On Mon, 20 Jul 2020 at 12:52, tugouxp <13824125...@163.com> wrote: > in flollowing comment of tcg.h, it says every helper max need 6 > input argument and 1 output argument, > but why here both multiply by 2 in here?
Because arguments might be 64 bits, and on a 32-bit host a 64-bit argument has to be passed as 2 separate operands. > what is the 14 mean? It's the maximum possible number of operands in the worst case: six 64-bit input arguments which need to be passed as 2 32-bit operands each, and a 64-bit return value which also needs to be passed as 2 32-bit operands. (6 * 2) + 2 == 14. The reason we care is described in the next part of the comment: we keep one bit of information (liveness) about the operands of each TCGop, and because 14 < 16 we can fit this information into a single 16-bit integer per TCG op. > 520 /* While we limit helpers to 6 arguments, for 32-bit hosts, with padding, > 521 this imples a max of 6*2 (64-bit in) + 2 (64-bit out) = 14 operands. > 522 There are never more than 2 outputs, which means that we can store all > 523 dead + sync data within 16 bits. */ thanks -- PMM
