DWARF: space-optimize loc. descr. for integer literals on 32-bit targets

This enhances location description generation so that the generated
opcodes for integer literals are as space-efficient when HOST_WIDE_INT
is 64-bits large than when it's 32-bits large. In particular, this
reduces the size of the opcodes generated to produce big unsigned
literals using small literal integers instead.

gcc/

	* dwarf2out.c (int_loc_descriptor): Generate opcodes for another
	equivalent 32-bit constant (modulo 2**32) when that yields
	smaller instructions.
	(size_of_int_loc_descriptor): Update accordingly.

gcc/testsuite/

	* gnat.dg/debug8.adb: New testcase.

From-SVN: r241024

gcc/ChangeLog

 2016-10-12  Pierre-Marie de Rodat  <derodat@adacore.com>
 
+	* dwarf2out.c (int_loc_descriptor): Generate opcodes for another
+	equivalent 32-bit constant (modulo 2**32) when that yields
+	smaller instructions.
+	(size_of_int_loc_descriptor): Update accordingly.
+
+2016-10-12  Pierre-Marie de Rodat  <derodat@adacore.com>
+
 	* dwarf2out.c (dwarf2out_early_global_decl): For nested
 	functions, call dwarf2out_decl on the parent function first.
 

gcc/dwarf2out.c

@@ -11985,20 +11985,35 @@ int_loc_descriptor (HOST_WIDE_INT i)
 	/* DW_OP_const1u X DW_OP_litY DW_OP_shl takes just 4 bytes,
 	   while DW_OP_const4u is 5 bytes.  */
 	return int_shift_loc_descriptor (i, HOST_BITS_PER_WIDE_INT - clz - 8);
+
+      else if (DWARF2_ADDR_SIZE == 4 && i > 0x7fffffff
+	       && size_of_int_loc_descriptor ((HOST_WIDE_INT) (int32_t) i)
+		  <= 4)
+	{
+	  /* As i >= 2**31, the double cast above will yield a negative number.
+	     Since wrapping is defined in DWARF expressions we can output big
+	     positive integers as small negative ones, regardless of the size
+	     of host wide ints.
+
+	     Here, since the evaluator will handle 32-bit values and since i >=
+	     2**31, we know it's going to be interpreted as a negative literal:
+	     store it this way if we can do better than 5 bytes this way.  */
+	  return int_loc_descriptor ((HOST_WIDE_INT) (int32_t) i);
+	}
       else if (HOST_BITS_PER_WIDE_INT == 32 || i <= 0xffffffff)
 	op = DW_OP_const4u;
+
+      /* Past this point, i >= 0x100000000 and thus DW_OP_constu will take at
+	 least 6 bytes: see if we can do better before falling back to it.  */
       else if (clz + ctz >= HOST_BITS_PER_WIDE_INT - 8
 	       && clz + 8 + 255 >= HOST_BITS_PER_WIDE_INT)
-	/* DW_OP_const1u X DW_OP_const1u Y DW_OP_shl takes just 5 bytes,
-	   while DW_OP_constu of constant >= 0x100000000 takes at least
-	   6 bytes.  */
+	/* DW_OP_const1u X DW_OP_const1u Y DW_OP_shl takes just 5 bytes.  */
 	return int_shift_loc_descriptor (i, HOST_BITS_PER_WIDE_INT - clz - 8);
       else if (clz + ctz >= HOST_BITS_PER_WIDE_INT - 16
 	       && clz + 16 + (size_of_uleb128 (i) > 5 ? 255 : 31)
 		  >= HOST_BITS_PER_WIDE_INT)
 	/* DW_OP_const2u X DW_OP_litY DW_OP_shl takes just 5 bytes,
-	   DW_OP_const2u X DW_OP_const1u Y DW_OP_shl takes 6 bytes,
-	   while DW_OP_constu takes in this case at least 6 bytes.  */
+	   DW_OP_const2u X DW_OP_const1u Y DW_OP_shl takes 6 bytes.  */
 	return int_shift_loc_descriptor (i, HOST_BITS_PER_WIDE_INT - clz - 16);
       else if (clz + ctz >= HOST_BITS_PER_WIDE_INT - 32
 	       && clz + 32 + 31 >= HOST_BITS_PER_WIDE_INT
@@ -12223,6 +12238,10 @@ size_of_int_loc_descriptor (HOST_WIDE_INT i)
 	       && clz + 8 + 31 >= HOST_BITS_PER_WIDE_INT)
 	return size_of_int_shift_loc_descriptor (i, HOST_BITS_PER_WIDE_INT
 						    - clz - 8);
+      else if (DWARF2_ADDR_SIZE == 4 && i > 0x7fffffff
+	       && size_of_int_loc_descriptor ((HOST_WIDE_INT) (int32_t) i)
+		  <= 4)
+	return size_of_int_loc_descriptor ((HOST_WIDE_INT) (int32_t) i);
       else if (HOST_BITS_PER_WIDE_INT == 32 || i <= 0xffffffff)
 	return 5;
       s = size_of_uleb128 ((unsigned HOST_WIDE_INT) i);

gcc/testsuite/ChangeLog

 2016-10-12  Pierre-Marie de Rodat  <derodat@adacore.com>
 
+	* gnat.dg/debug8.adb: New testcase.
+
+2016-10-12  Pierre-Marie de Rodat  <derodat@adacore.com>
+
 	* gnat.dg/debug9.adb: New testcase.
 
 2016-10-12  Richard Biener  <rguenther@suse.de>

gcc/testsuite/gnat.dg/debug8.adb

+-- { dg-do compile }
+-- { dg-options "-cargs -g -fgnat-encodings=minimal -dA" }
+-- { dg-final { scan-assembler-not "DW_OP_const4u" } }
+-- { dg-final { scan-assembler-not "DW_OP_const8u" } }
+
+--  The DW_AT_byte_size attribute DWARF expression for the
+--  DW_TAG_structure_type DIE that describes Rec_Type contains the -4u literal.
+--  Check that it is not created using an inefficient encoding (DW_OP_const1s
+--  is expected).
+
+procedure Debug8 is
+
+   type Rec_Type (I : Integer) is record
+      B : Boolean;
+      case I is
+         when 0 =>
+            null;
+         when 1 .. 10 =>
+            C : Character;
+         when others =>
+            N : Natural;
+      end case;
+   end record;
+
+   R : access Rec_Type := null;
+
+begin
+   null;
+end Debug8;