> On Jul 19, 2018, at 2:24 PM, Jakub Jelinek <ja...@redhat.com> wrote: > > On Thu, Jul 19, 2018 at 02:06:16PM -0500, Qing Zhao wrote: >>> If you expand it as (int) ((unsigned char *)p)[n] - (int) ((unsigned char >>> *)q)[n] >>> then aren't you relying on int type to have wider precision than unsigned >>> char >>> (or unit_mode being narrower than mode)? >> >> do you imply that we should only expand it as (int) ((unsigned char *)p)[n] >> - (int) ((unsigned char *)q)[n] when we are sure >> int type is wider than unsigned char? > > Yes. > >>> I don't see anywhere where you'd >>> give up on doing the inline expansion on targets where e.g. lowest >>> addressable unit would be 16-bit and int would be 16-bit too. >> >> even on this targets, is char type still 8-bit? >> then int type is still wider than char? > > C requires that int is at least 16-bit wide, so the sizeof (int) == sizeof > (char) case is only possible say with 16-bit char and 16-bit int, or 32-bit > char and 32-bit int etc. > >>> On targets where int is as wide as char, one would need to expand it instead >>> as something like: >>> if (((unsigned char *)p)[n] == ((unsigned char *)q)[n]) loop; >>> ret = ((unsigned char *)p)[n] < ((unsigned char *)q)[n] ? -1 : 1; >>> or similar or just use the library routine. >> >> >> even when int type is as wide as char, expand it as (int) ((unsigned char >> *)p)[n] - (int) ((unsigned char *)q)[n] >> should still be correct (even though not optimal), doesn’t it? > > No. Consider p[n] being e.g. 1 and q[n] being __SCHAR_MAX__ + 3U and 16-bit > int and 16-bit char. Then (unsigned char) 0x0001 < (unsigned char) 0x8002, > so it should return a negative number. But (int) (0x0001U - 0x8002U) is > 0x7fff, which is a positive int. Now, if int is 17-bit and char is 16-bit, > this works fine, because is then -0x8001 and thus negative.
Okay, I see now. really appreciate for your detailed explanation. > > The above really works only if int is at least one bit wider than unsigned > char. Then, I will add a check to exclude the inlining when int is NOT wider than unsigned char on the target. is the following the correct check: (exp is the call to strcmp) if (CHAR_TYPE_SIZE >= TYPE_PRECISION (TREE_TYPE (exp))) Thanks. Qing
