[RFC 2/2] [gdb/symtab] Don't expand type units for breakpoints

[Tom de Vries <tdevries@suse.de>]

Consider a cc1plus exec compiled with GCC and LTO [1].

If we try to set a breakpoint on do_rpo_vn:
...
$ gdb -q -batch cc1plus \
    -ex "b do_rpo_vn"
Breakpoint 1 at 0xfd75f0: do_rpo_vn. (2 locations)
...
we expand 435 units:
...
$ gdb -q -batch cc1plus \
    -ex "b do_rpo_vn" \
    -ex "maint print statistics" \
  | grep "Number of read units"
  Number of read units: 435
...

[ When compiling with LTO, GCC generates debug info in two phases:
- in the early phase, it generates a unit for each source file, containing the
  types in the source file.  Let's call these type-only units.
- in the late phase, it generates a so-called artificial unit for each
  optimized code blob.  The artificial unit doesn't contains types, but
  imports them from the type-only units.

The cc1plus exec contains 1150 units, 128 of which are artificials units:
...
$ readelf -wi --dwarf-depth=1 cc1plus | grep -c "Compilation Unit @"
1150
$ readelf -wi --dwarf-depth=1 cc1plus | grep -c "<artificial>"
128
... ]

Most of the expanded units are type-only units, which have no relevant
information when we're looking for function symbols.

They're merely expanded because the artificial units import them.

So the question is: can we skip expanding the type-only units when looking for
function symbols?

This patch contains a naive implementation of that approach, in process_queue.

Using it, we expand just 7 artificial units:
...
$ gdb -q -batch cc1plus \
    -ex "b do_rpo_vn" \
    -ex "maint print statistics" \
  | grep "Number of read units"
  Number of read units: 7
...

But there's a problem with this implementation, which we can illustrate using
DWZ:
...
$ cat test.c
struct s {
  int x0, x1, x2, x3, x4, x5, x6, x7, x8, x9;
};
struct s var1;
int main (void) {
  return 0;
}
$ gcc -g test.c; rm -f ab.dwz; cp a.out b.out; dwz -m ab.dwz a.out b.out
$ gdb -q -batch a.out -ex "b main" -ex "ptype struct s"
Breakpoint 1 at 0x40111a: file test.c, line 6.
No struct type named s.
...

First, we use DWZ to move struct s to a partial unit.

Then in the GDB session the following happens:
- setting a breakpoint on main expands the test.c CU
- the test.c CU has an import of the partial CU, but the partial CU is not
  expanded, because it contains only types
- printing struct s attempts to expand test.c CU, finds that it's already
  expanded, and the partial CU containing struct s remains unexpanded.
- gdb tries to find struct s in the expanded CUs, and struct s is not found.

RFC: what is the best way to fix this problem?

I wondered about expanding function compunit_symtab_set_p et al with a
domain_search_flags parameter, giving us a way to express that a CU is
expanded for SEARCH_FUNCTION_DOMAIN, but not for other domains.

But there may be a better way to achieve this.

[ FWIW, I've tested this on x86_64-linux, and ran into a number of FAILs:
...
      9 gdb.cp/cpexprs-debug-types.exp:
      1 gdb.dlang/circular.exp:
      2 gdb.dwarf2/ada-cold-name.exp:
      4 gdb.dwarf2/ada-linkage-name.exp:
     14 gdb.dwarf2/dw2-bad-abstract-origin.exp:
      1 gdb.dwarf2/dw2-inline-bt.exp:
      1 gdb.dwarf2/dw2-prologue-end-2.exp:
      1 gdb.dwarf2/dw2-ranges-psym.exp:
      7 gdb.dwarf2/dw2-step-between-different-inline-functions.exp:
      6 gdb.dwarf2/dw2-step-between-inline-func-blocks.exp:
     60 gdb.dwarf2/dw2-unexpected-entry-pc.exp:
      1 gdb.dwarf2/inlined_subroutine-inheritance.exp:
      1 gdb.dwarf2/struct-decl.exp:
... ]

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33922
---
 gdb/dwarf2/read.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index afd0b60006e..475dc03f0b0 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -1945,7 +1945,8 @@ dwarf2_base_index_functions::search_one
 
   compunit_symtab *symtab
     = dw2_instantiate_symtab (per_cu, per_objfile, false, domain);
-  gdb_assert (symtab != nullptr);
+  if (symtab == nullptr)
+    return true;
 
   if (listener != nullptr)
     {
@@ -3908,6 +3909,15 @@ process_queue (dwarf2_per_objfile *per_objfile, domain_search_flags domain)
 
       gdb_assert (!per_objfile->compunit_symtab_set_p (per_cu));
 
+      bool skip = (domain == SEARCH_FUNCTION_DOMAIN
+		   && !per_cu->addresses_seen);
+      if (skip)
+	{
+	  cu->queued = false;
+	  per_objfile->queue->pop ();
+	  continue;
+	}
+
       namespace chr = std::chrono;
 
       unsigned int debug_print_threshold;
-- 
2.51.0