Problems with GGC and bitmap/hash_set

[Michael Ploujnikov <michael dot ploujnikov at oracle dot com>]

Hi,

I've been doing some investigations that required using a bitmap to keep track of decl IDs and I ran into segmentation fault after my bitmap has been loaded from a PCH. Attached is a short patch that can reliably reproduce my problem. Looks like head->current is set to point to uninitialized memory for some reason. Could someone with a strong understanding of GGC take a look and let me know what I might be doing wrong and how to fix this?

I can reliably reproduce this with the following command:

gdb --args /gcc/build/./gcc/cc1plus -nostdinc++ -nostdinc++ -I /gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu -I /gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include -I /gcc/src/libstdc++-v3/libsupc++ -I /gcc/src/libstdc++-v3/include/backward -I /gcc/src/libstdc++-v3/testsuite/util -iprefix /gcc/build/gcc/../lib/gcc/x86_64-pc-linux-gnu/9.0.0/ -isystem /gcc/build/./gcc/include -isystem /gcc/build/./gcc/include-fixed -D_GNU_SOURCE -D _GNU_SOURCE -D LOCALEDIR=. -isystem /gcc-install/x86_64-pc-linux-gnu/include -isystem /gcc-install/x86_64-pc-linux-gnu/sys-include /gcc/src/libstdc++-v3/testsuite/17_intro/headers/c++1998/all_c++200x_compatibility.cc -quiet -dumpbase all_c++200x_compatibility.cc -mtune=generic -march=x86-64 -auxbase-strip all_c++200x_compatibility.s -g -O2 -Wc++11-compat -Werror -version -fdiagnostics-color=never -fchecking=1 -fmessage-length=0 -fno-show-column -ffunction-sections -fdata-sections -fno-diagnostics-show-caret -o all_c++200x_compatibility.s

Here's a sample GDB session:

Breakpoint 2, c_common_read_pch (pfile=0x31eafa0, 
    name=0x31fc950 "/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/extc++.h.gch/O2g.gch", fd=9, 
    orig_name=0x31ff0e0 "/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/extc++.h") at ../../src/gcc/c-family/c-pch.c:348
348       timevar_push (TV_PCH_RESTORE); 
(gdb) enable 1
(gdb) c
Continuing.

Breakpoint 1, allocate_decl_uid () at ../../src/gcc/tree.c:1024
1024      if (!all_ids)
(gdb) p *all_ids
$8 = {static crashme = {elements = 0x0, heads = 0x0, obstack = {chunk_size = 0, chunk = 0x0, 
      object_base = 0x0, next_free = 0x0, chunk_limit = 0x0, temp = {i = 0, p = 0x0}, 
      alignment_mask = 0, chunkfun = {plain = 0x0, extra = 0x0}, freefun = {plain = 0x0, 
        extra = 0x0}, extra_arg = 0x0, use_extra_arg = 0, maybe_empty_object = 0, 
      alloc_failed = 0}}, indx = 1790, tree_form = true, first = 0x1002dd9950, 
  current = 0x7fdf0f865370, obstack = 0x0}
(gdb) n
1029      gcc_assert (all_ids->tree_form);
(gdb) n
1030      int new_id = next_decl_uid++;
(gdb) n
1031      gcc_assert (!bitmap_bit_p (all_ids, new_id));
(gdb) s
bitmap_bit_p (head=0x10006cc3c0, bit=229123) at ../../src/gcc/bitmap.c:963
963       unsigned int indx = bit / BITMAP_ELEMENT_ALL_BITS;
(gdb) p indx
$10 = 32767
(gdb) s
968       if (!head->tree_form)
(gdb) s    
971         ptr = bitmap_tree_find_element (head, indx);
(gdb) s
bitmap_tree_find_element (head=0x10006cc3c0, indx=1790) at ../../src/gcc/bitmap.c:546
546       if (head->current == NULL
(gdb) p head->current
$11 = (bitmap_element *) 0x7fdf0f865370
(gdb) p *head->current
Cannot access memory at address 0x7fdf0f865370
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000000c4866b in bitmap_bit_p (head=0x10006cc3c0, bit=229123) at ../../src/gcc/bitmap.c:978
978       return (ptr->bits[word_num] >> bit_num) & 1;




Alternatively I tried using a hash_set to keep track of ids:

typedef int_hash <unsigned, UINT_MAX> unsigned_int_hasher;
static GTY(()) hash_set<unsigned_int_hasher> *all_ids;

But I couldn't figure out how to write the required gt_ggc_mx and gt_pch_nx methods.


Any help would be greatly appreciated!


- Michael

From 6b410e862017a677095ae094244c87ca1bbe72fd Mon Sep 17 00:00:00 2001
From: Michael Ploujnikov <michael.ploujnikov@oracle.com>
Date: Thu, 17 Jan 2019 13:01:01 -0500
Subject: [PATCH] Problem with restoring tree bitmap from PCH

---
 gcc/tree.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git gcc/tree.c gcc/tree.c
index 170ef1300e..70564c3b87 100644
--- gcc/tree.c
+++ gcc/tree.c
@@ -1014,12 +1014,23 @@ record_node_allocation_statistics (enum tree_code code, size_t length)
   tree_node_sizes[(int) kind] += length;
 }
 
+static GTY(()) bitmap all_ids;
+
 /* Allocate and return a new UID from the DECL_UID namespace.  */
 
 int
 allocate_decl_uid (void)
 {
-  return next_decl_uid++;
+  if (!all_ids)
+    {
+      all_ids = BITMAP_GGC_ALLOC ();
+      bitmap_tree_view (all_ids);
+    }
+  gcc_assert (all_ids->tree_form);
+  int new_id = next_decl_uid++;
+  gcc_assert (!bitmap_bit_p (all_ids, new_id));
+  bitmap_set_bit (all_ids, new_id);
+  return new_id;
 }
 
 /* Return a newly allocated node of code CODE.  For decl and type
-- 
2.19.1

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