[llvm-dev] Inserting MachineInstr's

[James Y Knight via llvm-dev]

On Sep 7, 2015, at 9:40 PM, Chris.Dewhurst via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi,
> 
> I have a task to complete and I’m getting stuck. I can’t find anything comparable in the documentation. The shortest explanation I can give is as follows: I need to use double-precision floating point values for floating-point multiplies. 
> 
>   Replace:
>   fmuls %f20,%f21,%f8
> 
>   with the sequence:
>   fstod %f20,%f0
>   fstod %f21,%f2
>   fmuld %f0,%f2,%f8

Are you sure you need to actually do this as a post-processing pass after code generation? Why not setOperationAction(ISD::FMUL, MVT::i32, Promote);  SparcISelLowering.cpp? I believe that'd avoid having it get generated in the first place (although I've not tested it).

Also: which erratum are you trying to work around? UT699's "FMULS precision loss after FDIVD/FSQRTD"? If so, I'd think that the workaround option of adding a "std %fNN, [%sp-8]" after fdivd/fsqrtd instructions may be better?