On Tuesday, 18 August 2015 at 10:45:49 UTC, 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 the ENTER instruction should be replaced by a full prologue:
-
https://github.com/D-Programming-Language/dmd/blob/ef24f9acd99aa52ed28e7221cb0997099ab85f4a/src/backend/cod3.c#L2939
-
http://stackoverflow.com/questions/5959890/enter-vs-push-ebp-mov-ebp-esp-sub-esp-imm-and-leave-vs-mov-esp-ebp
It seems that since the Pentium I, ENTER is always slower. But i
don't know if it's used as a kind of optimization for the binary
size. Actually before using DMD I had **never** seen an ENTER.
