https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85814
--- Comment #7 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> --- Author: rsandifo Date: Mon May 21 22:02:35 2018 New Revision: 260488 URL: https://gcc.gnu.org/viewcvs?rev=260488&root=gcc&view=rev Log: Fix tree-ssa-strlen handling of partial clobbers (PR85814) In this PR we have: c_5 = c_4(D) + 4; c_12 = c_5 + 1; *c_5 = 2; a = 2; // A c_21 = c_12 + 1; *c_12 = 2; a = 2; // B c_28 = c_21 + 1; *c_21 = 2; a = 2; c_7 = c_28 + 1; *c_28 = 2; where a is a global int. We decide that A can't clobber *c_5 == c_4[4] because the latter implies that c_4 is an object of 5 bytes or more, whereas a has exactly 4 bytes. The assumption for B and *c_5 is the same, but when considering B and *c_12, we only follow the definition of c_12 to c_5 + 1 (for good reason) and so have *c_12 == c_5[1]. We then don't have the same size guarantee and so assume that B could clobber *c_12. This leads to a situation in which the strinfo for c_5 is still valid but the next strinfo (c_12) isn't. We then segfaulted while trying to get the strinfo for c_21 + 1 == c_5 + 3 because get_stridx_plus_constant assumed that c_5's next strinfo (c_12) would be valid too. And of course it should be valid really. It doesn't make sense for the string based at c_5 to be valid but a substring of it to be invalid. I don't think we can guarantee that such weird corner cases never happen though, even if we tried to avoid this one. One possibility would be to mark c_12 as valid on the basis that c_5 is valid, but I'm not sure the complication is worth it given that it seems to trigger very rarely. A better optimisation would be to get the unroller to clean up after itself a bit more... Although this particular instance of the bug relies on r249880, I think we could have similar problems in GCC 7. It would be much harder to trigger though, especially since it relies on unfolded IR like the above. 2018-05-21 Richard Sandiford <richard.sandif...@linaro.org> gcc/ PR tree-optimization/85814 * tree-ssa-strlen.c (get_stridx_plus_constant): Cope with a null return from get_strinfo when unsharing the next strinfo in the chain. gcc/testsuite/ PR tree-optimization/85814 * gcc.dg/torture/pr85814.c: New test. Added: trunk/gcc/testsuite/gcc.dg/torture/pr85814.c Modified: trunk/gcc/ChangeLog trunk/gcc/testsuite/ChangeLog trunk/gcc/tree-ssa-strlen.c
