dmd codegen improvements

[Timon Gehr via Digitalmars-d]

On 08/18/2015 12:45 PM, Walter Bright wrote:
> Martin ran some benchmarks recently that showed that ddmd compiled with
> dmd was about 30% slower than when compiled with gdc/ldc. This seems to
> be fairly typical.
>
> I'm interested in ways to reduce that gap.
>
> There are 3 broad kinds of optimizations that compilers do:
>
> 1. source translations like rewriting x*2 into x<<1, and function inlining
>
> 2. instruction selection patterns like should one generate:
>
>      SETC AL
>      MOVZ EAX,AL
>
> or:
>      SBB EAX
>      NEG EAX
>
> 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.
>
> For example, recently I improved the usage of the SETcc instructions.
>
> https://github.com/D-Programming-Language/dmd/pull/4901
> https://github.com/D-Programming-Language/dmd/pull/4904
>
> A while back I improved usage of BT instructions, the way switch
> statements were implemented, and fixed integer divide by a constant with
> multiply by its reciprocal.

Maybe relevant: There's some work on automatically discovering peephole 
optimizations that a compiler misses, e.g. 
http://blog.regehr.org/archives/1109