dmd codegen improvements

[Ivan Kazmenko via Digitalmars-d]

On Tuesday, 18 August 2015 at 10:45:49 UTC, Walter Bright wrote:
> ...
> 3. data flow analysis optimizations like constant propagation, 
> dead code elimination, register allocation, loop invariants, 
> etc.
>
> Modern compilers (including dmd) do all three.
>
> So if you're comparing code generated by dmd/gdc/ldc, and 
> notice something that dmd could do better at (1, 2 or 3), 
> please let me know. Often this sort of thing is low hanging 
> fruit that is fairly easily inserted into the back end.
> ...

I've once tried to trace the slowdown cause in a simple program 
and reduced it to https://issues.dlang.org/show_bug.cgi?id=11821, 
I think if falls under point 3 in your post (redundant 
instruction in a simple loop).  Despite 1.5 years have passed, 
the issue still stands with 2.068.0.

That's for -m32.  The -m64 version of the loop does not look as 
having a redundant instruction to me, but is still longer than 
the output of GDC or LDC.