public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug c/116098] New: _Bool value from tagged union is incorrect when built with -O1
@ 2024-07-25 21:21 laria at laria dot me
  2024-07-25 21:46 ` [Bug tree-optimization/116098] [14/15 Regression] _Bool value from tagged union is incorrect when built with optimization pinskia at gcc dot gnu.org
                   ` (16 more replies)
  0 siblings, 17 replies; 18+ messages in thread
From: laria at laria dot me @ 2024-07-25 21:21 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116098

            Bug ID: 116098
           Summary: _Bool value from tagged union is incorrect when built
                    with -O1
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: laria at laria dot me
  Target Milestone: ---

Created attachment 58762
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58762&action=edit
preprocessed test-O1.i

I have encountered a weird behavior that seems like a bug in GCC to me.

I have a tagged union representing either a number (int) or a boolean (_Bool)
and a function that determines the "truthyness" of the value (true for all
numbers, the boolean value for booleans).

When I use this on a number and then negate the result, I get the wrong result,
when I compile this with -O1, but the expected result when compiling with -O0
or -O2.

Additionally when I return from main with `return res ? 0 : 1;`, I get neither
0
nor 1 as the exit code, but I instead get 3. (Also only with -O1).

======= BEGIN test-O1.c =======
int puts(const char *);

struct Value {
    enum ValueType {
        VALUE_BOOLEAN,
        VALUE_NUM,
    } type;
    union {
        _Bool boolean;
        int num;
        void *blank[2];
    };
};

static struct Value s_value;
static _Bool s_b;

_Bool
truthy(void)
{
    struct Value value = s_value;
    if (s_b) s_b = 0;
    // Will not reproduce when using an if or a ternary ?: instead.
    switch (value.type) {
    case VALUE_BOOLEAN:
        return value.boolean;
    default:
        return 1;
    }
}

int
main(void)
{
    s_b = 0;
    s_value = (struct Value) {
        .type = VALUE_NUM,

        // Seems to "work" with any value >= 2.
        // Tweak this number to get different return exit codes.
        .num = 2,
    };
    s_value = (struct Value) {
        .type = VALUE_BOOLEAN,
        .boolean = !truthy(), // truthy should be 1, so .boolean=0
    };
    _Bool b = truthy();
    puts(b ? "true" : "false"); // Should print "false", prints "true" instead
    return b ? 0 : 1; // Should return 1, returns 3 instead
}
======= END test-O1.c =======

As far as I can tell, I don't rely on any undefined behavior here. I only
retrieve the value from the union with the member I used to write to it the
last
time, which I make sure of using the stored tag / type.

The preprocessed *.i file for this is also attached.

The full compilation command is:

gcc -Wall -Werror -Wextra -pedantic -std=c11 -fno-strict-aliasing -fwrapv \
    -fno-aggressive-loop-optimizations -O1 test-O1.c

There are no compiler warnings / errors. The compiler exits successfully.

I'm running GCC on x86_64; GNU/Linux (6.9.8-200.fc40.x86_64; Fedora 40)

Compiling with -fsanitize=undefined does not produce runtime errors, but the
bug
then goes away.

Strangely enough a slightly less reduced source shows the same behavior, but
now
it only happens with -O2, not with -O1/-O0:

======= BEGIN test-O2.c =======
int puts(const char *);

struct Value {
    enum ValueType {
        VALUE_BOOLEAN,
        VALUE_NUM,
    } type;
    union {
        _Bool boolean;
        int num;
        void *blank[2];
    };
};

static struct Value s_value;
static _Bool s_b;

static void
val_set(struct Value value)
{
    s_b = 0;
    s_value = value;
}

static struct Value
val_get(void)
{
    struct Value value = s_value;
    if (s_b) s_b = 0;
    return value;
}

static _Bool
truthy(void)
{
    struct Value value = val_get();
    // Will not reproduce when using an if or a ternary ?: instead.
    switch (value.type) {
    case VALUE_BOOLEAN:
        return value.boolean;
    default:
        return 1;
    }
}

int
main(void)
{
    s_b = 0;
    val_set((struct Value) {
        .type = VALUE_NUM,

        // Seems to "work" with any value >= 2.
        // Tweak this number to get different return exit codes.
        .num = 2,
    });
    _Bool b1 = truthy(); // truthy should be 1
    val_set((struct Value) {
        .type = VALUE_BOOLEAN,
        .boolean = !b1, // SHould be 0
    });
    _Bool b2 = truthy();
    puts(b2 ? "true" : "false"); // Should print "false", prints "true" instead
    return b2 ? 0 : 1; // Should return 1, returns 3 instead
}
======= END test-O2.c =======

I have tested this with ...

