[Bug tree-optimization/110204] [14 Regression] Suspicous warning when compiling ranges-v3 using GCC trunk (iteration 9223372036854775807 invokes undefined behavior)

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110204

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #5)
> Yeah, the issue is that PRE figures out a new value here but we've already
> done value-numbering so we can't alter the "old" solution here.  So what we
> do
> is add a '0' with value '_42' (instead of value '0').  This "second order"
> value numbering leaves more opportunities on the plate.  It was IMHO
> cleaner to insert a
> 
>   pretmp_163 = 0;
> 
> than substituting '0' everywhere but then leaving around the third order
> optimizations in case we had _42 + 1 that would then simplify to sth = 1 ...
> 
> Previously we'd have inserted a degenerate PHI, now we get these kind of
> copies.  "Now" is for a long time so this isn't new for PRE at least.
> 
> We could "hack" this by doing
> 
> diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
> index 11061a374a2..effb4f4de73 100644
> --- a/gcc/tree-ssa-sccvn.cc
> +++ b/gcc/tree-ssa-sccvn.cc
> @@ -6615,6 +6615,13 @@ eliminate_dom_walker::eliminate_push_avail
> (basic_block, tree op)
>        if (avail[SSA_NAME_VERSION (valnum)])
>         pushop = avail[SSA_NAME_VERSION (valnum)];
>        avail_stack.safe_push (pushop);
> +      if (gassign *ass = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (op)))
> +       if (gimple_assign_rhs_class (ass) == GIMPLE_SINGLE_RHS)
> +         {
> +           tree rhs1 = gimple_assign_rhs1 (ass);
> +           if (CONSTANT_CLASS_P (rhs1) || TREE_CODE (rhs1) == SSA_NAME)
> +             op = rhs1;
> +         }
>        avail[SSA_NAME_VERSION (valnum)] = op;
>      }
>  }

Ah, that breaks the avail stack handling.  We can do the trick in
eliminate_avail though where it's more expensive or make the avail
stack more complicated.