Handle DW_TAG_variant_part and DW_TAG_variant

This changes dwarf2read to understand DW_TAG_variant_part and
DW_TAG_variant.

Note that DW_AT_discr_list is not handled.  I did not need this for
Rust.  I imagine this should not be too hard to add later, should
someone need it.  Meanwhile I have gdb emit a complaint if it is seen.

There is a lurking issue concerning the placement of the discriminant
in the DWARF.  For Rust, I ended up following the letter of the
standard and having the discriminant be a child of the
DW_TAG_variant_part.  However, GCC's Ada support does not do this.
Pierre-Marie filed this with the DWARF committee:

    http://dwarfstd.org/ShowIssue.php?issue=180123.1

However as that is read-only, if you have comments you might consider
adding them to the GCC bug:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83935

Finally, there is a DWARF extension lurking in here.  In Rust, a
univariant enum will not have a discriminant.  However, in order to
unify the representation of all data-carrying enums, I've made LLVM
(and my forthcoming rustc patch) emit a univariant enum using a
DW_TAG_variant with a single variant part and without DW_AT_discr.
The lack of this DW_AT_discr is the extension.  I will submit an issue
on dwarfstd.org about this.

2018-02-26  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (struct variant_field): New.
	(struct nextfield) <variant>: New field.
	(dwarf2_add_field): Handle DW_TAG_variant_part.
	(dwarf2_attach_fields_to_type): Attach a discriminant_info to a
	discriminated union.
	(read_structure_type): Handle DW_TAG_variant_part.
	(handle_struct_member_die): New function, extracted from
	process_structure_scope.  Handle DW_TAG_variant.
	(process_structure_scope): Handle discriminated unions.  Call
	handle_struct_member_die.

2018-02-26  Tom Tromey  <tom@tromey.com>

	* gdb.dwarf2/variant.c: New file.
	* gdb.dwarf2/variant.exp: New file.

3 files changed, 276 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 8d7792f..755fe94 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2018-02-26  Tom Tromey  <tom@tromey.com>
 
+	* gdb.dwarf2/variant.c: New file.
+	* gdb.dwarf2/variant.exp: New file.
+
+2018-02-26  Tom Tromey  <tom@tromey.com>
+
 	* gdb.rust/simple.exp: Accept more possible results in enum test.
 
