When I was adding pipeline to my backend, some instructions needed to be fused and I found that there was no suitable interface to implement my requirements.
My hope is that 1. Do instruction scheduling and combine any two instructions, and sometimes the two instructions can be treated as 1 when they are issued 2. The two instructions only work better when they are immediately adjacent to each other 3. An instruction can only be fused once, i.e. if the current instruction has been fused with the previous one, the next one cannot be fused with the current one. I have referred to numerous interfaces in the “GCC INTERNALS” which implement some of my requirements, but all of which just happen not to cover my needs completely. These interfaces are： - bool TARGET_SCHED_MACRO_FUSION_PAIR_P (rtx insn *prev, rtx insn *curr) The name of the interface looks a lot like what I need. But in reality I found that this interface only fuses instructions that are already adjacent to each other and does not do scheduling (not satisfy 1). And this interface may fuse 3 or more instructions (not satisfy 3). - void TARGET_SCHED_FUSION_PRIORITY (rtx insn *insn, int max_pri, int *fusion_pri, int *pri) This interface is very powerful, but with only one insn being processed at a time, this interface does not seem to be suitable for context sensitive situations. - Use (define_bypass number out_insn_names in_insn_names [guard]) The “bypass” does not guarantee that the instruction being dispatched is immediately adjacent to (not satisfy 2). Moreover, bypass only handles instructions with true dependence. - int TARGET_SCHED_REORDER (FILE *file, int verbose, rtx insn **ready, int *n_readyp, int clock) and TARGET_SCHED_REORDER2() This interface allows free adjustment of ready instructions, but it is not eay to get the last scheduled instruction. The last scheduled instruction needs to be taken into account for fusion. - Use define_peephole2 Since the fused instructions are somehow identical to one instruction, it is thought that a peephole might be a good choice. But “define_peephole2” also does not schedule instructions. In summary, I have not found an interface that does both scheduling and fusion. Maybe we should enhance one of the above interfaces, or maybe we should provide a new one. I think it is necessary and beneficial to have an interface that does both scheduling and fusion.
