Hi Richard, Thanks very much for comments. > > Ping? > > > > Hi Ramana, could you help me review this patch? > > Hi Eric, Richard, could you help me review the change in regcprop.c? > > Subtraction of zero isn't canonical rtl though. Passes after peephole2 would > be well within their rights to simplify the expression back to a move. > From that point of view, making the passes recognise (plus X 0) and (minus X 0) > as special cases would be inconsistent. > > Rather than make the Thumb 1 CC usage implicit in the rtl stream, and carry > the current state around in cfun->machine, it seems like it would be better to > get md_reorg to rewrite the instructions into a form that makes the use of > condition codes explicit.
Here is a problem that two versions MOV instruction are supported in Thumb1 instruction set. The flag-setting version MOV can only take low register operands, while non-flag-setting version MOV can take high register operands. So we cannot rewrite non-flag-setting version MOV(with high register operands) into explicitly flag-setting one, and that's why it is rewritten into subtract of zero instruction now. > > md_reorg also sounds like a better place in the pipeline than peephole2 to be > doing this kind of transformation, although I admit I have zero evidence to > back that up... Yes, it may be feasible to rewrite the instruction in machine reorg pass, rather than peephole2. But that need bigger change in ARM back end. Hi Ramana, Richard, what's your opinion on this? Thanks very much.
