[clang] Disable FTZ/DAZ when compiling shared libraries by default. (PR #80475)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> Just to be clear: I'm not proposing entirely eliminating the "link against crtfastmath.o" behavior, when linking a binary with `-ffast-math` (though, separately from _this_ review, that may be worth considering!). I only meant we should stop attempting to infer anything about `-fdenormal-fp-math` due to using `-ffast-math`. (as per the other paragraph in my last comment).

Are youe suggesting that we should continue linking against crtfastmath.o when it is available and fast-math is enabled but we should set the "denormal-fp-math" attributes to "ieee" or "dynamic"? I want to make sure I'm understanding you correctly.

Personally, I don't like linking with crtfastmath.o to get this behavior. I would prefer to insert code into whatever function(s), if any, we identify as a relevant entry point. That would require having the attribute set, at least for the entry function(s). Linking with crtfastmath.o depends on finding it, which I think makes it dependent on the GNU toolchain, and the static initializer is less visible to users debugging their program.

For x86-based CPU targets, we really shouldn't be making assumptions about the FTZ/DAZ state for any function where we aren't setting it, but if it's useful for other targets, we shouldn't take that away.

https://github.com/llvm/llvm-project/pull/80475