On Thu, 24 Nov 2016, Senthil Kumar Selvaraj wrote: > Hi, > > I've been analyzing a failing regtest (gcc.dg/strlenopt-8.c) for the avr > target. I found that the (dump) failure is because there are 4 > instances of memcpy, while the testcase expects only 2 for a > non-strict align target like the avr. > > Comparing that with a dump generated by x64_64-pc-linux, I found that > the extra memcpy's come from the forwprop pass, when it replaces > strcat with strlen and memcpy. For x86_64, the memcpy generated gets > folded into a load/store in gimple_fold_builtin_memory_op. That > doesn't happen for the avr because len (2) happens to be bigger than > MOVE_MAX (1). > > The avr can only move 1 byte efficiently from reg <-> memory, but it's > more efficient to load and store 2 bytes than to call memcpy, so > MOVE_MAX_PIECES is set to 2. > > Given that gimple_fold_builtin_memory_op gets to choose between > leaving the memcpy call as is, or breaking it down to a by-pieces > move, shouldn't it use MOVE_MAX_PIECES instead of > MOV_MAX? > > That is what the below patch does, and that makes the test > pass. Does this sound right?
No, as we handle both memcpy and memmove this way we rely on the whole storage fit in a single register so we do the right thing for overlapping memory. Richard. > Regards > Senthil > > Index: gcc/gimple-fold.c > =================================================================== > --- gcc/gimple-fold.c (revision 242741) > +++ gcc/gimple-fold.c (working copy) > @@ -703,7 +703,7 @@ > src_align = get_pointer_alignment (src); > dest_align = get_pointer_alignment (dest); > if (tree_fits_uhwi_p (len) > - && compare_tree_int (len, MOVE_MAX) <= 0 > + && compare_tree_int (len, MOVE_MAX_PIECES) <= 0 > /* ??? Don't transform copies from strings with known length this > confuses the tree-ssa-strlen.c. This doesn't handle > the case in gcc.dg/strlenopt-8.c which is XFAILed for that > > -- Richard Biener <rguent...@suse.de> SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)
