[llvm-dev] Clang Optimizer freaks out on "simple" goto code?

[mats petersson via llvm-dev]

On 5 December 2016 at 12:08, Philip Pfaffe via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Dennis,
>
> While Clang's code is significantly larger,  that is probably on purpose:
> Clang has vectorized the goto-loop.
>
> To validate whether that was correct and a good idea, plug both results
> into a benchmark and look at the actual performance data.
>

And that, in turn, depends on the length of the loop. If the values are
known at compile-time for `count`, the compiler will know whether
performing SSE operations or not "is worth it".

I had a case where I was passing an array of int and a length to a
function, and clang generated a whole lot of instructions to unroll the
loop and make it SSE - not realizing that the common value for `length` was
1 and never bigger than some small number (16 or 32). I didn't make any
effort to imrpove the compiled code, as I realized it was relatively simple
to inline the whole piece of code in LLVM-IR (it was part of my Pascal
compiler project). But I believe if I had added a `assert(length < 16)` to
the code, it would have done a decent job with it. [Inlining it helps my
Pascal compiler beat the FreePascal implementation by about 2-3x using that
particular algorithm for solving suduko - the call itself was quite an
overhead, and "not having a loop when you don't need to" helps even more]

--
Mats

>
> Philip
>
> 2016-12-05 12:32 GMT+01:00 Dennis Luehring via llvm-dev <
> llvm-dev at lists.llvm.org>:
>
>> FYI
>>
>> found this example while reading:
>> https://github.com/jameysharp/corrode/issues/30#issuecomment-231969365
>> and compared it with current gcc 6.2, clang 3.9
>>
>> gcc 6.2 result is quite small - clang 3.9 produces much much more code
>> for this example
>> https://godbolt.org/g/uWxr8F
>>
>> is that a missing optimization opportunity or just wrong behavior of the
>> optimizer?
>>
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>
>
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