-private/tmprojqrs
at/output.obj -c -Os -pipe -fno-plt -march=la464 -D_FILE_OFFSET_BITS=64 -O0 -Werror=implicit-function-declaration -Werror=unknown-warning-option -Werror=unused
-command-line-argument -Werror=ignored-optimization-argument -std=gnu99` -> 1
stderr:
cc: error: argument unused during compilation: '-fno-plt' [-Werror,-Wunused-command-line-argument]
-----------
Checking for type "uid_t" : NO 

[ziyao233]

Clang 20 starts to error on void functions whose parameters are applied with [[lifetimebound]] modifier.

This affects libtg_owt: issue desktop-app/tg_owt#150, workaround PR #3991

Reference: llvm/llvm-project#113460
Reference: https://github.com/llvm/llvm-project/blob/d102e90de5d1fd0ea1e0395d64d4be7af88669c7/clang/include/clang/Basic/AttrDocs.td#L4225-L4286