jcranmer-intel added a comment.

Some general comments on the documentation here:

One of the lesser-known issues of `-ffast-math` is the fact that it (or 
`-funsafe-math-optimizations`) causes `crtfastmath.o` to be linked, which adds 
a static constructor that sets the FTZ/DAZ bits in MXCSR, affecting not only 
the current compilation unit but all static and shared libraries included in 
the program. This behavior deserves to be mentioned somewhere in the 
documentation. Additionally, the effects of fast-math/unsafe-math-optimizations 
on fdenormal-fp-math{32} should be noted. I realize this is orthogonal to the 
actual change you're doing here, but while we're improving the documentation of 
`-f[no-]fast-math`, we should probably fix this missing documentation at the 
same time.



================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:2778
+  StringRef LastSeenFfpContractOption;
+  StringRef LastSeenFfastMathOption;
   if (!JA.isDeviceOffloading(Action::OFK_Cuda) &&
----------------
Given that this will only be set to "" or "fast", it probably makes sense to 
make for this be a `bool SeenFfastMathOption` variable instead.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D123630/new/

https://reviews.llvm.org/D123630

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