Automatically download LLVM build based on host platform

[giordano]

Tested on Linux with x86-64 and aarch64 architectures, and macOS Apple Silicon, for both LLVM 20.1.8 and 21.1.8 (related to #22).

CC @gbaraldi @topolarity.

[giordano]

The only annoying thing is that llvm-config in the macOS build hardcodes absolute paths referring to Homebrew prefix, so that works only if one actually uses Homebrew, which I don't on my laptop, but I found another machine with Homebrew and it worked there.

Also, for the record I had issues on a couple of different aarch64-linux machines where compilation with LLVM_VER=21.1.8 failed with

In file included from tools/gen_target_tables.cpp:9:
In file included from /home/mose/repo/cpufeatures/build/llvm-21.1.8/include/llvm/ADT/StringMap.h:17:
In file included from /home/mose/repo/cpufeatures/build/llvm-21.1.8/include/llvm/ADT/StringMapEntry.h:19:
In file included from /home/mose/repo/cpufeatures/build/llvm-21.1.8/include/llvm/ADT/StringRef.h:12:
/home/mose/repo/cpufeatures/build/llvm-21.1.8/include/llvm/ADT/DenseMapInfo.h:17:10: fatal error: 'cassert' file not found
   17 | #include <cassert>
      |          ^~~~~~~~~
1 error generated.

The problem was that Clang was finding the wrong GCC installation, I fixed that by manually passing --gcc-install-dir=/usr/lib/gcc/aarch64-linux-gnu/13 (on these machines Clang was picking GCC14 installation, which for some reason didn't have all the C++ header files). This was a very system-specific issue (it worked out-of-the-box on machines where Clang picks a working GCC installation), but I shared the solution just in case someone else runs into the same.

[gbaraldi]

Is this because different targets have different headers?

[christiangnrd]

It's a fix to get LLVM 21 tables generating

[giordano]

It was always a bug:

cpufeatures/tools/gen_target_tables.cpp

Line 155 in 27da145

OS << "0x" << Twine::utohexstr(Word) << "ULL";

refers to Twine without including the necessary header file. Previous versions relied on accidental forward inclusion, but that's not very robust (and finally broke with LLVM 21).

[gbaraldi]

Looks reasonable

[giordano]

Looks reasonable

If this is good to go, can you please merge? I can't do it myself 🙂

[gbaraldi]

I can merge it but not give you the bit seemingly