Fix fixed-point regression with recent GCC

A recent version of GCC changed how fixed-point types are described.
For example, a denominator in one test case now looks like:

    GNU_denominator      (exprloc)
     [ 0] implicit_value: 16 byte block: 00 00 b8 9d 0d 69 55 a0 01 00 00 00 00 00 00 00

... the difference being that this now uses exprloc and emits a
DW_OP_implicit_value for the 16-byte block.  (DWARF 5 still uses
DW_FORM_data16.)

This change was made here:

    https://gcc.gnu.org/pipermail/gcc-patches/2020-December/560897.html

This patch updates gdb to handle this situation.

Note that, before GCC 11, this test would not give the same answer.
Earlier versions of GCC fell back to GNAT encodings for this case.

gdb/ChangeLog
2021-01-25  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (get_mpz): New function.
	(get_dwarf2_rational_constant): Use it.

gdb/testsuite/ChangeLog
2021-01-25  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/fixed_points.exp: Add regression test.
	* gdb.ada/fixed_points/fixed_points.adb (FP5_Var): New variable.
	* gdb.ada/fixed_points/pck.adb (Delta5, FP5_Type): New.

6 files changed, 65 insertions, 19 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 3a4844f..0c30d75 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2021-01-25  Tom Tromey  <tromey@adacore.com>
 
+	* dwarf2/read.c (get_mpz): New function.
+	(get_dwarf2_rational_constant): Use it.
+
+2021-01-25  Tom Tromey  <tromey@adacore.com>
+
 	* ada-lang.c (resolve_subexp): Handle array context.
 
 2021-01-23  Tom Tromey  <tom@tromey.com>
diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 309ff83..ae95c65 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -18228,6 +18228,46 @@ read_typedef (struct die_info *die, struct dwarf2_cu *cu)
   return this_type;
 }
 
+/* Helper for get_dwarf2_rational_constant that computes the value of
+   a given gmp_mpz given an attribute.  */
+
+static void
+get_mpz (struct dwarf2_cu *cu, gdb_mpz *value, struct attribute *attr)
+{
+  /* GCC will sometimes emit a 16-byte constant value as a DWARF
+     location expression that pushes an implicit value.  */
+  if (attr->form == DW_FORM_exprloc)
+    {
+      dwarf_block *blk = attr->as_block ();
+      if (blk->size > 0 && blk->data[0] == DW_OP_implicit_value)
+	{
+	  uint64_t len;
+	  const gdb_byte *ptr = safe_read_uleb128 (blk->data + 1,
+						   blk->data + blk->size,
+						   &len);
+	  if (ptr - blk->data + len <= blk->size)
+	    {
+	      mpz_import (value->val, len,
+			  bfd_big_endian (cu->per_objfile->objfile->obfd) ? 1 : -1,
+			  1, 0, 0, ptr);
+	      return;
+	    }
+	}
+
+      /* On failure set it to 1.  */
+      *value = gdb_mpz (1);
+    }
+  else if (attr->form_is_block ())
+    {
+      dwarf_block *blk = attr->as_block ();
+      mpz_import (value->val, blk->size,
+		  bfd_big_endian (cu->per_objfile->objfile->obfd) ? 1 : -1,
+		  1, 0, 0, blk->data);
+    }
+  else
+    *value = gdb_mpz (attr->constant_value (1));
+}
+
 /* Assuming DIE is a rational DW_TAG_constant, read the DIE's
    numerator and denominator into NUMERATOR and DENOMINATOR (resp).
 
