http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50183
--- Comment #10 from William J. Schmidt <wschmidt at gcc dot gnu.org> 2011-09-13 16:40:05 UTC --- The problem arises during canonicalization in the presence of nested loops. Loops are processed outward-in. When each loop is processed, its single exit edge's destination block is cleansed of duplicate close-phis. The problem in this case is that subsequent processing of the inner loop reintroduces duplicate close-phis that are not then cleansed. Specifically, block 43 contains the close-phis for the inner loop, and block 45 contains the close-phis for the outer loop: <bb 43>: # err2.395_561 = PHI <err2.395_219(41)> # err2_lsm.820_573 = PHI <err2.395_219(41)> ... <bb 45>: # err2.395_571 = PHI <err2.395_561(43)> # err2_lsm.820_574 = PHI <err2_lsm.820_573(43)> ... When the outer loop is processed, the two close-phis are not duplicates (they have different phi-arguments). When the inner loop is processed, the two close-phis are found to be duplicates. The first is chosen as the representative. All immediate uses of err2_lsm.820_573 are replaced with uses of err2.395_561, and the second phi in block 43 is removed. The result is that block 45 now contains two duplicate close-phis. I believe this can be easily fixed by checking whether a replaced immediate use is itself a PHI statement with a single argument (a close-phi). If so, make_close_phi_nodes_unique () should be called again for the block containing the replaced use. I'll experiment with this.
