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

[Pierre-Marie de Rodat <derodat at adacore dot com>]

Hello,

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.

Bootstrapped and regtested on x86-linux (no regression).  I also checked
that the new testcase fails with mainline.  Ok to commit?  Thank you in
advance!

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/dwarf2out.c                  | 31 ++++++++++++++++++++++++++-----
 gcc/testsuite/gnat.dg/debug8.adb | 29 +++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/gnat.dg/debug8.adb

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 0fdab9a..f175ea1 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -11954,20 +11954,36 @@ 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 (HOST_BITS_PER_WIDE_INT > 32
+	       && 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
@@ -12192,6 +12208,11 @@ 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 (HOST_BITS_PER_WIDE_INT > 32
+	       && 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);
diff --git a/gcc/testsuite/gnat.dg/debug8.adb b/gcc/testsuite/gnat.dg/debug8.adb
new file mode 100644
index 0000000..fabcc22
--- /dev/null
+++ b/gcc/testsuite/gnat.dg/debug8.adb
@@ -0,0 +1,29 @@
+-- { 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;
-- 
2.9.3