[cfe-dev] Difference in generated code between variadic parameter pack and manual version

[Bart Samwel via cfe-dev]

On Mon, Sep 21, 2020 at 1:44 PM Arthur O'Dwyer <arthur.j.odwyer at gmail.com>
wrote:

> On Mon, Sep 21, 2020 at 7:02 AM Bart Samwel via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hi there folks,
>>
>> I wonder if anybody can shed some light on this. I'm looking at a
>> function with a parameter pack argument and one without, that should do the
>> exact same thing.
>>
>> https://godbolt.org/z/Keqzcj
>>
>> However, the version with the parameter pack expands (at -O3
>> -march=broadwell, on clang 10.0.1, on godbolt) into a loop per 128 bytes,
>> plus a loop per 64 bytes, plus nonvectorized instructions to process the
>> remaining <=63 bytes. The manual version expands to just a loop per 128
>> bytes (256-bit vectors, unrolled 4x), and nonvectorized instructions to
>> process the remaining <=127 bytes.
>>
>
> It's about the fold expression.
> https://godbolt.org/z/EPETj9
>
> With C++17 fold-expressions, (args | ...) doesn't mean (arg1 | arg2 |
> arg3); it means (arg1 | (arg2 | arg3)).  So with the right-fold you wrote,
> you're telling the compiler to OR the values together "right-to-left",
> whereas the non-template version does it "left-to-right": ((arg1 | arg2) |
> arg3). And apparently this makes some huge difference to the codegen (which
> is still mysterious to me, but out of my depth).
>

That is just plain weird, and probably interesting for the codegen folks to
look at. :) Thanks a lot for figuring this out!


>
> Switch the right-fold to a left-fold and the codegen becomes identical, at
> least to my eyes. (In the above Godbolt, put -DVARIADIC in one compiler
> frame and nothing in the other.)
>
> –Arthur
>


-- 
Bart Samwel
bart.samwel at databricks.com
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