[llvm-bugs] [Bug 43439] New: Invalid type for bit-field smaller than 32 bits

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Sep 25 00:46:13 PDT 2019


            Bug ID: 43439
           Summary: Invalid type for bit-field smaller than 32 bits
           Product: clang
           Version: 9.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: alex_lop at walla.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Here is the reproduction of the bug: https://godbolt.org/z/_IWIqC

The compilation fails on the static_assert, while it shouldn't based on the C11
standard (see quoting below in this post).

The code example below was compiled with the following options:  -O3 -std=c11
-Wall -Wextra -v

#include <stdint.h>
#include <assert.h>

#define CHECK_NUM(num)    _Generic((num),               \
                                    uint64_t : 0,       \
                                    int64_t : 0,        \
                                    default : 1)
int main(void)
    struct _s
        uint64_t        val : 1;
    } s = {0};

    static_assert(CHECK_NUM(s.val), "Variable too large #1"); // <strong>Should
not fail here</strong>

    return 0;

The compilation output:

clang version 9.0.0 (tags/RELEASE_900/final 372344)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/compiler-explorer/clang-9.0.0/bin
Found candidate GCC installation:
Selected GCC installation:
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64

 "/opt/compiler-explorer/clang-9.0.0/bin/clang-9" -cc1 -triple
x86_64-unknown-linux-gnu -S -disable-free -disable-llvm-verifier
-discard-value-names -main-file-name example.c -mrelocation-model static
-mthread-model posix -fmath-errno -masm-verbose -mconstructor-aliases
-munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info
-debug-info-kind=limited -dwarf-version=4 -debugger-tuning=gdb
-momit-leaf-frame-pointer -v -coverage-notes-file /home/ubuntu/./output.gcno
-resource-dir /opt/compiler-explorer/clang-9.0.0/lib/clang/9.0.0
-internal-isystem /usr/local/include -internal-isystem
-internal-externc-isystem /usr/include/x86_64-linux-gnu
-internal-externc-isystem /include -internal-externc-isystem /usr/include -O3
-Wall -Wextra -std=c11 -fdebug-compilation-dir /home/ubuntu -ferror-limit 19
-fmessage-length 0 -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -vectorize-loops -vectorize-slp -mllvm
--x86-asm-syntax=intel -faddrsig -o ./output.s -x c <source>

clang -cc1 version 9.0.0 based upon LLVM 9.0.0 default target

ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:

End of search list.

<source>:15:5: error: static_assert failed "Variable too large #1"

    static_assert(CHECK_NUM(s.val), "Variable too large #1"); // Should not
fail here

    ^             ~~~~~~~~~~~~~~~~

/usr/include/assert.h:143:24: note: expanded from macro 'static_assert'
# define static_assert _Static_assert


1 error generated.
Compiler returned: 1


Based on the C11 standard section Boolean, characters and integers :

If an int can represent all values of the original type (as restricted by the
width, for a bit-field), the value is converted to an int; otherwise, it is
converted to an unsigned int. These are called the integer promotions.) All
other types are unchanged by the integer promotions.

I would expect _Generic((s.val), ...) to return 1 for int in this case.

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