5565 – [64-bit] Wrong Floating Point Results, Related to Mixing With size_t

The following code produces erratic results. I don't know where exactly the culprit is, but I'm hoping I've reduced it enough that someone with a decent mental model of how the compiler works will be able to finish the job.
import std.stdio;
double foo(int[] data, double q = 0.25) {
 immutable double N = data.length;
 // This actually affects the result, making it more severe and obvious
 // in addition to showing that the value is correct.
 stderr.writeln(N); // Prints 8. Correct.
 // lowEnd should be floor(7 * 0.25) == 1.
 immutable lowEnd = cast(size_t) ((N - 1) * q);
 // highEnd should be floor(7 * 0.75) - 1 = 4.
 immutable highEnd = cast(size_t) ((N - 1) * (1.0 - q) - lowEnd);
 // highFract should be 7 * 0.75 - 4 - 1 = 0.25
 immutable highFract = (N - 1) * (1.0 - q) - lowEnd - highEnd;
 stderr.writeln(lowEnd, '\t', highEnd, '\t', highFract);
 return highFract;
}
void main() {
 writeln(foo([1,2,3,4,5,6,7,8]));
}
Prints:
8
0	4545916	-4.54591e+06
-4.54591e+06

Forgot to mention: This test case doesn't reproduce the bug if -O is enabled, though the test case I reduced it from did, I think. (I'm not 100% sure that I wasn't just seeing two different bugs simultaneously.)

https://github.com/D-Programming-Language/dmd/commit/95b70d83ac51496eb171c1b408b5781eeee1a29d
https://github.com/D-Programming-Language/dmd/commit/fe147641a0334bca8d5ac3def7a0aba8529a0a66 

I think this was actually two bugs that I thought had a single root cause. You only got one of them. Now, the result from running this code with -m64 and without -O on linux is:
8
1	4545912	-4.54591e+06
-4.54591e+06
On the line with the three columns, the first one is now correct. The other two are still wrong, unless -O is enabled.

https://github.com/D-Programming-Language/dmd/commit/2421fd325b8e7528f5bafd2c4a4d6fd5008f3ab1
https://github.com/D-Programming-Language/dmd/commit/c51de3ab026f811744a4dc81b6f50c2525fc07a5