What a shame for clang and gcc! They still don't implement perfect hashes for large sparse switch tables. I did that recently for my rcc C compiler, and it was just a single day of work. Esp. beneficial for unicode work. The LLVM patch was rejected a decade ago. Gcc has several unicode patches sitting in branches, which would also need that
> The switch in sparse_8 could be simplified by dividing x by eight before the jump. Interestingly, gcc doesn't find this simplification.
It could not because that would ensure that 7 values for each case would be handled by function calls instead of the default case - 0/8 is 0, but 3/8 is also 0
In order to peak into hardware speculation/BTB, having a look atswitch lowering seems a good way. Big switches will go into serious convolution to please that hardware since by definition this is a source of a lot of branch mispredicitions.
4 comments:
What a shame for clang and gcc! They still don't implement perfect hashes for large sparse switch tables. I did that recently for my rcc C compiler, and it was just a single day of work. Esp. beneficial for unicode work. The LLVM patch was rejected a decade ago. Gcc has several unicode patches sitting in branches, which would also need that
> The switch in sparse_8 could be simplified by dividing x by eight before the jump. Interestingly, gcc doesn't find this simplification.
It could not because that would ensure that 7 values for each case would be handled by function calls instead of the default case - 0/8 is 0, but 3/8 is also 0
This one was interesting, especially its name :)
In order to peak into hardware speculation/BTB, having a look atswitch lowering seems a good way. Big switches will go into serious convolution to please that hardware since by definition this is a source of a lot of branch mispredicitions.