I use DMD and GDC. DMD for debug builds and (since it is the reference compiler) to ensure language conformance. GDC for performance tests and release. In other terms, I don't expect the W3 reference browser to be the fastest, but to set the required standard for HTML interpretation. If you asked me, I'd keep all smart compiler optimizations out of DMD for sake of stability, compilation speed and maintenance effort. Some of what GCC does is amazing, but probably requires heaps of difficult to read code. (I once saw it SSE optimize my code where I was using a 4-byte struct with 3 used bytes that I did computations on in a loop.)
-- Marco
