Re: DMD is faster than LDC and GDC

[John Colvin via Digitalmars-d]

On Thursday, 12 November 2015 at 11:59:50 UTC, Daniel Kozak wrote:
code:
import std.stdio;

auto fmttable(immutable(string[][]) table) {

    import std.array : appender, uninitializedArray;
    import std.range : take, repeat;
    import std.exception : assumeUnique;

    auto res = appender(uninitializedArray!(char[])(128));
    res.clear();

    if (table.length == 0) return "";
    // column widths
    auto widths = new int[](table[0].length);

    foreach (rownum, row; table) {
        foreach (colnum, cell; row) {
            if (cell.length > widths[colnum])
                widths[colnum] = cast(int)cell.length;
        }
    }

    foreach (row; table) {
        res ~= "|";
        foreach (colnum, cell; row) {
            int l = widths[colnum] - cast(int)cell.length;
            res ~= cell;
            if (l)
                res ~= ' '.repeat().take(l);
            res ~= "|";
        }
        res.put("\n");
    }

     return res.data.assumeUnique();
}

void main() {

    immutable table = [
        ["row1.1", "row1.2  ", "row1.3"],
        ["row2.1", "row2.2", "row2.3"],
        ["row3.1", "row3.2", "row3.3  "],
        ["row4.1", "row4.2", "row4.3"],
        ["row5.1", "row5.2", "row5.3"],
    ];

    writeln(fmttable(table));
    int i;
    for (i=0; i < 1000000; ++i) {
        fmttable(table);
    }
    writeln(i);
}

timings:

DMD (-O -release -inline -boundscheck=off):
real    0m0.003s
user    0m0.000s
sys     0m0.000s

LDMD2-ldc2 (-O -release -inline -boundscheck=off):
real    0m1.071s
user    0m1.067s
sys     0m0.000s


GDC (-O3 -finline -frelease -fno-bounds-check):
real    0m0.724s
user    0m0.720s
sys     0m0.003s

To test the speed of fmttable itself I split fmttable and main in 
to different modules, made fmttable extern(C) so I could just 
prototype it in the main module (no import), then compiled them 
separately before linking. This should prevent any possible 
inlining/purity cleverness. ~1s for ldmd2, ~2s for dmd, which is 
business as normal.

dmd is being clever and spotting that fmttable is pure, it would 
be good if ldc/gdc could spot this to.