Speed up psymbol reading by removing a copy

I noticed that cp_canonicalize_string and friends copy a
unique_xmalloc_ptr to a std::string.  However, this copy isn't
genuinely needed anywhere, and it serves to slow down DWARF psymbol
reading.

This patch removes the copy and updates the callers to adapt.

This speeds up the reader from 1.906 seconds (mean of 10 runs, of gdb
on a copy of itself) to 1.888 seconds (mean of 10 runs, on the same
copy as the first trial).

gdb/ChangeLog
2020-05-08  Tom Tromey  <tom@tromey.com>

	* symtab.h (class demangle_result_storage) <set_malloc_ptr>: New
	overload.
	<swap_string, m_string>: Remove.
	* symtab.c (demangle_for_lookup, completion_list_add_symbol):
	Update.
	* stabsread.c (define_symbol, read_type): Update.
	* linespec.c (find_linespec_symbols): Update.
	* gnu-v3-abi.c (gnuv3_get_typeid): Update.
	* dwarf2/read.c (dwarf2_canonicalize_name): Update.
	* dbxread.c (read_dbx_symtab): Update.
	* cp-support.h (cp_canonicalize_string_full)
	(cp_canonicalize_string, cp_canonicalize_string_no_typedefs):
	Return unique_xmalloc_ptr.
	* cp-support.c (inspect_type): Update.
	(cp_canonicalize_string_full): Return unique_xmalloc_ptr.
	(cp_canonicalize_string_no_typedefs, cp_canonicalize_string):
	Likewise.
	* c-typeprint.c (print_name_maybe_canonical): Update.
	* break-catch-throw.c (check_status_exception_catchpoint):
	Update.

1 files changed, 7 insertions, 10 deletions

diff --git a/gdb/symtab.h b/gdb/symtab.h
index 5c5db0faba..764c567a90 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -2299,20 +2299,18 @@ bool iterate_over_symbols_terminated
 /* Storage type used by demangle_for_lookup.  demangle_for_lookup
    either returns a const char * pointer that points to either of the
    fields of this type, or a pointer to the input NAME.  This is done
-   this way because the underlying functions that demangle_for_lookup
-   calls either return a std::string (e.g., cp_canonicalize_string) or
-   a malloc'ed buffer (libiberty's demangled), and we want to avoid
-   unnecessary reallocation/string copying.  */
+   this way to avoid depending on the precise details of the storage
+   for the string.  */
 class demangle_result_storage
 {
 public:
 
-  /* Swap the std::string storage with STR, and return a pointer to
-     the beginning of the new string.  */
-  const char *swap_string (std::string &str)
+  /* Swap the malloc storage to STR, and return a pointer to the
+     beginning of the new string.  */
+  const char *set_malloc_ptr (gdb::unique_xmalloc_ptr<char> &&str)
   {
-    std::swap (m_string, str);
-    return m_string.c_str ();
+    m_malloc = std::move (str);
+    return m_malloc.get ();
   }
 
   /* Set the malloc storage to now point at PTR.  Any previous malloc
@@ -2326,7 +2324,6 @@ public:
 private:
 
   /* The storage.  */
-  std::string m_string;
   gdb::unique_xmalloc_ptr<char> m_malloc;
 };