[gcc r15-995] Implement wrap-around arithmetics in DWARF expressions

[Eric Botcazou]

https://gcc.gnu.org/g:f3d6d60d2ae584a23bb7c681cbd511202953c391

commit r15-995-gf3d6d60d2ae584a23bb7c681cbd511202953c391
Author: Eric Botcazou <ebotcazou@adacore.com>
Date:   Mon Jun 3 17:44:13 2024 +0200

    Implement wrap-around arithmetics in DWARF expressions
    
    For the following Ada package declaring a simple variable-sized record type:
    
    package P is
    
      type Enum is (Zero, One, Two, Three, Four, Five, Six, Seven, Eight, Nine);
    
      type Rec (Kind : Enum := Zero) is record
          case Kind is
            when Four .. Seven =>
              S : String (1 .. 32);
            when others =>
              null;
          end case;
        end record;
    
    end P;
    
    the compiler builds a "size function" in GENERIC which is at -Og:
    
    sizetype _GLOBAL.SZ5_p (p__enum p0)
    {
      return (UNSIGNED_8) p0 + 252 <= 3 ? 32 : 0;
    }
    
    The UNSIGNED_8-based trick makes it possible to eliminates one branch but
    relies on the wrap-around arithmetics of UNSIGNED_8.  This size function
    is then translated into a DWARF procedure, but the wrap-around arithmetics
    is dropped on the floor, leading to a wrong size calculation when the DWARF
    procedure is executed.
    
    The fix also contains an optimization of unsigned comparisons in DWARF for
    the case where the type is smaller than the "generic type" like here.
    
    gcc/
            * dwarf2out.cc (loc_list_from_tree_1) <CEIL_DIV_EXPR>; Add const.
            <do_comp_binop>: Use a signed comparison for small unsigned types.
            Implement wrap-around arithmetics for small integer types.

Diff:
---
 gcc/dwarf2out.cc | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/gcc/dwarf2out.cc b/gcc/dwarf2out.cc
index 5b064ffd78a..89efa5474d3 100644
--- a/gcc/dwarf2out.cc
+++ b/gcc/dwarf2out.cc
@@ -19383,7 +19383,7 @@ loc_list_from_tree_1 (tree loc, int want_address,
     case ROUND_DIV_EXPR:
       if (TYPE_UNSIGNED (TREE_TYPE (loc)))
 	{
-	  enum machine_mode mode = TYPE_MODE (TREE_TYPE (loc));
+	  const enum machine_mode mode = TYPE_MODE (TREE_TYPE (loc));
 	  scalar_int_mode int_mode;
 
 	  if ((dwarf_strict && dwarf_version < 5)
@@ -19518,6 +19518,15 @@ loc_list_from_tree_1 (tree loc, int want_address,
     do_comp_binop:
       if (TYPE_UNSIGNED (TREE_TYPE (TREE_OPERAND (loc, 0))))
 	{
+	  const enum machine_mode mode
+	    = TYPE_MODE (TREE_TYPE (TREE_OPERAND (loc, 0)));
+	  scalar_int_mode int_mode;
+
+	  /* We can use a signed comparison if the sign bit is not set.  */
+	  if (is_a <scalar_int_mode> (mode, &int_mode)
+	      && GET_MODE_SIZE (int_mode) < DWARF2_ADDR_SIZE)
+	    goto do_binop;
+
 	  list_ret = loc_list_from_tree (TREE_OPERAND (loc, 0), 0, context);
 	  list_ret1 = loc_list_from_tree (TREE_OPERAND (loc, 1), 0, context);
 	  list_ret = loc_list_from_uint_comparison (list_ret, list_ret1,
@@ -19544,6 +19553,7 @@ loc_list_from_tree_1 (tree loc, int want_address,
       add_loc_list (&list_ret, list_ret1);
       if (list_ret == 0)
 	return 0;
+
       add_loc_descr_to_each (list_ret, new_loc_descr (op, 0, 0));
       break;
 
@@ -19668,6 +19678,28 @@ loc_list_from_tree_1 (tree loc, int want_address,
   if (!ret && !list_ret)
     return 0;
 
+  /* Implement wrap-around arithmetics for small integer types.  */
+  if ((TREE_CODE (loc) == PLUS_EXPR
+       || TREE_CODE (loc) == MINUS_EXPR
+       || TREE_CODE (loc) == MULT_EXPR
+       || TREE_CODE (loc) == NEGATE_EXPR
+       || TREE_CODE (loc) == LSHIFT_EXPR)
+      && INTEGRAL_TYPE_P (TREE_TYPE (loc))
+      && TYPE_OVERFLOW_WRAPS (TREE_TYPE (loc)))
+    {
+      const enum machine_mode mode = TYPE_MODE (TREE_TYPE (loc));
+      scalar_int_mode int_mode;
+
+      if (is_a <scalar_int_mode> (mode, &int_mode)
+	  && GET_MODE_SIZE (int_mode) < DWARF2_ADDR_SIZE)
+	{
+	   const unsigned HOST_WIDE_INT mask
+	    = (HOST_WIDE_INT_1U << GET_MODE_BITSIZE (int_mode)) - 1;
+	   add_loc_descr_to_each (list_ret, uint_loc_descriptor (mask));
+	   add_loc_descr_to_each (list_ret, new_loc_descr (DW_OP_and, 0, 0));
+	}
+    }
+
   if (want_address == 2 && !have_address
       && (dwarf_version >= 4 || !dwarf_strict))
     {