dmd makes D appear slow

[H. S. Teoh via Digitalmars-d]

On Fri, May 29, 2015 at 06:13:02PM +0000, weaselcat via Digitalmars-d wrote:
> In nearly every benchmark I see D in, the default compiler used is dmd
> which runs computationally intense tasks 4-5x+ slower than GDC/LDC

As I keep saying, in my own compute-intensive projects I have
consistently found that dmd-generated code (dmd -O -inline -release) is
about 20-30% slower on average, sometimes even up to 50% slower,
compared to gdc-generated code (gdc -O3 -finline -frelease). This is
measured by actual running time in an actual application, not
benchmark-specific code.

I have looked at the generated assembly before, and it's clear that the
gdc optimizer is way ahead of dmd's. The dmd optimizer starts failing to
inline inner loop code after about 1-2 levels of function call nesting,
not to mention it's unable to factor out a lot of loop boilerplate code.

The gdc optimizer, by contrast, not only factors out almost all loop
boilerplate code and inlines inner loop function calls several levels
deep, it also unrolls loops in a CPU-specific way, does major loop
restructuring, compounded with much more linear code optimization than
dmd does, instruction reordering and then refactoring after that, etc.,
in some cases reducing the size of inner loop code (as in, the amount of
code that runs per iteration) by up to 90%.

I don't know the internal workings of the dmd optimizer, but it's clear
that at present, with almost nobody working on it except Walter, it's
never going to catch up. (Maybe this statement will provoke Walter into
working his magic? :-P)


[...]
> This isn't the first time I've seen this, in basically every benchmark
> featuring D I have to submit a patch/make a comment that dmd shouldn't
> be used. Make no mistake, this is damaging to D's reputation - how
> well does D's "native efficiency" go over when people are saying it's
> slower than Scala and F#(mono)?
> 
> LDC and GDC need promoted more.
[...]

This will probably offend some people, but I think LDC/GDC should be the
default download on dlang.org, and dmd should be provided as an
alternative for those who want the latest language version and don't
mind the speed compromise.


T

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