[PATCH] [x86] make reciprocal estimate code generation more flexible

[Sanjay Patel]

After thinking it over, I decided that a simple enum of reciprocal operation bools wasn't going to do the job. In addition to wanting to enable specific ops on specific data types, we have users asking to customize the number of N-R refinements calculated...and they want different counts based on the type of op.

The best way to achieve this level of flexibility (and further customization requests that are sure to follow) is with a 'TargetRecip' class. So that's what I'm proposing to add in this updated patch.

From the llc command-line, you can now do something like this:
$ llc -recip=vec-divf,sqrtd.2

That translates to: allow vector float division and scalar double square root codegen. Use the target default setting for N-R refinement steps for the div but override the refinement steps for the sqrt to be '2'.

To minimize change in the updated x86 backend, I've defaulted to disabling all recip codegen, but for enabled operations, we use 1 N-R step.

This means only users that were targeting AMD Jaguar will see a functional change from this patch (that chip had defaulted both sqrt and div codegen for scalars and vectors on). We can easily change the x86 target defaults in a follow-on patch. We'll also want to update the PowerPC and R600 backends to use this new command-line functionality.

I wanted to add more testing for the llc parameter parsing, but I'm not finding a way to do it. If there's a way to do that with a generic target, please let me know.

I have a dependent clang driver / front-end patch to pass similar command-line params through to the backend in progress.


http://reviews.llvm.org/D8982

Files:
  include/llvm/CodeGen/CommandFlags.h
  include/llvm/Target/TargetOptions.h
  include/llvm/Target/TargetRecip.h
  lib/Target/CMakeLists.txt
  lib/Target/TargetRecip.cpp
  lib/Target/X86/X86.td
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86Subtarget.cpp
  lib/Target/X86/X86Subtarget.h
  lib/Target/X86/X86TargetMachine.cpp
  test/CodeGen/X86/recip-fastmath.ll
  test/CodeGen/X86/sqrt-fastmath.ll

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