Questions about march/mcpu/mattr and good generic defaults

[David Nadlinger]

On Thursday, 19 December 2013 at 23:28:50 UTC, Mikko Ronkainen 
wrote:
> Do -m32/m64 just mean -march=x86/x86-64? Or do they something 
> else too?

They generally switch between 32/64 bit subtargets for different 
processor families, not just x86. But yes, they just set the 
general target architecture. For the default -mcpu settings this 
implies on the various operating systems, see: 
https://github.com/ldc-developers/ldc/blob/master/driver/targetmachine.cpp#L30

> To create a generic 32-bit program, are these sane choices:
>
> -m32 -mcpu=i686 -mattr=sse2 ?

Depends on what level of backward-compatibility you need. All 64 
bit processors support SSE2 (which is why it is enabled by 
default there).

> And with 64-bit:
>
> -m64 -mcpu=x86_64 -mattr=avx ?
>
> Not sure about the avx though, doesn't seem to help my program 
> anymore than sse2.

You definitely can't expect 64 bit machines out in the wild to 
have AVX yet. For example, the Core i7 in the mid-2010 MacBook 
Pro I'm typing this message on doesn't support it.

On the other hand, if you want to compile your program only for a 
few machines with similar processors you control, you could also 
try using -mcpu=native to let LDC auto-detect the available 
features.

David