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

Jon Chesterfield via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 15 12:10:50 PDT 2017

OK, I'm clear on scalars. Data races are thankfully OK in this context.

Densely packing vectors sounds efficient and is clear in the case where
lanes * width is a multiple of 8 bits. I don't think I understand how it
works in other cases.

If we could take store <4 x i8> truncating to <4 x i7> as an example. This
can be converted into four scalar i8 -> i7 stores with corresponding
increments to the address, in which case the final layout in memory
is 0b01111111011111110111111101111111. Or it can be written as a packed
vector which I think would resemble 0b00001111111111111111111111111111.

This would mean the memory layout changes depending on how/whether the
legaliser breaks large vectors down into smaller types. Is this the case?
For example, <4xi32> => <4 x i31> converts to two <2 x i32> => <2 x i31>
stores on a target with <2 x i32> legal but would not be split if <4 x i32>
were declared legal.



On Fri, Sep 15, 2017 at 7:41 PM, Friedman, Eli <efriedma at codeaurora.org>

> 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
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
> Foundation Collaborative Project
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