On Wed, Aug 22, 2012 at 8:49 AM, Zhenqiang Chen <zhenqiang.c...@arm.com> wrote: >> The patch is to enable bbro for -Os. When optimizing for size, it >> * avoid duplicating block. >> * keep its original order if there is no chance to fall through. >> * ignore edge frequency and probability. >> * handle predecessor first if its index is smaller to break long trace.
You do this by inserting the index as a key. I don't fully understand this change. You're assuming that a block with a lower index has a lower pre-order number in the CFG's DFS spanning tree, IIUC (i.e. the blocks are numbered sequentially)? I'm not sure that's always true. I think you should add an explanation for this heuristic. >> * only connect Trace n with Trace n + 1 to reduce long jump. ... >> * bb-reorder.c (connect_better_edge_p): New added. >> (find_traces_1_round): When optimizing for size, ignore edge >> frequency >> and probability, and handle all in one round. >> (bb_to_key): Use bb->index as key for size. >> (better_edge_p): The smaller bb index is better for size. >> (connect_traces): Connect block n with block n + 1; >> connect trace m with trace m + 1 if falling through. >> (copy_bb_p): Avoid duplicating blocks. >> (gate_handle_reorder_blocks): Enable bbro when optimizing for -Os. This probably fixes PR54364. > @@ -1169,6 +1272,10 @@ copy_bb_p (const_basic_block bb, int code_may_grow) > int max_size = uncond_jump_length; > rtx insn; > > + /* Avoid duplicating blocks for size. */ > + if (optimize_function_for_size_p (cfun)) > + return false; > + > if (!bb->frequency) > return false; This shouldn't be necessary, due to the CODE_MAY_GROW argument, and this change should result in a code size increase because jumps to conditional jumps aren't removed anymore. What did you make this change for, do you have a test case where code size increases if you allow copy_bb_p to return true? Ciao! Steven
