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

[Friedman, Eli via llvm-dev]

On 9/25/2017 12:13 PM, Björn Pettersson A wrote:
> So what is the correct behavior of the store <2 x i2>. Storing two bytes with a zext:ed i2 in each, or a bit packed vector?
>
> I can't remember any documentation mentioning that vectors are bit packed. But if LLVM is supposed to bit pack vectors, should we do it for any element size, or only when element size is less than the byte size, or only for i1 vectors?
For lack of any formal documentation, the most canonical source of 
information about the size of a type is DataLayout.  And currently 
DataLayout::getTypeSizeInBits() assumes vectors are bit-packed.
> Maybe bit packing should be optional (target specific)?

That's... maybe possible?  It would break the current rule that whether 
a bitcast is legal is independent of the target, and I don't see a 
compelling reason.  (If you're trying to comply with some external ABI, 
you can always explicitly extend a value before you store it.)

-Eli

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