Performance test of short-circuiting AliasSeq

[Stefan Koch]

On Tuesday, 2 June 2020 at 09:19:57 UTC, Stefan Koch wrote:
> On Tuesday, 2 June 2020 at 06:52:00 UTC, Walter Bright wrote:
>> On 6/1/2020 1:23 PM, Stefan Koch wrote:
>>> TLDR; Performance patch caused a slowdown.
>>> Why? Because the checking for the case which it wants to 
>>> optimize takes more time than you safe by optimizing it.
>>
>> You'll need a finer grained profile to reach this conclusion 
>> than the gross measurement of the compiler runtime. I suggest 
>> using Vtune.
>
> Vtune doesn't do intrusive profiling as far as I know.
> Therefore it's measurements _can_ miss functions which are 
> called often,
> but are very short running.
> A profile  obtained by periodic sampling is has the potential 
> of misrepresenting the situation.
>
> In this case finer grained analysis is of course needed to do a 
> 100% definite statement, however the measurement itself is 
> quite telling.
> Your short-circuiting patch was the only thing that changed 
> between the versions.
>
> I do have a fully instrumented version of dmd.
> I know exactly which functions spends how much time processing 
> which symbol.
>
> I will post those results soon.

So I have a little profiling tool in DMD which tells me how often 
certain functions are being called.
This is the output for the test with the patch:

  === Phase Time Distribution : ===
  phase                                                            
                           avgTime    absTime       freq

  dmd.dsymbolsem.dsymbolSemantic                                   
                           127270.86  37145399296   291861
  dmd.dsymbolsem.templateInstanceSemantic                          
                           355060.25  17450856448   49149
  dmd.expressionsem.symbolToExp                                    
                           993.92     113991488     114689
  dmd.dtemplate.TemplateInstance.syntaxCopy                        
                           1327.78    92444120      69623
  dmd.dmangle.Mangler.mangleType                                   
                           293.39     60637312      206680
  dmd.dtemplate.TemplateDeclaration.findExistingInstance           
                           950.36     19462416      20479
  dmd.mtype.TypeIdentifier.syntaxCopy                              
                           203.80     12570752      61681
  Type::mangleToBuffer                                             
                           83.38      2107624       25277
  dmd.dmangle.Mangler.mangleSymbol                                 
                           573.35     108936        190
  dmd.func.FuncDeclaration.functionSemantic                        
                           374.00     374           1

  === Phase Time Distribution : ===
  phase                                                            
                           avgTime    absTime       freq
  dmd.dsymbolsem.dsymbolSemantic                                   
                           124654.58  36400631808   292012
  dmd.dsymbolsem.templateInstanceSemantic                          
                           348265.00  17116876800   49149
  dmd.expressionsem.symbolToExp                                    
                           1013.76    116267384     114689
  dmd.dtemplate.TemplateInstance.syntaxCopy                        
                           1317.11    91701088      69623
  dmd.dmangle.Mangler.mangleType                                   
                           288.24     59609648      206809
  dmd.dtemplate.TemplateDeclaration.findExistingInstance           
                           954.04     19537828      20479
  dmd.mtype.TypeIdentifier.syntaxCopy                              
                           205.71     12690568      61693
  Type::mangleToBuffer                                             
                           91.08      2307294       25332
  dmd.dmangle.Mangler.mangleSymbol                                 
                           832.58     169014        203
  dmd.func.FuncDeclaration.functionSemantic                        
                           680.00     680           1

As you can see the templateInstanceSemantic is called the same 
number of times.
That would imply the optimization does never actually get 
triggered.