Patch to (x86) asm/system.h to suppress warning by GCC 3.3.
I would like to suggest a patch to x86 asm/system.h of the kernel
2.4.21. Patch is attached at the end.
GCC version.
gcc -v
Reading specs from /usr/lib/gcc-lib/i386-linux/3.3/specs
Configured with: ../src/configure -v --enable-languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib --enable-nls --without-included-gettext --enable-__cxa_atexit --enable-clocale=gnu --enable-debug --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc i386-linux
Thread model: posix
gcc version 3.3 (Debian)
Background:
linux kernel 2.4.21 asm/system.h header produces warnings when
compiled using GCC 3.3 (Debian).
For example, when I compile lm_sensors package
( See the following URL for lm_sensors
http://www2.lm-sensors.nu/~lm78/index.html )
I get many warnings similar to the following.
--- quote from compilation ---
gcc -D__KERNEL__ -DMODULE -DEXPORT_SYMTAB -fomit-frame-pointer -I. -Ikernel/include -I/usr/local/include -I/lib/modules/2.4.21/build/include -nostdinc -I /usr/lib/gcc-lib/i386-linux/3.3/include -Wall -O2 -c kernel/busses/i2c-nforce2.c -o kernel/busses/i2c-nforce2.o
In file included from /lib/modules/2.4.21/build/include/asm/semaphore.h:39,
from /lib/modules/2.4.21/build/include/linux/fs.h:200,
from /lib/modules/2.4.21/build/include/linux/capability.h:17,
from /lib/modules/2.4.21/build/include/linux/binfmts.h:5,
from /lib/modules/2.4.21/build/include/linux/sched.h:9,
from /lib/modules/2.4.21/build/include/linux/mm.h:4,
from /lib/modules/2.4.21/build/include/linux/slab.h:14,
from /lib/modules/2.4.21/build/include/asm/pci.h:32,
from /lib/modules/2.4.21/build/include/linux/pci.h:654,
from kernel/busses/i2c-nforce2.c:35:
/lib/modules/2.4.21/build/include/asm/system.h: In function `__set_64bit_var':
/lib/modules/2.4.21/build/include/asm/system.h:190: warning: dereferencing type-punned pointer will break strict-aliasing rules
/lib/modules/2.4.21/build/include/asm/system.h:190: warning: dereferencing type-punned pointer will break strict-aliasing rules
kernel/busses/i2c-nforce2.c: In function `nforce2_access':
kernel/busses/i2c-nforce2.c:148: warning: `len' might be used uninitialized in this function
--- end quote.
Note the warning message "dereferencing type-punned pointer will break
strict-aliasing rules".
>From (x86) asm/system.h, we find the following code on line 190
#define ll_low(x) *(((unsigned int*)&(x))+0)
#define ll_high(x) *(((unsigned int*)&(x))+1)
static inline void __set_64bit_var (unsigned long long *ptr,
unsigned long long value)
{
---> __set_64bit(ptr,ll_low(value), ll_high(value));
}
We see that the "unsigned long long" value is handled using ll_low()
and ll_high() macros, which indeed play loose with pointers.
My suggested patch is basically as follows.
static inline void __set_64bit_var (unsigned long long *ptr,
unsigned long long value)
{
union {
unsigned long long ull;
unsigned int ui2[2];
} t;
t.ull = value;
__set_64bit(ptr,t.ui2[0], t.ui2[1]);
}
Verification:
I checked that the generated assembly language code
for my modification is not that bad.
Indeed the modified code generates a little cleaner output
than the current C code does IMHO.
(Or am I missing something? Maybe )
The modified code also works correctly as far as I can see.
So I suggest that we modify the asm/system.h (for x86)
(Maybe someone can offer better fix which may not
break hidden assumptions/requirements missed by me.
In any case, suppressing warnings from a system-supplied
header ought to be a priority.)
Testing was done by the following C code.
