On 07/19/2011 08:57 AM, Paulo J. Matos wrote: > On 19/07/11 16:41, Richard Henderson wrote: >> (or fails to set the flags in a way that >> is useful for the comparison). > > I am not sure I understand the above. Could you give an example where > certain flags might be set but are not useful for comparison?
You pasted one before -- the RX definition of ABS. The instruction description of the O flag for ABS reads: # The flag is set if src before the operation was 80000000h; otherwise it is cleared. The upshot is that the entire flags register is set as if the operation was "0 - src", i.e. (compare:CC (const_int 0) (reg:SI src)) However, the comparison that we're actually going to try to generate is (compare:CC (abs:SI (reg:SI src)) (const_int 0)) Which is of course not the same thing. However, if we can ignore the O flag for a given comparison, then we can still usefully make use of some of the flags. E.g. the Z flag in testing for (in)equality against zero. The other very common case is subtract, where the flags are set as if the comparison is (compare:CC input1 input2) whereas the comparison that we generate is (compare:CC output 0) Have a look at all the uses of rx_match_ccmode in rx.md. r~
