[llvm-dev] RFC: Complex in LLVM

[Troy Johnson via llvm-dev]

> Why? I'd prefer we avoid introducing even more special cases. Is there 
> any reason why we should not define "complex <scalar type>", or to be 
> more restrictive, "complex <floating-point type>"? I really don't like 
> the idea of excluding 128-bit complex types, and I think that we can
> have a generic facility.

Hal, we had 128-bit complex in an earlier draft of David's proposal, but thought it to be an unnecessary distraction from our main interest, which is complex composed of 32-bit and 64-bit FP.  I'll take responsibility for having prodded him to remove it.  :)

We're most interested in seeing complex supported in LLVM to help with compiling complex types in C and C++, where there is general understanding of complex when it's commonly composed of a pair of floats or doubles.  Once you get beyond 32-bit and 64-bit FP as the constituent type though, you're in "complex long double" territory, and that introduces confusion.  Many expect complex long double to have 128-bit parts, but long double can mean 80-bit FP.  Although we personally haven't seen much practical use of a complex type composed of 80-bit FP, we thought that proposing complex for 128-bit FP might require also covering the 80-bit FP case.  What long double means is, after all, a Clang issue and not an LLVM issue, but we figured that proposing more types would have a harder path to success than proposing fewer types.

Do you think it's best to have the full set (c32, c64, c80, c128) or just (c32, c64, c128)?

-Troy