The changes here are based on the corresponding diffs for allowing FMF on 'select':
As discussed there, we want to have fast-math-flags be a property of an FP value because the alternative (having them on things like fcmp) leads to logical inconsistency such as:
The earlier patch for select made almost no practical difference because most unoptimized conditional code begins life as a phi (based on what I see in clang).
Similarly, I don't expect this patch to do much on its own either because SimplifyCFG promptly drops the flags when converting to select on a minimal example like:
But once we have this plumbing in place, we should be able to wire up the FMF propagation and start solving cases like that.
The change to RecurrenceDescriptor::AddReductionVar() is required to prevent a regression in a LoopVectorize test. We are intersecting the FMF of any FPMathOperator there, so if a phi is not properly annotated, new math instructions may not be either. Once we fix the propagation in SimplifyCFG, it may be safe to remove that hack.