Re: [PATCH 27/30] build: move compiler version check to meson

[Paolo Bonzini]

On 12/9/22 12:52, Peter Maydell wrote:
> The new code makes it much harder to move our compiler version
> requirements forward in future, because there's no longer a simple
> "check for normal clang X or apple clang Y" test where we can
> bump up X and Y based on what's provided in the various host
> platforms we have to support. Doesn't meson provide a way to do
> the version check on the version number the way we were doing
> previously?

Yes, I could just do the check with #error just like before.

For GCC I used the nicer "compiler.version().version_compare('>=x.y')" 
check.  For clang we have to deal with both upstream and Apple version 
numbers; it's also possible to check for __apple_build_version__ to 
distinguish between the two and then do version_compare(), but at that 
point it's easier to just use cc.compiles() and #error.

While it's possible to learn the upstream version corresponding to Apple 
compilers with "c++ --verbose", it is not really machine-friendly so 
Meson does not attempt to parse it and neither did QEMU's configure script.

So I went for the trick of using -Wpragma-pack, which was extracted from 
https://github.com/simd-everywhere/simde/blob/master/simde/simde-detect-clang.h. 
 The advantage is that you only have one check, the disadvantage is of 
course that it obscures what's going on.  I can add a clearer comment 
around the check, or switch to #error and add a pointer to 
https://trac.macports.org/wiki/XcodeVersionInfo where the mapping can be 
found, or really anything else that is your preference.

Paolo