 2018-02-25  Simon Marchi  <simon.marchi@polymtl.ca>
diff --git a/gdb/testsuite/gdb.dwarf2/variant.c b/gdb/testsuite/gdb.dwarf2/variant.c
new file mode 100644
index 0000000..e145e78
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/variant.c
@@ -0,0 +1,47 @@
+/* Copyright 2018 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+unsigned char buffer[8];
+unsigned char buffer2[8];
+
+static void
+func (void)
+{
+}
+
+int
+main (void)
+{
+  /* Write the expected values into the buffer.  */
+  unsigned int x = 23;
+  if (*(char *) &x)
+    {
+      /* Little endian.  */
+      buffer[0] = 23;
+      buffer[4] = 23;
+      buffer2[0] = 255;
+      buffer2[4] = 23;
+    }
+  else
+    {
+      buffer[3] = 23;
+      buffer[7] = 23;
+      buffer2[0] = 255;
+      buffer2[7] = 23;
+    }
+
+  func ();
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dwarf2/variant.exp b/gdb/testsuite/gdb.dwarf2/variant.exp
new file mode 100644
index 0000000..b61659f
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/variant.exp
@@ -0,0 +1,224 @@
+# Copyright 2018 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test DW_TAG_variant_part and DW_TAG_variant.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use
+# gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .c variant.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    upvar cu_lang cu_lang
+
+    declare_labels uinteger_label float_label int8_label
+    declare_labels discr_1_label discr_2_label discr_3_label
+    declare_labels one_label two_label
+
+    # Creating a CU with 4-byte addresses lets this test link on
+    # both 32- and 64-bit machines.
+    cu { addr_size 4 } {
+	compile_unit {
+	    {name file1.txt}
+	    {language @DW_LANG_Rust}
+	} {
+            uinteger_label: DW_TAG_base_type {
+                {DW_AT_byte_size 4 DW_FORM_sdata}
+                {DW_AT_encoding  @DW_ATE_unsigned}
+                {DW_AT_name      {unsigned integer}}
+            }
+
+            int8_label: DW_TAG_base_type {
+                {DW_AT_byte_size 1 DW_FORM_sdata}
+                {DW_AT_encoding  @DW_ATE_signed}
+                {DW_AT_name      i8}
+            }
+
+	    float_label: base_type {
+		{name float}
+		{encoding @DW_ATE_float}
+		{byte_size 4 DW_FORM_sdata}
+	    }
+
+	    one_label: structure_type {
+		{name One}
+		{byte_size 4 DW_FORM_sdata}
+	    } {
+		member {
+		    {name __0}
+		    {type :$uinteger_label}
+		    {data_member_location 0 data1}
+		}
+	    }
+
+	    two_label: structure_type {
+		{name Two}
+		{byte_size 4 DW_FORM_sdata}
+	    } {
+		member {
+		    {name __0}
+		    {type :$float_label}
+		    {data_member_location 0 data1}
+		}
+	    }
+
+	    structure_type {
+		{name Simple}
+		{byte_size 8 DW_FORM_sdata}
+	    } {
+		variant_part {
+		    {discr :$discr_1_label DW_FORM_ref4}
+		} {
+		    discr_1_label: member {
+			{type :$uinteger_label}
+			{data_member_location 0 data1}
+		    }
+
+		    variant {
+			{discr_value 23 udata}
+		    } {
+			member {
+			    {type :$one_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+
+		    variant {
+			{discr_value 1 udata}
+		    } {
+			member {
+			    {type :$two_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+		}
+	    }
+
+	    structure_type {
+		{name Defaulted}
+		{byte_size 8 DW_FORM_sdata}
+	    } {
+		variant_part {
+		    {discr :$discr_2_label DW_FORM_ref4}
+		} {
+		    discr_2_label: member {
+			{type :$uinteger_label}
+			{data_member_location 0 data1}
+		    }
+
+		    variant {
+		    } {
+			member {
+			    {type :$one_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+
+		    variant {
+			{discr_value 1 udata}
+		    } {
+			member {
+			    {type :$two_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+		}
+	    }
+
+	    structure_type {
+		{name Univariant}
+		{byte_size 8 DW_FORM_sdata}
+	    } {
+		variant_part {
+		} {
+		    variant {
+		    } {
+			member {
+			    {type :$one_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+		}
+	    }
+
+	    # Rust won't emit a negative discriminant like this, but
+	    # we want to test the code path anyway.
+	    structure_type {
+		{name Negative}
+		{byte_size 8 DW_FORM_sdata}
+	    } {
+		variant_part {
+		    {discr :$discr_3_label DW_FORM_ref4}
+		} {
+		    discr_3_label: member {
+			{type :$int8_label}
+			{data_member_location 0 data1}
+		    }
+
+		    variant {
+			{discr_value -1 sdata}
+		    } {
+			member {
+			    {type :$one_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+
+		    # Make this the default value so we'll see an
+		    # incorrect result if we mishandle signed
+		    # discriminants.
+		    variant {
+		    } {
+			member {
+			    {type :$two_label}
+			    {data_member_location 4 data1}
+			}
+		    }
+		}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] debug] } {
+    return -1
+}
+
+if ![runto func] {
+    return -1
+}
+
+# Get the values into history so we can use it from Rust.
+gdb_test "print (void *) buffer" "\\\$1 = .void .. $hex .buffer."
+gdb_test "print (void *) buffer2" "\\\$2 = .void .. $hex .buffer2."
+
+gdb_test "set language rust"
+gdb_test "print *(\$1 as *mut Simple)" " = One\\(23\\)" \
+    "print as Simple"
+gdb_test "print *(\$1 as *mut Defaulted)" " = One\\(23\\)" \
+    "print as Defaulted"
+gdb_test "print *(\$1 as *mut Univariant)" " = One\\(23\\)" \
+    "print as Univariant"
+
+gdb_test "print *(\$2 as *mut Negative)" " = One\\(23\\)" \
+    "print as Negative"