Invalid type for bit-field smaller than 32 bits

[llvmbot]

Bugzilla Link	43439
Resolution	INVALID
Resolved on	Sep 25, 2019 23:21
Version	9.0
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid

Extended Description

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 #&#8203;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: /opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0
Selected GCC installation: /opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0
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 /opt/compiler-explorer/clang-9.0.0/lib/clang/9.0.0/include -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

clang -cc1 version 9.0.0 based upon LLVM 9.0.0 default target x86_64-unknown-linux-gnu

ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/opt/compiler-explorer/clang-9.0.0/lib/clang/9.0.0/include
/usr/include/x86_64-linux-gnu
/usr/include

End of search list.

: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 6.3.1.1 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.

[llvmbot]

Not related to integer promotion.