[gdb/symtab] Prefer var def over decl

Consider the DWARF as generated by gcc with the tentative patch to fix gcc
PR91507 - "wrong debug for completed array with previous incomplete
declaration":
...
 <1><f4>: Abbrev Number: 2 (DW_TAG_array_type)
    <f5>   DW_AT_type        : <0xff>
    <f9>   DW_AT_sibling     : <0xff>
 <2><fd>: Abbrev Number: 3 (DW_TAG_subrange_type)
 <2><fe>: Abbrev Number: 0
 <1><ff>: Abbrev Number: 4 (DW_TAG_pointer_type)
    <100>   DW_AT_byte_size   : 8
    <101>   DW_AT_type        : <0x105>
 <1><105>: Abbrev Number: 5 (DW_TAG_base_type)
    <106>   DW_AT_byte_size   : 1
    <107>   DW_AT_encoding    : 6       (signed char)
    <108>   DW_AT_name        : (indirect string, offset: 0x19f): char
 <1><10c>: Abbrev Number: 6 (DW_TAG_variable)
    <10d>   DW_AT_name        : zzz
    <111>   DW_AT_decl_file   : 1
    <112>   DW_AT_decl_line   : 1
    <113>   DW_AT_decl_column : 14
    <114>   DW_AT_type        : <0xf4>
    <118>   DW_AT_external    : 1
    <118>   DW_AT_declaration : 1
 <1><118>: Abbrev Number: 2 (DW_TAG_array_type)
    <119>   DW_AT_type        : <0xff>
    <11d>   DW_AT_sibling     : <0x128>
 <1><12f>: Abbrev Number: 8 (DW_TAG_variable)
    <130>   DW_AT_specification: <0x10c>
    <134>   DW_AT_decl_line   : 2
    <135>   DW_AT_decl_column : 7
    <136>   DW_AT_type        : <0x118>
    <13a>   DW_AT_location    : 9 byte block: 3 30 10 60 0 0 0 0 0      (DW_OP_addr: 601030)
...

The DWARF will result in two entries in the symbol table, a decl with type
char *[] and a def with type char*[2].

When trying to print the value of zzz:
...
$ gdb a.spec.out -batch -ex "p zzz"
...
the decl (rather than the def) will be found in the symbol table, which is
missing the location information, and consequently we get:
...
$1 = 0x601030 <zzz>
...

[ There is a fallback mechanism that finds the address of the variable in the
minimal symbol table, but that's not used here, because the type of the decl
does not specify a size.  We could use the symbol size here to get the size
of the type, but that's currently not done: PR exp/24989.  Still, fixing that
PR would not fix the generic case, where minimal symbol info is not
available. ]

Fix this by preferring defs over decls when searching in the symbol table.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2019-12-06  Tom de Vries  <tdevries@suse.de>

	PR symtab/24971
	* block.c (best_symbol, better_symbol): New function.
	(block_lookup_symbol_primary, block_lookup_symbol): Prefer def over
	decl.

gdb/testsuite/ChangeLog:

2019-12-06  Tom de Vries  <tdevries@suse.de>

	* gdb.dwarf2/varval.exp: Add decl before def test.

Change-Id: Id92326cb8ef9903b121ef9e320658eb565d0f5a9

1 files changed, 70 insertions, 4 deletions

diff --git a/gdb/block.c b/gdb/block.c
index a4592e3..b49c548 100644
--- a/gdb/block.c
+++ b/gdb/block.c
@@ -657,6 +657,43 @@ block_iter_match_next (const lookup_name_info &name,
   return block_iter_match_step (iterator, name, 0);
 }
 
