[llvm-dev] One element vectors versus scalars

[Martin J. O'Riordan via llvm-dev]

Thanks Hal.  In our case the ABI issue doesn't arise, but I'll have to think about the legalization to see if maybe we could write our type definitions better.

	MartinO

-----Original Message-----
From: Hal Finkel [mailto:hfinkel at anl.gov] 
Sent: 10 November 2015 12:34

----- Original Message -----
> From: "Martin J. O'Riordan via llvm-dev" <llvm-dev at lists.llvm.org>
> 
> I am curious as to why LLVM has explicit support for 1 element 
> vectors, for example ‘ v1i32 ’. Are there some contexts where handling 
> a value as a 1 element vector can yield better code quality than the 
> corresponding scalar ‘ i32 ’?

Hi Martin,

No, but I see two reasons we support 1-element vectors:

 1. Having 1-element vectors makes a lot of the SDAG legalization code simpler, allowing us to separate the code that splits vector types from the code that transforms 1-element vectors into scalars.

 2. On some platforms, there are ABI differences between the two types. (e.g. i128 and <1 x i128> are passed in different registers across function-call boundaries).

 -Hal