Reply To: The Belt

You are right that the congruence arguments to branch instructions provide a replacement for the result specifiers of legacy ISAs. You are wrong when you assume that the costs are equivalent.

Legacy result specifiers are needed on any instruction that has a result. A congruence argument is needed only for values that are still live across a branch at the end of a basic block, and then only if the branch is going to a control flow join point. The counts of specifiers and of congruence arguments can and do diverge dramatically in real code. I see codes like sha1 where there are hundreds of results dropped in a block, and only two are passed to the next block. I’ve never actually instrumented the tools to measure, but by eyeball I’d guess that there is one congruence argument for every eight drops on average.

Just to give you a better guess I took a look at the code I happened to have been working on before writing this note: it’s five functions from a buddy-allocator package and I had been chasing an case of less than optimal asm coming from the specializer. In the whole package there were exactly three congruence arguments, one each on three branches; as it happened all three were control variables of loops. The code of the five functions dropped 52 times total.

This ratio is so high for a mix of three reasons. First is that many values die before they get to a branch; only the live ones need congruence. Second is that the specializer doesn’t use congruence for values that are live across loops but not used within the loop: instead it spills at point of creation and fills at point of use. That allocator code has five spills and six fills on the Silver config. A legacy machine with lots of registers could have just left the values in register. Of course those would have to be spilled across calls; as Heinlein said, TANSTAAFL. Third, the specializer goes to a lot of work to eliminate branches. That’s to reduce the cost of prediction and mispredicts, but as a by product increases the benefif of congruence vs. specifiers.