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.ploujni...@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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