Valgrind '--show-leak-kinds=all'

Thomas Schwinge thomas@codesourcery.com
Tue Aug 17 13:00:37 GMT 2021


Hi!

On 2021-08-06T17:10:36+0200, Richard Biener <richard.guenther@gmail.com> wrote:
> On August 6, 2021 4:09:37 PM GMT+02:00, Thomas Schwinge <thomas@codesourcery.com> wrote:
>>I'm working on plugging a memory leak in an entirely different
>>compartment of GCC, but also ran into this issue:
>>
>>On 2021-02-12T08:35:52+0100, Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>>> On Thu, Feb 11, 2021 at 7:35 PM Martin Sebor <msebor@gmail.com> wrote:
>>>> On 2/11/21 12:59 AM, Richard Biener wrote:
>>>> > On Wed, Feb 10, 2021 at 6:16 PM Martin Sebor <msebor@gmail.com> wrote:
>>>> >> [...] Valgrind shows more leaks in this code that
>>>> >> I'm not sure what to do about:
>>>> >>
>>>> >> 1) A tree built by build_type_attribute_qual_variant() called from
>>>> >>      attr_access::array_as_string() to build a temporary type only
>>>> >>      for the purposes of formatting it.
>>>> >>
>>>> >> 2) A tree (an attribute list) built by tree_cons() called from
>>>> >>      build_attr_access_from_parms() that's used only for the duration
>>>> >>      of the caller.
>>>> >>
>>>> >> Do these temporary trees need to be released somehow or are the leaks
>>>> >> expected?
>>>> >
>>>> > You should configure GCC with --enable-valgrind-annotations to make
>>>> > it aware of our GC.
>>>>
>>>> I did configure with that option:
>>>>
>>>> $ /src/gcc/master/configure --enable-checking=yes
>>>> --enable-languages=all,jit,lto --enable-host-shared
>>>> --enable-valgrind-annotations
>>
>>>> $ /build/gcc-master/gcc/xgcc -B /build/gcc-master/gcc -S -Wall
>>>> /src/gcc/master/gcc/testsuite/gcc.dg/Wvla-parameter.c -wrapper
>>>> valgrind,--leak-check=full,--show-leak-kinds=all,--track-origins=yes,--log-file=valgrind-out.txt
>>>>
>>>> Do you not see the same leaks?
>>
>>I do; also stuff like:
>>
>>    56 bytes in 1 blocks are still reachable in loss record 152 of 875
>>       at 0x483DD99: calloc (vg_replace_malloc.c:762)
>>       by 0x1753240: xcalloc (xmalloc.c:162)
>>       by 0x669C83: ggc_internal_alloc(unsigned long, void (*)(void*), unsigned long, unsigned long) (ggc-page.c:918)
>>       by 0x89E07D: ggc_internal_cleared_alloc(unsigned long, void (*)(void*), unsigned long, unsigned long) (ggc-common.c:117)
>>       by 0xF65D0D: make_node(tree_code) (ggc.h:143)
>>       by 0xF6632B: build_decl(unsigned int, tree_code, tree_node*, tree_node*) (tree.c:5264)
>>       by 0xA28ADC: build_builtin_function(unsigned int, char const*, tree_node*, int, built_in_class, char const*, tree_node*) (langhooks.c:681)
>>       by 0xA29FDD: add_builtin_function(char const*, tree_node*, int, built_in_class, char const*, tree_node*) (langhooks.c:716)
>>       by 0x622BFB: def_builtin_1(built_in_function, char const*, built_in_class, tree_node*, tree_node*, bool, bool, bool, tree_node*, bool) [clone .constprop.25] (lto-lang.c:650)
>>       by 0x640709: lto_define_builtins(tree_node*, tree_node*) (omp-builtins.def:46)
>>       by 0x641EE3: lto_init() (lto-lang.c:1339)
>>       by 0x61E26A: toplev::main(int, char**) (toplev.c:1921)
>>
>>... and many, many more.
>>
>>> Err, well.  --show-leak-kinds=all is probably the cause.
>>
>>Before finding this email, I too had convinced myself that everying that
>>came by 'ggc_*' I may ignore, because:
>>
>>> We
>>> definitely do not force-release
>>> all reachable GC allocated memory at program end.
>>
>>... of this: these blocks simply had not been GCed at program end.
>>
>>It's however a bit tedious to filter, in my case, 11864 lines of Valgrind
>>output.

(Actually, might use something like the "mitigated as follows" that I've
added here: <https://gcc.gnu.org/wiki/DebuggingGCC#Valgrind>.)

>>> Not sure if
>>> valgrind annotations can
>>> make that obvious to valgrind.
>>
>>Or, if that's not feasible (I don't know much about Valgrind...), then
>>instead would it help to force a final GC at program end if we're running
>>in "valgrind mode"?  If that's a plausible thing to do, would guarding
>>that by GCC having been configured with '--enable-valgrind-annotations'
>>be OK, or do we need a '--param', or something else?
>
> Well, instead of a final GC we could explicitly release all GC managed memory.

Heh, of course, a "final GC at program end" doesn't help (much), given
that (most of) all the blocks are still reachable via the usual GC roots.

So I tried looking into how we might release all GCC memory
unconditionally, via adapting 'ggc_mark_roots' (to not add back roots via
'ggc_mark_root_tab'), 'clear_marks', 'sweep_pages', 'release_pages',
etc., but couldn't get this to work.  It doesn't help, of course, that I
don't know much about how the GC really works internally.  Possibly my
non-understanding of the "context depth" is highly relevant.

Anyway, this isn't really important for me right now, having otherwise
resolve my original issue, so I'm not intending to spend a lot more time
on this.


Calling 'memory_block_pool::trim (0);' at the end of 'gcc/main.c:main'
does have some effect, too, but isn't sufficient/useful on its own, of
course.


Grüße
 Thomas
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