- ... the GCC that came on my Fedora 40 system: 14.1.1 20240701
      (Red Hat 14.1.1-7)
- ... GCC built from the release tarball: 14.1.0 (GCC)
- ... GCC built from the current git trunk (commit 679086172b84be):
      15.0.0 20240724

All these versions show this behavior.

The bug however does *not* appear in gcc 13.3.1 20240522 (Red Hat 13.3.1-1) on.

-------------------------

Here are the gcc -v outputs:

14.1.1 installed on my system (Fedora 40):

    Using built-in specs.
    COLLECT_GCC=/usr/bin/gcc
    COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/14/lto-wrapper
    OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
    OFFLOAD_TARGET_DEFAULT=1
    Target: x86_64-redhat-linux
    Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,m2,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --enable-libstdcxx-backtrace
--with-libstdcxx-zoneinfo=/usr/share/zoneinfo --with-linker-hash-style=gnu
--enable-plugin --enable-initfini-array
--with-isl=/builddir/build/BUILD/gcc-14.1.1-20240701/obj-x86_64-redhat-linux/isl-install
--enable-offload-targets=nvptx-none,amdgcn-amdhsa --enable-offload-defaulted
--without-cuda-driver --enable-gnu-indirect-function --enable-cet
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
--with-build-config=bootstrap-lto --enable-link-serialization=1
    Thread model: posix
    Supported LTO compression algorithms: zlib zstd
    gcc version 14.1.1 20240701 (Red Hat 14.1.1-7) (GCC)

GCC 14.1.0 built from source

    Using built-in specs.
    COLLECT_GCC=/home/laria/local/gcc/usr/local/bin/gcc
   
COLLECT_LTO_WRAPPER=/home/laria/local/gcc/usr/local/bin/../libexec/gcc/x86_64-pc-linux-gnu/14.1.0/lto-wrapper
    Target: x86_64-pc-linux-gnu
    Configured with: ./configure --disable-multilib
    Thread model: posix
    Supported LTO compression algorithms: zlib zstd
    gcc version 14.1.0 (GCC)

Current (well, yesterday's :) ) git trunk
(679086172b84be18c55fdbb9cda7e97806e7c083)

    Using built-in specs.
    COLLECT_GCC=/home/laria/src/gcc/build/install/usr/local/bin/gcc
   
COLLECT_LTO_WRAPPER=/home/laria/src/gcc/build/install/usr/local/bin/../libexec/gcc/x86_64-pc-linux-gnu/15.0.0/lto-wrapper
    Target: x86_64-pc-linux-gnu
    Configured with: ../configure --disable-multilib --enable-languages=c
    Thread model: posix
    Supported LTO compression algorithms: zlib zstd
    gcc version 15.0.0 20240724 (experimental) (GCC)

^ permalink raw reply	[flat|nested] 18+ messages in thread
end of thread, other threads:[~2024-09-06 19:56 UTC | newest]

Thread overview: 18+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2024-07-25 21:21 [Bug c/116098] New: _Bool value from tagged union is incorrect when built with -O1 laria at laria dot me
2024-07-25 21:46 ` [Bug tree-optimization/116098] [14/15 Regression] _Bool value from tagged union is incorrect when built with optimization pinskia at gcc dot gnu.org
2024-07-25 21:50 ` sjames at gcc dot gnu.org
2024-07-25 21:56 ` pinskia at gcc dot gnu.org
2024-07-26  3:50 ` pinskia at gcc dot gnu.org
2024-07-26  4:31 ` pinskia at gcc dot gnu.org
2024-07-26  6:04 ` rguenth at gcc dot gnu.org
2024-07-26  6:07 ` pinskia at gcc dot gnu.org
2024-07-26  6:08 ` rguenth at gcc dot gnu.org
2024-07-27  0:38 ` [Bug tree-optimization/116098] [14/15 Regression] _Bool value from tagged union is incorrect when built with optimization since r14-1597-g64d90d06d2db43 pinskia at gcc dot gnu.org
2024-07-27 11:01 ` rguenther at suse dot de
2024-08-01  9:41 ` jakub at gcc dot gnu.org
2024-08-30 16:28 ` pinskia at gcc dot gnu.org
2024-08-30 21:54 ` pinskia at gcc dot gnu.org
2024-08-31 16:17 ` cvs-commit at gcc dot gnu.org
2024-08-31 16:18 ` [Bug tree-optimization/116098] [14 " pinskia at gcc dot gnu.org
2024-09-06 19:48 ` laria at laria dot me
2024-09-06 19:56 ` [Bug tree-optimization/116098] [14/15 " pinskia at gcc dot gnu.org

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).