https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98572
Bug ID: 98572
Summary: Unexpected signed extension for unsigned bit fields
Product: gcc
Version: 8.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: vonchun at gmail dot com
Target Milestone: ---
Consider the following C code:
=============================================================================
#include <stdio.h>
#include <stdint.h>
//#pragma pack(1)
typedef struct {
unsigned bits24:24;
unsigned _:8;
} my_type_t;
int main()
{
uintptr_t p;
my_type_t t1;
t1.bits24 = 0xfffff;
p = t1.bits24 << 12;
printf("p=%llx\n", p);
uint32_t t2 = 0xfffff;
p = t2 << 12;
printf("p=%llx\n", p);
return 0;
}
==============================================================================
The output will be:
p=fffffffffffff000
p=fffff000
which means signed extension occurrs here:
p = t1.bits24 << 12;
This signed extension is unexpected since bits24 has been declared as
"unsigned". Even if integer promotion happens, it should be promoted as
"unsigned int" as well.
VERSION information:
gcc --version
gcc (GCC) 8.3.1 20191121 (Red Hat 8.3.1-5)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
OS:
[localhost test]$ uname -a
Linux localhost.localdomain 4.18.0 #1 SMP Thu Jul 23 13:20:43 PDT 2020 x86_64
x86_64 x86_64 GNU/Linux
