gimple-iterator, ubsan: Fix ICE during instrumentation of returns_twice calls [PR112709]

ubsan, asan (both PR112709) and _BitInt lowering (PR113466) want to
insert some instrumentation or adjustment statements before some statement.
This unfortunately creates invalid IL if inserting before a returns_twice
call, because we require that such calls are the first statement in a basic
block and that we have an edge from the .ABNORMAL_DISPATCHER block to
the block containing the returns_twice call (in addition to other edge(s)).

The following patch adds helper functions for such insertions and uses it
for now in ubsan (I'll post a follow up which uses it in asan and will
work later on the _BitInt lowering PR).

In particular, if the bb with returns_twice call at the start has just
2 edges, one EDGE_ABNORMAL from .ABNORMAL_DISPATCHER and another
(non-EDGE_ABNORMAL/EDGE_EH) from some other bb, it just inserts the
statement or sequence on that other edge.
If the bb has more predecessor edges or the one not from
.ABNORMAL_DISPATCHER is e.g. an EH edge (this latter case likely shouldn't
happen, one would need labels or something like that), the patch splits the
block with returns_twice call such that there is just one edge next to
.ABNORMAL_DISPATCHER edge and adjusts PHIs as needed to make it happen.
The functions also replace uses of PHIs from the returns_twice bb with
the corresponding PHI arguments, because otherwise it would be invalid IL.

E.g. in ubsan/pr112709-2.c (qux) we have before the ubsan pass
  <bb 10> :
  # .MEM_5(ab) = PHI <.MEM_4(9), .MEM_25(ab)(11)>
  # _7(ab) = PHI <_20(9), _8(ab)(11)>
  # .MEM_21(ab) = VDEF <.MEM_5(ab)>
  _22 = bar (*_7(ab));
where bar is returns_twice call and bb 11 has .ABNORMAL_DISPATCHER call,
this patch instruments it like:
  <bb 9> :
  # .MEM_4 = PHI <.MEM_17(ab)(4), .MEM_10(D)(5), .MEM_14(ab)(8)>
  # DEBUG BEGIN_STMT
  # VUSE <.MEM_4>
  _20 = p;
  # .MEM_27 = VDEF <.MEM_4>
  .UBSAN_NULL (_20, 0B, 0);
  # VUSE <.MEM_27>
  _2 = __builtin_dynamic_object_size (_20, 0);
  # .MEM_28 = VDEF <.MEM_27>
  .UBSAN_OBJECT_SIZE (_20, 1024, _2, 0);

  <bb 10> :
  # .MEM_5(ab) = PHI <.MEM_28(9), .MEM_25(ab)(11)>
  # _7(ab) = PHI <_20(9), _8(ab)(11)>
  # .MEM_21(ab) = VDEF <.MEM_5(ab)>
  _22 = bar (*_7(ab));
The edge from .ABNORMAL_DISPATCHER is there just to represent the
returning for 2nd and later times, the instrumentation can't be
done at that point as there is no code executed during that point.
The ubsan/pr112709-1.c testcase includes non-virtual PHIs to cover
the handling of those as well.

2024-03-13  Jakub Jelinek  <jakub@redhat.com>

	PR sanitizer/112709
	* gimple-iterator.h (gsi_safe_insert_before,
	gsi_safe_insert_seq_before): Declare.
	* gimple-iterator.cc: Include gimplify.h.
	(edge_before_returns_twice_call, adjust_before_returns_twice_call,
	gsi_safe_insert_before, gsi_safe_insert_seq_before): New functions.
	* ubsan.cc (instrument_mem_ref, instrument_pointer_overflow,
	instrument_nonnull_arg, instrument_nonnull_return): Use
	gsi_safe_insert_before instead of gsi_insert_before.
	(maybe_instrument_pointer_overflow): Use force_gimple_operand,
	gimple_seq_add_seq_without_update and gsi_safe_insert_seq_before
	instead of force_gimple_operand_gsi.
	(instrument_object_size): Likewise.  Use gsi_safe_insert_before
	instead of gsi_insert_before.

	* gcc.dg/ubsan/pr112709-1.c: New test.
	* gcc.dg/ubsan/pr112709-2.c: New test.