Hi,
在 2024/5/10 20:50, Richard Biener 写道:
> IMO give we're dispatching to the rtx_cost hook eventually it needs
> documenting there or alternatively catching zero and adjusting its
> result there. Of course cost == 0 ? 1 : cost is wrong as it makes
> zero vs. one the same cost - using cost + 1 when from rtx_cost
> might be more correct, at least preserving relative costs.
I tested the draft patch which sets "cost > 0 ? cost + 1 : 1;". Some
regression cases are found on x86. The main problems are:
The cost compare with COSTS_N_INSNS (1) doesn't works any more with
the patch. As all costs are added with 1, the following compare
returns true when the cost is 5 but false originally.
if (cost > COSTS_N_INSNS (1))
Another problem is the cost is from set_src_cost, it doesn't take dest
into consideration. For example, the cost of a store "[`x']=r109:SI"
is set to 1 as it only measure the cost of set_src. It seems
unreasonable.
IMHO, the cost less than COSTS_N_INSNS (1) is meaningful in rtx_cost
calculation but unreasonable for an insn. Should the minimum cost of
an insn be set to COSTS_N_INSNS (1)?
Thanks
Gui Haochen
