llvm dev - Oct 2012 - [LLVMdev] TargetDescription string

In "Writing an LLVM Compiler Backend", there's some discussion of
the
TargetDescription string, but it doesn't explain the examples I look
at. For instance, in the description of the PowerPC, I see

    "E-p:64:64-f64:64:64-i64:64:64-f128:64:128-n32:64"

What's "preferred alignment" versus "ABI alignment"?

What are the 3 figures following the "i"? (the documentation suggests
there will only be 2).

I see two entries for "f".  Do I read these as info about
"f64" and "f128"?

What is "n32:64" about?

Thanks,
Preston

On Fri, Oct 26, 2012 at 5:11 PM, Preston Briggs
<preston.briggs at gmail.com> wrote:
> In "Writing an LLVM Compiler Backend", there's some
discussion of the
> TargetDescription string, but it doesn't explain the examples I look
> at. For instance, in the description of the PowerPC, I see
>
>     "E-p:64:64-f64:64:64-i64:64:64-f128:64:128-n32:64"
>
> What's "preferred alignment" versus "ABI
alignment"?
>
> What are the 3 figures following the "i"? (the documentation
suggests
> there will only be 2).
>
> I see two entries for "f".  Do I read these as info about
"f64" and "f128"?
>
> What is "n32:64" about?
http://llvm.org/docs/LangRef.html#datalayout

-Eli