dmd codegen improvements

Walter Bright via Digitalmars-d digitalmars-d at puremagic.com
Tue Aug 18 03:45:49 PDT 2015


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.


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