[PATCH] sccvn: Avoid UB in ao_ref_init_from_vn_reference [PR105533]

[Jakub Jelinek]

Hi!

When compiling libgcc or on e.g.
int a[64];
int p;

void
foo (void)
{
  int s = 1;
  while (p)
    {
      s -= 11;
      a[s] != 0;
    }
}
sccvn invokes UB in the compiler as detected by ubsan:
../../gcc/poly-int.h:1089:5: runtime error: left shift of negative value -40
The problem is that we still use C++11..C++17 as the implementation language
and in those C++ versions shifting negative values left is UB (well defined
since C++20) and above in
           offset += op->off << LOG2_BITS_PER_UNIT;
op->off is poly_int64 with -40 value (in libgcc with -8).
I understand the offset_int << LOG2_BITS_PER_UNIT shifts but it is then well
defined during underlying implementation which is done on the uhwi limbs,
but for poly_int64 we use
                offset += pop->off * BITS_PER_UNIT;
a few lines earlier and I think that is both more readable in what it
actually does and triggers UB only if there would be signed multiply
overflow.  In the end, the compiler will treat them the same at least at the
RTL level (at least, if not and they aren't the same cost, it should).

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2024-03-07  Jakub Jelinek  <ja...@redhat.com>

        PR middle-end/105533
        * tree-ssa-sccvn.cc (ao_ref_init_from_vn_reference) <case ARRAY_REF>:
        Multiple op->off by BITS_PER_UNIT instead of shifting it left by
        LOG2_BITS_PER_UNIT.

--- gcc/tree-ssa-sccvn.cc.jj    2024-02-28 22:57:18.318658827 +0100
+++ gcc/tree-ssa-sccvn.cc       2024-03-06 14:52:16.819229719 +0100
@@ -1221,7 +1221,7 @@ ao_ref_init_from_vn_reference (ao_ref *r
          if (maybe_eq (op->off, -1))
            max_size = -1;
          else
-           offset += op->off << LOG2_BITS_PER_UNIT;
+           offset += op->off * BITS_PER_UNIT;
          break;
 
        case REALPART_EXPR:


        Jakub