[PATCH] D67517: Create UsersManual section entitled 'Controlling Floating Point Behavior'

[Cameron McInally via Phabricator via llvm-commits]

cameron.mcinally added inline comments.


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Comment at: clang/docs/UsersManual.rst:1180
+
+   Defaults to ``ieee``.
+
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andrew.w.kaylor wrote:
> rjmccall wrote:
> > mibintc wrote:
> > > I'm not certain this is correct description of the Default.  Previously in the documentation the default ws not described.  I looked at the code and in Target.h it is initialized to ieee but possibly other specializations would replace the initial value. 
> > If it's target-specific, that seems relevant to describe.
> It looks like this is currently only supported for ARM targets. I also saw a CUDA-specific flag for this.
> 
> X86 targets won't be able to support "positive-zero" mode and "preserve-sign" (which we typically just call "ftz") would only take effect for SSE instructions (i.e. not x87), but we would like to add "denormals-are-zero" (or "daz") which means that denormal operands will be treated as if they were zero by FP instructions. Also "ftz' and "daz" can be used together so I guess we'll need a "daz+ftz" option. All of this depends on having a backend implementation of course, but we're going to want to do this soon. If we can figure out the command line interface, I can talk to someone about getting the x86 codegen part implemented.
> All of this depends on having a backend implementation of course, but we're going to want to do this soon. If we can figure out the command line interface, I can talk to someone about getting the x86 codegen part implemented.

This is currently handled for -ffast-math/-Ofast by including crtfastmath.o on the link line. IIRC, all it does is set FTZ+DAZ. Can we piggyback off that?

That's done because we want FTZ+DAZ to be toggled at start-up, so static initializers and such get the benefit. I suppose it would be possible to do in CodeGen, but sounds tricky...


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