On 11/18/2017 10:47 AM, Peter Bergner wrote: > On 11/18/17 12:16 AM, Jeff Law wrote: >> You might wander a bit and see if/how cse handles other similar >> circumstances. For example (not (not (x)) (neg (neg x)) and (bswap >> (bswap (x)) > > I actually tried examples like that to see what CSE would do, but I > could never come up with a test case, where those would survive until > CSE. I suppose I could try disabling the optimizations that remove > them before CSE to see what happens, but... Or alternately use the RTL front-end to pass in hand crafted RTL for what you want to test and have compilation start with cse1. You can see an example of this in testsuite/gcc.dg/rtl/aarch64/71779.c.
> > > >> THe last would seem particularly interesting -- as a hack see if you can >> generate a bswap instead of vec_select at expansion time, then see if >> CSE is able to fix it up. Or perhaps set it as a REG_EQUAL note. >> Again, it's a hack, but you just want to see if CSE can see the >> equivalence if it's in a more common form. Though I'm not sure if BSWAP >> is handled significantly better than an equivalence VEC_SELECT. > > ...the problem is that CSE only creates equivalences between two > expressions when one is assigned to the other. It doesn't seem to > look deeper into the expressions to try and find other equivalences, > so even if we see A = (not (not (B))), the only equivalence it makes > is that one. We don't get A is equivalent to B. But IIRC there are simplifications and checks for equivalent forms that are attached via REG_EQUAL notes. Though as you say that may all be irrelevant. Again, it's been a real long time since I looked at this code with any regularity. > > > Do you have any input on the following hunk, which seems to be an > independent change to the rest of the patch? > > @@ -2640,7 +2699,7 @@ exp_equiv_p (const_rtx x, const_rtx y, i > return 1; > > for (i = regno; i < endregno; i++) > - if (REG_IN_TABLE (i) != REG_TICK (i)) > + if (REG_IN_TABLE (i) != -1 && REG_IN_TABLE (i) != REG_TICK (i)) > return 0; My concern here is that Y is supposed to already be in the table. If Y is already in the table, then REG_IN_TABLE (i) shouldn't be -1 IIUC. jeff