--- #include <stdio.h> #include <stdlib.h>/* The following inlines were taken straight from asm/system.h */
static inline void __set_64bit (unsigned long long * ptr, unsigned int low, unsigned int high) { __asm__ __volatile__ ( "\n1:\t" "movl (%0), %%eax\n\t" "movl 4(%0), %%edx\n\t" "lock cmpxchg8b (%0)\n\t" "jnz 1b" : /* no outputs */ : "D"(ptr), "b"(low), "c"(high) : "ax","dx","memory"); }
static inline void __set_64bit_constant (unsigned long long *ptr, unsigned long long value) { __set_64bit(ptr,(unsigned int)(value), (unsigned int)((value)>>32ULL)); } #define ll_low(x) *(((unsigned int*)&(x))+0) #define ll_high(x) *(((unsigned int*)&(x))+1)
static inline void __set_64bit_var (unsigned long long *ptr, unsigned long long value) { __set_64bit(ptr,ll_low(value), ll_high(value)); }
/* Here is my modification. Does not produce warning from the compiler GCC 3.3, and seem to generate cleaner code. */
static inline void __set_64bit_var_2 (unsigned long long *ptr, unsigned long long value) { union { unsigned long long ull; unsigned int ui2[2]; } t; t.ull = value; __set_64bit(ptr,t.ui2[0], t.ui2[1]); }
unsigned long long g_ull; unsigned long long *g_ullptr;
int test2() { __set_64bit_var(g_ullptr, g_ull); return 0; } int test3() {
__set_64bit_var_2(g_ullptr, g_ull); return 0; }
int main() { unsigned long long t; g_ullptr = &t;
g_ull = 0x1100220033004400ull;
test2();
if(t != g_ull) abort();
g_ull = 0x0011002200330044ull;
test3();
if( t != g_ull ) abort();
exit(0); }
--- gcc -Wall -O2 -S test-macro.c test-macro.c: In function `__set_64bit_var': test-macro.c:35: warning: dereferencing type-punned pointer will break strict-aliasing rules test-macro.c:35: warning: dereferencing type-punned pointer will break strict-aliasing rules cat test-macro.s .file "test-macro.c" .text .p2align 2,,3 .globl test2 .type test2, @function test2: pushl %ebp movl %esp, %ebp pushl %edi pushl %ebx subl $8, %esp movl g_ull, %eax movl g_ull+4, %edx movl %eax, -16(%ebp) movl g_ullptr, %edi movl %edx, -12(%ebp) movl -16(%ebp), %ebx movl -12(%ebp), %ecx #APP 1: movl (%edi), %eax movl 4(%edi), %edx lock cmpxchg8b (%edi) jnz 1b #NO_APP addl $8, %esp popl %ebx xorl %eax, %eax popl %edi leave ret .size test2, .-test2 .p2align 2,,3 .globl test3 .type test3, @function test3: pushl %ebp movl %esp, %ebp pushl %edi pushl %ebx movl g_ullptr, %edi movl g_ull, %ebx movl g_ull+4, %ecx #APP 1: movl (%edi), %eax movl 4(%edi), %edx lock cmpxchg8b (%edi) jnz 1b #NO_APP popl %ebx xorl %eax, %eax popl %edi leave ret .size test3, .-test3 .p2align 2,,3 .globl main .type main, @function main: pushl %ebp movl %esp, %ebp pushl %esi pushl %ebx subl $16, %esp andl $-16, %esp leal -16(%ebp), %eax movl %eax, g_ullptr movl $855655424, g_ull movl $285221376, g_ull+4 call test2 movl -12(%ebp), %ebx movl -16(%ebp), %ecx movl g_ull, %eax movl g_ull+4, %edx movl %ebx, %esi xorl %edx, %esi xorl %ecx, %eax orl %eax, %esi jne .L10 movl $3342404, g_ull movl $1114146, g_ull+4 call test3 movl -12(%ebp), %ebx movl -16(%ebp), %ecx movl g_ull, %eax movl g_ull+4, %edx movl %ebx, %esi xorl %edx, %esi xorl %ecx, %eax orl %eax, %esi je .L9 .L10: call abort .L9: subl $12, %esp pushl $0 call exit .size main, .-main .comm g_ull,8,8 .comm g_ullptr,4,4 .ident "GCC: (GNU) 3.3 (Debian)" --- Run the program.
gcc -Wall -O2 test-macro.c test-macro.c: In function `__set_64bit_var': test-macro.c:35: warning: dereferencing type-punned pointer will break strict-aliasing rules test-macro.c:35: warning: dereferencing type-punned pointer will break strict-aliasing rules
./a.out
(no error.)
PS: i am not on linux-kernel and I would appreciate it if follow-up is cc: to me as well.
PPS: I wondered why linux kernel compilatio itsefl didn't produce the warning and found the use of -fno-strict-aliasing on gcc command line. Hmm...
PPPS: In any case, the code produced seem to be cleaner (and possibly faster. Will someone count the cycles?) and so the patch seems preferable.
[end of memo]
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This archive was generated by hypermail 2b29 : Mon Jun 30 2003 - 22:00:16 EST