[llvm-dev] What should a truncating store do?

[Friedman, Eli via llvm-dev]

On 9/15/2017 11:30 AM, Jon Chesterfield wrote:
> Interesting, thank you. I expected both answers to be "unchanged" so 
> was surprised by the zero extend in the legaliser.
>
> The motivation here is that it's faster for us to load N bytes, apply 
> whatever masks are necessary to reproduce the truncating store then 
> store all N bytes. This is only a good plan if there's no change to 
> the semantics :)

See http://llvm.org/docs/LangRef.html#store-instruction .  In general, 
you have to be careful to avoid data races, but that might not apply to 
your target.

> Are scalar integer types zero extended to the next multiple of 8 or to 
> the next power of 2 greater than 7? For example, i17 => i24 or i17 => i32?

Multiple of 8.

> I think this means truncating stores of vector types will introduce 
> zero bits at the end of each element instead grouping all the zeros at 
> the end. For example, <i6 63, i6 63> writes to sixteen bits as 
> 0b0011111100111111, not as 0b0000111111111111?

Vector types are tightly packed, so <8 x i1> is 1 byte, not 8 bytes.

-Eli

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