http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51721
Richard Guenther <rguenth at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |NEW Last reconfirmed| |2012-01-02 Ever Confirmed|0 |1 --- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-02 10:19:13 UTC --- (In reply to comment #2) > This is a common problem with the -Warray-bounds warning, first jump threading > (during vrp1) optimizes it into just a single s == 17 check, followed by > a[11] = 0; b[11] = 0; c[17] = 0; d[11] = 0; if true and a[s] = 0; etc. if > false > (well, at the end of vrp1 the constants aren't in the array refs yet, but they > are propagated there afterwards), and as no optimization figures out the weird > if (s >> 1 == 0) check (if (s < 2) would DTRT) to determine that s is not 17, > vrp2 warns about those accesses. > Perhaps for -Warray-bounds (at least if not -Warray-bounds=2 or similar) we > shouldn't warn on code that has been jump threaded, anyway, I don't think that > is solvable for 4.7 easily. > > What we perhaps could do more easily for this testcase (and could improve code > too) is during VRP for: > <bb 2>: > D.1716_2 = s_1(D) >> 1; > if (D.1716_2 == 0) > goto <bb 3>; > else > goto <bb 12>; > (or any other constant after >>, both signed and unsigned right shift, and == > or !=) insert ASSERT_EXPRs into both bbs, saying that the SSA_NAME in rhs1 of > the > shift is in/out of second ==/!= operand << rhs2 of shift, -""- + ((1 << rhs2) > - > 1) range. In this case it would be ASSERT_EXPRs that s_1(D) <= 1 at the start > of bb 3 (and if bb 12 had only one predecessor, also that s_1(D) > 1 at bb 12 > start). Richard, what do you think about that? Yeah, if that turns out to be a common pattern, though maybe restrict it to ==/!= 0 tests? (if that simplifies the patch)