Link time optimization in D

[Walter Bright]

retard wrote:
> Mon, 31 May 2010 18:23:18 +0200, Pelle wrote:
> 
>> On 05/31/2010 05:43 PM, retard wrote:
>>> DMD is much slower than Sun Javac/Jvm 7, GNU GCC 4.5, and LLVM.
>> For very special values of much, I suppose.
> 
> Polymorphic method calls, auto-vectorization, link-time optimization, 
> floating point performance, etc.

Link time optimization? Let's examine it:

https://lwn.net/Articles/387122/

"When source files are compiled and linked using -flto, GCC applies 
optimizations as if all the source code were in a single file. This allows GCC 
to perform more aggressive optimizations across files, such as inlining the body 
of a function from one file that is called from a different file, and 
propagating constants across files. In general, the LTO framework enables all 
the usual optimizations that work at a higher level than a single function to 
also work across files that are independently compiled."

D has had that for 8 years now, it's just done in the compiler instead of the 
linker. You can specify as many D source modules as you want on the command line 
to the compiler, and it will compile & optimizer & inline them all together and 
generate one object file.

Essentially, link time optimization is a hack to get around the language problem 
C/C++ have with their separate compilation model.

To say D code is slower because it lacks link-time optimization is a false 
supposition.