On 3 December 2013 10:08, <lu...@proxima.alt.za> wrote: > I seem to remember > Charles suggesting that it is difficult to do. And as it caught a > rather embarrassing case of sloppy programming >
It's not sloppy programming, but an underlying assumption about RISC machines in the (then) future having a respectable number of (truly) general-purpose registers, and the x86 does not. The S-U numbers guide register allocation, but there is no spilling code, except for nested function calls, and a few special cases that don't use the S-U numbers. Given that the out of fixed registers diagnostic occurs rarely in the normal source, and the S-U numbers might or might not be strictly accurate, the potential disadvantage of introducing inefficiency in existing compiled code by spill code misguided by inaccurate estimates made it seem not worthwhile at the time.
