On Fri, 18 Jul 2025, ywgrit wrote: > For now, if combine pass can combine the simple nested comparison branches, > e.g. > if (a != b) > if (c == d) > These cond bbs must have only the conditional, which is too harsh. > > We often meet code like this: > if (a != b) > if (m[index] == k[index]) > m and c are arrays, so the 2nd branch belongs to a bb that has mem_ref > gimples and these stmts could trap. So these stmts won't pass the > bb_no_side_effects_p check, the branches can't be merged and performance > gains are lost, what are the way to merge these branches bb? > I think there are extremely many such nested branches and probably the > prediction accuracy of such nested branches is not very high, so doing > branch merging will result in high performance gain.
Without actual data I do not believe such general claim. But the issue is that we cannot speculate the loads from m[index] or k[index] when they might trap, so there is no way to merge the branches. Intel APX introduces conditional moves that hide traps, so with that you could do flag = a != b; cmov<flag> m[index], reg1 cmov<flag> k[index], reg2 if (flag && reg1 == reg2) but there is no way to do this in ifcombine on GIMPLE. It would also be slower in case if (a != b) is well predicted and mostly false. Richard.
