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
