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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