+/* Return true if symbol A is the best match possible for DOMAIN.  */
+
+static bool
+best_symbol (struct symbol *a, const domain_enum domain)
+{
+  return (SYMBOL_DOMAIN (a) == domain
+	  && SYMBOL_CLASS (a) != LOC_UNRESOLVED);
+}
+
+/* Return symbol B if it is a better match than symbol A for DOMAIN.
+   Otherwise return A.  */
+
+static struct symbol *
+better_symbol (struct symbol *a, struct symbol *b, const domain_enum domain)
+{
+  if (a == NULL)
+    return b;
+  if (b == NULL)
+    return a;
+
+  if (SYMBOL_DOMAIN (a) == domain
+      && SYMBOL_DOMAIN (b) != domain)
+    return a;
+  if (SYMBOL_DOMAIN (b) == domain
+      && SYMBOL_DOMAIN (a) != domain)
+    return b;
+
+  if (SYMBOL_CLASS (a) != LOC_UNRESOLVED
+      && SYMBOL_CLASS (b) == LOC_UNRESOLVED)
+    return a;
+  if (SYMBOL_CLASS (b) != LOC_UNRESOLVED
+      && SYMBOL_CLASS (a) == LOC_UNRESOLVED)
+    return b;
+
+  return a;
+}
+
 /* See block.h.
 
    Note that if NAME is the demangled form of a C++ symbol, we will fail
@@ -684,7 +721,9 @@ block_lookup_symbol (const struct block *block, const char *name,
 
       ALL_BLOCK_SYMBOLS_WITH_NAME (block, lookup_name, iter, sym)
 	{
-	  if (SYMBOL_DOMAIN (sym) == domain)
+	  /* See comment related to PR gcc/debug/91507 in
+	     block_lookup_symbol_primary.  */
+	  if (best_symbol (sym, domain))
 	    return sym;
 	  /* This is a bit of a hack, but symbol_matches_domain might ignore
 	     STRUCT vs VAR domain symbols.  So if a matching symbol is found,
@@ -692,7 +731,7 @@ block_lookup_symbol (const struct block *block, const char *name,
 	     exactly the same domain.  PR 16253.  */
 	  if (symbol_matches_domain (SYMBOL_LANGUAGE (sym),
 				     SYMBOL_DOMAIN (sym), domain))
-	    other = sym;
+	    other = better_symbol (other, sym, domain);
 	}
       return other;
     }
@@ -746,7 +785,34 @@ block_lookup_symbol_primary (const struct block *block, const char *name,
        sym != NULL;
        sym = mdict_iter_match_next (lookup_name, &mdict_iter))
     {
-      if (SYMBOL_DOMAIN (sym) == domain)
+      /* With the fix for PR gcc/debug/91507, we get for:
+	 ...
+	 extern char *zzz[];
+	 char *zzz[ ] = {
+	   "abc",
+	   "cde"
+	 };
+	 ...
+	 DWARF which will result in two entries in the symbol table, a decl
+	 with type char *[] and a def with type char *[2].
+
+	 If we return the decl here, we don't get the value of zzz:
+	 ...
+	 $ gdb a.spec.out -batch -ex "p zzz"
+	 $1 = 0x601030 <zzz>
+	 ...
+	 because we're returning the symbol without location information, and
+	 because the fallback that uses the address from the minimal symbols
+	 doesn't work either because the type of the decl does not specify a
+	 size.
+
+	 To fix this, we prefer def over decl in best_symbol and
+	 better_symbol.
+
+	 In absence of the gcc fix, both def and decl have type char *[], so
+	 the only option to make this work is improve the fallback to use the
+	 size of the minimal symbol.  Filed as PR exp/24989.  */
+      if (best_symbol (sym, domain))
 	return sym;
 
       /* This is a bit of a hack, but symbol_matches_domain might ignore
@@ -755,7 +821,7 @@ block_lookup_symbol_primary (const struct block *block, const char *name,
 	 exactly the same domain.  PR 16253.  */
       if (symbol_matches_domain (SYMBOL_LANGUAGE (sym),
 				 SYMBOL_DOMAIN (sym), domain))
-	other = sym;
+	other = better_symbol (other, sym, domain);
     }
 
   return other;