A very strange bug. DMD 2.058 64-bit Linux
Caligo
iteronvexor at gmail.com
Sun Feb 26 12:06:06 PST 2012
bug.d
---------------->8---------------->8----------------
@trusted:
import std.datetime : benchmark;
import std.stdio : writefln, writeln;
alias double Real;
void ben(alias fun)(string msg, uint n = 1_000_000) {
auto b = benchmark!fun(n);
writefln(" %s %s ms", msg, b[0].to!("msecs", int));
}
struct Matrix(int row, int col) {
private:
alias row Row;
alias col Col;
alias Real[Row * Col] Data;
public:
Data _data = void;
alias _data this;
this(const Real[Row*Col] data) pure nothrow { _data = data; }
}
M inverse(M)(const auto ref M m) {
writeln(m[]);
M minv = m;
return minv;
}
unittest {
alias Matrix!(4, 4) Matrix4x4;
auto m9 = Matrix4x4([4, 0, 0, 0, 0, 0, 2, 0, 0, 1, 2, 0, 1, 0, 0, 1]);
ben!( {auto r = inverse(m9);} )("4x4 inverse:");
}
----------------8<----------------8<----------------
t1.d
---------------->8---------------->8----------------
import std.stdio;
void main(){ }
----------------8<----------------8<----------------
It took me a long time to pinpoint this because it's tricky to trigger the bug.
Once you have those two files, compile with this:
dmd -unittest t1.d bug.d
and then run t1:
./t1
The output you get should look like this:
...
[0, 4, 0, 0, 0, 0, 0, 2, 0, 0, 1, 2, 0, 1, 0, 0]
Obviously the output is wrong. 'm9' for some reason is getting
overwritten. In my project this caused big problems because there are
other m# with different values, and their values would literally get
copied to m9. Calling inverse() on m9 then would fail because the
other matrices are not invertible. Placing a writeln() in inverse()
helped me realize that what was being passed to inverse() was being
modified somewhere. I'm still now sure how m9 is being modified.
Another point, compiling with this:
dmd -unittest bug.d t1.d
and then running bug:
./bug
doesn't trigger the bug.
Could someone else please confirm this behavior?
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