https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68317
--- Comment #8 from Jiong Wang <jiwang at gcc dot gnu.org> --- (In reply to Richard Biener from comment #7) > (In reply to Jiong Wang from comment #6) > > Created attachment 36741 [details] > > prototype-fix > > > > (In reply to Richard Biener from comment #3) > > > (gdb) p debug_generic_expr (max) > > > 4294443008(OVF) > > > + # ivtmp.8_8 = PHI <4294443008(OVF)(2), ivtmp.8_11(4)> > > > + _5 = (int) ivtmp.8_8; > > > fn2 (_5); > > > - i_7 = i_1 + -1; > > > > > > <bb 4>: > > > + ivtmp.8_11 = ivtmp.8_8 - 524288; > > > goto <bb 3>; > > > > > > } > > > > > > note that the infinite loop contains undefined overflow. > > > > > > IVOPTs should simply strip the overflow flag (using drop_tree_overflow). > > > > And my further investigation shows PR68326 is caused by the same issue. > > > > # ivtmp.8_8 = PHI <4294443008(OVF)(2), ivtmp.8_11(4)> > > > > the new phi node destination is with unsigned int type, the constant > > value 4294443008 can fit into it, it's marked as OVF because > > it's treated as signed type. For the simple testcase in PR68326, > > the overflow number is 4294967286 which is -10, while there happen be > > another signed integer with initial value -10. So, looks like the unsigned > > 4294967286 somehow inherited the signed type from the other value in some > > tree pass, then some valid constant is marked with OVF unnecessarily. > > > > Anyway, below is my fix, does it looks the correct approach to you? > > > > drop_tree_overflow is invoked during create_iv, if the constant can actually > > fit into the type. I only checked INTEGER_CST, not for others like REAL, as > > I > > though they won't suffer from the unsigned/signed type issue. > > > > x86-64 bootstrap is OK with this patch, will do more testing if the approach > > is OK. > > > > diff --git a/gcc/tree-ssa-loop-manip.c b/gcc/tree-ssa-loop-manip.c > > index b614412..55a6334 100644 > > --- a/gcc/tree-ssa-loop-manip.c > > +++ b/gcc/tree-ssa-loop-manip.c > > @@ -136,6 +136,11 @@ create_iv (tree base, tree step, tree var, struct loop > > *loop, > > gsi_insert_seq_on_edge_immediate (pe, stmts); > > > > phi = create_phi_node (vb, loop->header); > > + if (TREE_OVERFLOW (initial) > > + && TREE_CODE (initial) == INTEGER_CST > > + && int_fits_type_p (initial, TREE_TYPE (vb))) > > + initial = drop_tree_overflow (initial); > > + > > add_phi_arg (phi, initial, loop_preheader_edge (loop), UNKNOWN_LOCATION); > > add_phi_arg (phi, va, loop_latch_edge (loop), UNKNOWN_LOCATION); > > } > > I think it's better to track down where the constant is generated. OK, I will do a further tracking. > Once the overflow value is returned from fold_* () it should be stripped > off its overflow flag. Unconditionally so with just > > if (TREE_OVERFLOW_P (..)) > .. = drop_tree_overflow (..); I don't understand the scope of OVERFLOW flag will affect, so was dropping the flag only when it's really a overflow.
