Nigel Stephens <nigel@xxxxxxxx> writes: > I have a patch against gcc-3.4 which makes the 64-bit inline shifts > somewhat smaller on ISAs which include the conditional move > (movz/movn) instructions, but more importantly removes all branches > from the inline code - which can be very expensive on long pipeline > CPUs, since in this sort of code they tend to cause many branch > mispredicts. Let me know if you want me to extract the patch - here's > a table of the number of instructions generated by the original md > pattern and the patched version: > > Instructions > Old New > ashldi3 12 9 > ashrdi3 12 12 > lshrdi3 12 9 > > > If people really don't like the inline expansion, then maybe it could be > enabled or disabled by a new -m option. IMO, controlling with optimize_size would be enough. But it sounds from your description like the patch just adds a new hard-coded multi-insn asm string. Is that right? If so, I'd really like to avoid that. It would much better IMO if we handle this in the target-independent parts of the compiler. We can already open-code certain non-native operations, it's "just" that wide shifts are a missing case. If we handle it in a target-independent way, with each insn exposed separately, we will be able to optimize special cases better. We'll also get the usual scheduling benefits. Richard
