Disappointing performance from DMD/Phobos

[Nicholas Wilson]

On Tuesday, 26 June 2018 at 02:10:17 UTC, Manu wrote:
> Some code:
> ---------------------------------
> struct Entity
> {
>   enum NumSystems = 4;
>   struct SystemData
>   {
>     uint start, length;
>   }
>   SystemData[NumSystems] systemData;
>   @property uint systemBits() const { return 
> systemData[].map!(e =>
> e.length).sum; }
> }
> Entity e;
> e.systemBits(); // <- call the function, notice the codegen
> ---------------------------------
>
> This property sum's 4 ints... that should be insanely fast. It 
> should
> also be something like 5-8 lines of asm.
> Turns out, that call to sum() is eating 2.5% of my total perf
> (significant among a substantial workload), and the call tree 
> is quite
> deep.
>
> Basically, inliner tried, but failed to seal the deal, and 
> leaves a call stack 7 levels deep.
>
> Pipeline programming is hip and also *recommended* D usage. The
> optimiser must do a good job. This is such a trivial workloop, 
> and
> with constant length (4).
> I expect 3 integer adds to unroll and inline. A call-tree 7 
> levels
> deep is quite a ways from the mark.
>
> Maybe this is another instance of Walter's "phobos begat 
> madness" observation?
> The unoptimised callstack is mental. Compiling with -O trims 
> most of
> the noise in the call tree, but it fails to inline the 
> remaining work
> which ends up 7-levels down a redundant call-tree.

Then use LDC! ;)

But seriously, DMD's inliner is a) in the wrong spot in the 
compilation pipeline (at the AST level) and b) is timid to say 
the least.