Always false float comparisons

[Ethan Watson via Digitalmars-d]

On Wednesday, 18 May 2016 at 08:55:03 UTC, Walter Bright wrote:
> MSVC doesn't appear to have a switch that does what you ask for

I'm still not entirely sure what the /fp switch does for x64 
builds. The documentation is not clear in the slightest and I 
haven't been able to find any concrete information. As near as I 
can tell it has no effect as the original behaviour was tied to 
how it handles the x87 control words. But it might also be 
possible that the SSE instructions emitted can differ depending 
on what operation you're trying to do. I have not dug deep to see 
exactly how the code gen differs. I can take a guess that 
/fp:precise was responsible for promoting my float to a double to 
call CRT functions, but I have not tested that so that's purely 
theoretical at the moment.

Of course, while this conversation has mostly been for compile 
time constant folding, the example of passing a value from the EE 
and treating it as a constant in the VU is still analagous to 
calculating a value at compile time in D at higher precision than 
the instruction set the runtime code is compiled to work with.

/arch:sse2 is the default with MSVC x64 builds (Xbox One defaults 
to /arch:avx), and it sounds like the DMD has defaulted to sse2 
for a long time. The exception being the compile time behaviour. 
That compile time behaviour conforming to the the runtime 
behaviour is an option I want, with the default being whatever is 
decided in here. Executing code at compile time at a higher 
precision than what SSE dictates is effectively undesired 
behaviour for our use cases.

And in cases where we compile code for another architecture on 
x64 (let's say ARM code with NEON instructions, as it's the most 
common case thanks to iOS development) then it would be forced to 
fallback to the default. Fine for most use cases as well. It 
would be up to the user to compile their ARM code on an ARM 
processor to get the code execution match if they need it.