Speeding up importing Phobos files

[Neia Neutuladh]

On Mon, 21 Jan 2019 19:10:11 +0000, Vladimir Panteleev wrote:
> On Monday, 21 January 2019 at 19:01:57 UTC, Steven Schveighoffer wrote:
>> I still find it difficult to believe that calling exists x4 is a huge
>> culprit. But certainly, caching a directory structure is going to be
>> more efficient than reading it every time.
> 
> For large directories, opendir+readdir, especially with stat, is much
> slower than open/access.

We can avoid stat() except with symbolic links.

Opendir + readdir for my example would be about 500 system calls, so it 
breaks even with `import std.stdio;` assuming the cost per call is 
identical and we're reading eagerly. Testing shows that this is the case.

With a C preprocessor, though, you're dealing with /usr/share with 
thousands of header files.

> This whole endeavor generally seems like poorly reimplementing what the
> OS should already be doing.

The OS doesn't have a "find a file with one of this handful of names among 
these directories" call.