[PATCH] make reciprocal estimate code generation more flexible by adding command-line options

[Sanjay Patel]

================
Comment at: lib/Target/X86/X86TargetMachine.cpp:108
@@ -107,2 +107,3 @@
 
+  this->Options.Reciprocals.setDefaults("all", false, 1);
   initAsmInfo();
----------------
hfinkel wrote:
> spatel wrote:
> > hfinkel wrote:
> > > Why false? Do you want a target feature here?
> > We had target features to control these, but I think it would be better to behave like gcc unless we have reason to diverge. 
> > 
> > That said, this does not match gcc behavior yet; that would be my next patch. For x86 at least, we would turn the following on by default when using -ffast-math:
> > sqrt
> > vec-sqrt
> > vec-div
> > 
> > I didn't set these defaults in this patch because it would change -ffast-math codegen for all CPUs other than btver2 (which had the recip codegen enabled for all eligible x86 recip types via target features).
> I thought that getRsqrtEstimate, etc. were only called when fast-math is on?
That's correct; everything is gated by -ffast-math or an unsafe algebra equivalent. Recip-est codegen should only be active after that check.

With gcc, once you have -ffast-math, you also get -mrecip=sqrt,vec-sqrt,vec-div by default. AFAIK, that is independent of arch or CPU subtarget.

The lone exception is 'div' (scalar division). Estimating scalar div on x86 (no FMA until recently) breaks a lot of real world code, so I want to keep that off by default (and again match gcc default behavior).

http://reviews.llvm.org/D8982

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