Re: [libmicrohttpd] Build failure with Link-Time Optimization

[Sergey Sudnitsyn]

Here you go - but as I mentioned earlier - extracting the relevant pieces of the application is next to impossible - and I am not getting the same errors when running microhttpd's 

`make check`.

g++ -m64 -O3 -DNDEBUG -fdata-sections -ffunction-sections -s -flto -fPIE -fPIC -fstack-protector-strong -D_FORTIFY_SOURCE=2 -std=c++20 -Waddress -Waggressive-loop-optimizations -Wall -Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment -Wconversion -Wdisabled-optimization -Wdouble-promotion -Werror -Wextra -Wfatal-errors -Wfloat-equal -Wformat-nonliteral -Wformat-security -Wformat-y2k -Wformat=2 -Wimport -Winit-self -Wno-inline -Winvalid-pch -Wlogical-op -Wmissing-braces -Wmissing-declarations -Wmissing-field-initializers -Wmissing-format-attribute -Wmissing-noreturn -Woverlength-strings -Wparentheses -Wpedantic -Wpointer-arith -Wredundant-decls -Wreturn-type -Wsequence-point -Wshadow -Wsign-compare -Wsign-conversion -Wstack-protector -Wstrict-aliasing -Wstrict-aliasing=2 -Wswitch -Wswitch-default -Wswitch-enum -Wtrigraphs -Wundef -Wuninitialized -Wunknown-pragmas -Wunreachable-code -Wunsafe-loop-optimizations -Wunused -Wunused-function -Wunused-label -Wunused-parameter -Wunused-value -Wunused-variable -Wvariadic-macros -Wvolatile-register-var -Wwrite-strings -Wno-deprecated-declarations -Wc++11-compat -Wconversion-null -Wctor-dtor-privacy -Weffc++ -Winvalid-pch -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wsign-promo -Wstrict-null-sentinel -Wno-useless-cast -Wzero-as-null-pointer-constant -Wstack-protector -fstack-protector-all -Wno-non-virtual-dtor -Wno-reorder -Wno-sign-conversion -Wno-float-equal -Wno-old-style-cast -Wno-zero-as-null-pointer-constant -m64 -Wl,--gc-sections -s -flto -pie -z noexecstack -z relro -z now -fuse-ld=gold -s main.cpp.o -o bin/main myapplication_libs.a -lmicrohttpd -lcurl -lgnutls /usr/lib64/libz.so -lpthread -lgcrypt -lgpg-error -lhogweed -lnettle -lgmp

src/microhttpd/connection.c: In function ‘connection_alloc_memory’:
src/microhttpd/connection.c:281:10: error: ‘need_to_free’ may be used uninitialized [-Werror=maybe-uninitialized]
  281 |       if (c->read_buffer_size - c->read_buffer_offset >= need_to_free)
      |          ^
compilation terminated due to -Wfatal-errors.
lto1: all warnings being treated as errors

On Thu, 28 Dec 2023 at 06:26, Christian Grothoff <grothoff@gnunet.org> wrote:

I think it might be helpful if you could share the *exact* compiler
invocation (make V=1) and error message you are getting...

On 12/27/23 11:38, Sergey Sudnitsyn wrote:
> Thank you.
> I don't know what's so special about my setup - I am getting the same
> error with GCC 12 & 13 - compiling for both x86_64 & aarch64.
> Compiler flags - nothing unusual - -O3 -flto  - but maybe warnings are a
> bit overzealous (-Wuninitialized -Wstack-protector).
> I'll try producing a sample that reproduces this - and will use
> suggested workaround in the meantime.
>
> On Tue, 26 Dec 2023 at 17:21, Evgeny Grin <k2k@yandex.ru
> <mailto:k2k@yandex.ru>> wrote:
>
>     Hi Sergey,
>
>     Are you using some special compiler or linker parameters? Some special
>     compiler or linker versions?
>
>     I regularly test MHD with LTO. I've retested the current git master
>     with
>     LTO and cannot reproduce.
>     No errors, nor even warnings.
>
>     If you check the code you will find that both mentioned variables are
>     not used before initialisation. (Please provide the code path if I'm
>     wrong).
>
>     Unless variables are really used as uninitialised (which should be a
>     bug
>     and needs to be fixed), I'd avoid unneeded initialisation to save the
>     code size (and negligible performance).
>
>     If your compiler/linker complaint is false positive, you probably may
>     workaround it with '-ftrivial-auto-var-init' parameter.
>
>     Let me know the details, please.
>
>     --
>     Evgeny
>
>     On 26/12/2023 16:14, Sergey Sudnitsyn wrote:
>      > When LTO is enabled, linking fails complaining about potentially
>      > uninitialized variables;
>      > It's hard to provide a small sample reproducing this, but I hope the
>      > patch can be applied even without it since it's innocent enough
>     (diff
>      > against commit 8bb481f ):
>      >
>      > diff --git a/src/microhttpd/connection.c
>     b/src/microhttpd/connection.c
>      > index f9892a55..e5dd1a45 100644
>      > --- a/src/microhttpd/connection.c
>      > +++ b/src/microhttpd/connection.c
>      > @@ -653,7 +653,7 @@ MHD_connection_alloc_memory_ (struct
>     MHD_Connection
>      > *connection,
>      >   {
>      >     struct MHD_Connection *const c = connection; /* a short alias */
>      >     struct MemoryPool *const pool = c->pool; /* a short alias */
>      > - size_t need_to_be_freed; /**< The required amount of additional
>     free
>      > memory */
>      > + size_t need_to_be_freed = 0; /**< The required amount of
>     additional
>      > free memory */
>      >     void *res;
>      >
>      >     res = MHD_pool_try_alloc (pool, size, &need_to_be_freed);
>      > @@ -4468,7 +4468,7 @@ process_request_body (struct MHD_Connection
>      > *connection)
>      >         { /* Need the parse the chunk size line */
>      >           /** The number of found digits in the chunk size number */
>      >           size_t num_dig;
>      > - uint64_t chunk_size;
>      > + uint64_t chunk_size = 0;
>      >           bool broken;
>      >           bool overflow;
>      >
>