@@ -18254,25 +18294,8 @@ get_dwarf2_rational_constant (struct die_info *die, struct dwarf2_cu *cu,
   if (num_attr == nullptr || denom_attr == nullptr)
     return;
 
-  if (num_attr->form_is_block ())
-    {
-      dwarf_block *blk = num_attr->as_block ();
-      mpz_import (numerator->val, blk->size,
-		  bfd_big_endian (cu->per_objfile->objfile->obfd) ? 1 : -1,
-		  1, 0, 0, blk->data);
-    }
-  else
-    *numerator = gdb_mpz (num_attr->constant_value (1));
-
-  if (denom_attr->form_is_block ())
-    {
-      dwarf_block *blk = denom_attr->as_block ();
-      mpz_import (denominator->val, blk->size,
-		  bfd_big_endian (cu->per_objfile->objfile->obfd) ? 1 : -1,
-		  1, 0, 0, blk->data);
-    }
-  else
-    *denominator = gdb_mpz (denom_attr->constant_value (1));
+  get_mpz (cu, numerator, num_attr);
+  get_mpz (cu, denominator, denom_attr);
 }
 
 /* Same as get_dwarf2_rational_constant, but extracting an unsigned
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 7fa1074..dad00fe 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,11 @@
 2021-01-25  Tom Tromey  <tromey@adacore.com>
 
+	* gdb.ada/fixed_points.exp: Add regression test.
+	* gdb.ada/fixed_points/fixed_points.adb (FP5_Var): New variable.
+	* gdb.ada/fixed_points/pck.adb (Delta5, FP5_Type): New.
+
+2021-01-25  Tom Tromey  <tromey@adacore.com>
+
 	* gdb.ada/local-enum.exp: Add enumerator resolution test.
 
 2021-01-25  Tom Tromey  <tromey@adacore.com>
diff --git a/gdb/testsuite/gdb.ada/fixed_points.exp b/gdb/testsuite/gdb.ada/fixed_points.exp
index 0d24453..ac45ef9 100644
--- a/gdb/testsuite/gdb.ada/fixed_points.exp
+++ b/gdb/testsuite/gdb.ada/fixed_points.exp
@@ -123,5 +123,10 @@ foreach_with_prefix scenario {all minimal} {
 	gdb_test "print fp4_var * 1" $fp4
 	gdb_test "print 1 * fp4_var" $fp4
 	gdb_test "print fp4_var / 1" $fp4
+
+	# This only started working in GCC 11.
+	if {[test_compiler_info {gcc-11-*}]} {
+	    gdb_test "print fp5_var" " = 3e-19"
+	}
     }
 }
diff --git a/gdb/testsuite/gdb.ada/fixed_points/fixed_points.adb b/gdb/testsuite/gdb.ada/fixed_points/fixed_points.adb
index cc2c637..4298cdf 100644
--- a/gdb/testsuite/gdb.ada/fixed_points/fixed_points.adb
+++ b/gdb/testsuite/gdb.ada/fixed_points/fixed_points.adb
@@ -55,6 +55,8 @@ procedure Fixed_Points is
    Overprecise_Object : Overprecise_Fixed_Point :=
      Overprecise_Fixed_Point'Small;
 
+   FP5_Var : FP5_Type := 3 * Delta5;
+
 begin
    Base_Object := 1.0/16.0;   -- Set breakpoint here
    Subtype_Object := 1.0/16.0;
@@ -64,4 +66,5 @@ begin
    Do_Nothing (FP2_Var'Address);
    Do_Nothing (FP3_Var'Address);
    Do_Nothing (FP4_Var'Address);
+   Do_Nothing (FP5_Var'Address);
 end Fixed_Points;
diff --git a/gdb/testsuite/gdb.ada/fixed_points/pck.ads b/gdb/testsuite/gdb.ada/fixed_points/pck.ads
index b5c1bc0..3bdf059 100644
--- a/gdb/testsuite/gdb.ada/fixed_points/pck.ads
+++ b/gdb/testsuite/gdb.ada/fixed_points/pck.ads
@@ -30,6 +30,10 @@ package Pck is
       with Small => Delta4 / 3.0;
    FP4_Var : FP4_Type := 2 * Delta4;
 
+   Delta5 : constant := 0.000_000_000_000_000_000_1;
+   type FP5_Type is delta Delta5 range 0.0 .. Delta5 * 10
+      with Small => Delta5 / 3.0;
+
    procedure Do_Nothing (A : System.Address);
 end pck;