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digitalmars.D.bugs - [Issue 14943] New: dmd should inline more aggressively


          Issue ID: 14943
           Summary: dmd should inline more aggressively
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: hsteoh quickfur.ath.cx

/* Crude benchmark for comparing dmd/gdc output. */

import std.algorithm : map, filter, reduce;
import std.conv : to;
import std.range : iota;
import std.stdio : writeln;

auto fun(int n)
        return iota(n)
                .map!(a => a*7)
                .filter!(a => a % 2 == 0)
                .reduce!((a,b) => a/2 + b);

void main() {


Compiled with:

        dmd -release -O -inline test.d -oftest.dmd
        gdc -frelease -finline -O3 test.d -o test.gdc

(dmd git HEAD, gdc 5.2.1)

Execution times:

        % time test.dmd ; time test.gdc

        real    0m0.628s
        user    0m0.627s
        sys     0m0.001s

        real    0m0.168s
        user    0m0.167s
        sys     0m0.000s

As can be seen, the executable produced by gdc runs about 3.7 times faster than
the executable produced by dmd. Why?

Looking at the disassembly, the first thing that stands out is that gdc has
inlined the call to writeln, whereas dmd calls a separate function. While this
isn't the bottleneck, it gives a hint of the things to come. We look next at
fun(), which in both cases are standalone functions.

The dmd version of fun() is pretty straightforward: it calls iota() to create
the range, then map(), then filter(), and finally reduce() where most of the
work takes place. This is pretty much a direct translation of the code.

The gdc version of fun() is markedly different. We immediately notice that the
only function call in it is a call to enforce(). Not only the first level calls
to iota, map, filter, reduce have been inlined, pretty much their *entire* call
trees have been inlined, except for the call to enforce().

Here I'd like to highlight the fact that in the dmd version of the code, the
call to iota() has another function call to the ctor of the returned range. So
we see that gdc has inlined two levels of function calls where dmd has inlined
none, even though one would expect that with -inline, at least the call from
iota() to the ctor of the range should have been inlined, since it's the only
place where that ctor would be called; iota() itself being merely a thin
wrapper around it. (The ctor itself is also pretty simple; I'm not sure why dmd
fails to inline it.)

Similar remarks apply to the calls to map() and filter() as well.

Now let's look at reduce(), which is where the actual action takes place. The
dmd version, of course, involves a separate function call, which in the grand
scheme of things isn't all that important, since it's only a single function
call. However, a look at the disassembly of reduce() shows that dmd has not
inlined the calls to .empty, .front, and .popFront. In fact, the function calls
yet another function -- reduceImpl -- where the main loop sits. Inside this
main loop, .empty, .front, and .popFront are again called with no inlining --
even though .empty has a trivial body, .front involves only 1 multiplication,
and .popFront only 1 multiplication and a single odd/even test.  On top of
this, each of these nested function calls involve a certain amount of
boilerplate: twiddling with the stack registers, shuffling call arguments
about, etc., that add up to quite a large overhead in reduceImpl's inner loop.

The gdc version, by contrast, inlines *everything*, except the call to
enforce() which is outside the inner loop. This aggressive inlining allowed gdc
to trim the loop body down to about only 18 instructions with no function
calls.  While the dmd inner loop itself has only 15 instructions, it involves 3
function calls, with .front having 8 instructions, .empty also 8 instructions,
and .popFront 13 instructions, making a total of 44 instructions per iteration.
A significant percentage of these instructions are function call boilerplate.
The entire inner loop in the gdc version would fit in about 4-5 CPU cache
lines, whereas the dmd version would require a lot more.

To dmd's credit, it did manage to inline the nested calls in .empty, .front,
and .popFront(), which would have involved more function calls when no inlining
at all is done (each wrapper range forwards the calls to the next). This
probably helped to reduce the cost of running their respective function bodies.
However, this isn't quite enough, since the overhead of 3 function calls in the
inner loop is pretty expensive when the cost could have been eliminated
completely, as gdc had done.


Aug 20 2015