Date: Fri, 11 Jun 2010 23:31:57 +0200 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: Tijl Coosemans <tijl@coosemans.org> Cc: freebsd-current@freebsd.org Subject: Re: Cleanup for cryptographic algorithms vs. compiler optimizations Message-ID: <86y6els1bm.fsf@ds4.des.no> In-Reply-To: <201006112304.10952.tijl@coosemans.org> (Tijl Coosemans's message of "Fri, 11 Jun 2010 23:04:10 %2B0200") References: <20100611162118.GR39829@acme.spoerlein.net> <867hm5tl6u.fsf@ds4.des.no> <201006112304.10952.tijl@coosemans.org>
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Tijl Coosemans <tijl@coosemans.org> writes:
> Dag-Erling Sm=C3=B8rgrav <des@des.no> writes:
> > #define FORCE_ASSIGN(type, var, value) \
> > *(volatile type *)&(var) =3D (value)
> memset can be optimised away as well. The only way is to declare those
> variables volatile.
Assigning through a volatile pointer, as in FORCE_ASSIGN(), also works,
even if the variable itself is not volatile. Unfortunately, you can't
trick the compiler into not optimizing away memset(), since you can't
pass a volatile pointer to memset().
% cat >zero.c <<EOF
#include <string.h>
int
main(void)
{
char a, b, c;
a =3D 1;
memset(&b, 2, sizeof b);
*(volatile char *)&c =3D 3;
return 0;
}
EOF
% for O in 0 1 2 ; do c99 -O$O -o zero-O$O zero.c ; done
% for O in 0 1 2 ; do echo disassemble main | gdb -batch -x /dev/stdin -q z=
ero-O$O ; done
Dump of assembler code for function main:
0x0000000000400560 <main+0>: push %rbp
0x0000000000400561 <main+1>: mov %rsp,%rbp
0x0000000000400564 <main+4>: sub $0x10,%rsp
0x0000000000400568 <main+8>: movb $0x1,0xffffffffffffffff(%rbp)
0x000000000040056c <main+12>: lea 0xfffffffffffffffe(%rbp),%rdi
0x0000000000400570 <main+16>: mov $0x1,%edx
0x0000000000400575 <main+21>: mov $0x2,%esi
0x000000000040057a <main+26>: callq 0x40042c <memset>
0x000000000040057f <main+31>: lea 0xfffffffffffffffd(%rbp),%rax
0x0000000000400583 <main+35>: movb $0x3,(%rax)
0x0000000000400586 <main+38>: mov $0x0,%eax
0x000000000040058b <main+43>: leaveq=20
0x000000000040058c <main+44>: retq=20=20=20
0x000000000040058d <main+45>: nop=20=20=20=20
0x000000000040058e <main+46>: nop=20=20=20=20
0x000000000040058f <main+47>: nop=20=20=20=20
End of assembler dump.
Dump of assembler code for function main:
0x0000000000400520 <main+0>: movb $0x3,0xffffffffffffffff(%rsp)
0x0000000000400525 <main+5>: mov $0x0,%eax
0x000000000040052a <main+10>: retq=20=20=20
0x000000000040052b <main+11>: nop=20=20=20=20
End of assembler dump.
Dump of assembler code for function main:
0x0000000000400520 <main+0>: xor %eax,%eax
0x0000000000400522 <main+2>: movb $0x3,0xffffffffffffffff(%rsp)
0x0000000000400527 <main+7>: retq=20=20=20
End of assembler dump.
In the -O0 case, all three assignments are carried out. In the -O1 and
-O2 cases, the first two assignments and the corresponding variables are
optimized away, while the third (which uses the volatile pointer trick)
is not.
Clang produces significantly worse code than gcc in all cases:
% for O in 0 1 2 ; do clang -O$O -o zero-O$O zero.c ; done
% for O in 0 1 2 ; do echo disassemble main | gdb -batch -x /dev/stdin -q z=
ero-O$O ; done
Dump of assembler code for function main:
0x0000000000400550 <main+0>: push %rbp
0x0000000000400551 <main+1>: mov %rsp,%rbp
0x0000000000400554 <main+4>: movl $0x0,0xfffffffffffffffc(%rbp)
0x000000000040055b <main+11>: movb $0x1,0xfffffffffffffffb(%rbp)
0x000000000040055f <main+15>: movb $0x2,0xfffffffffffffffa(%rbp)
0x0000000000400563 <main+19>: movb $0x3,0xfffffffffffffff9(%rbp)
0x0000000000400567 <main+23>: movl $0x0,0xfffffffffffffffc(%rbp)
0x000000000040056e <main+30>: mov 0xfffffffffffffffc(%rbp),%eax
0x0000000000400571 <main+33>: pop %rbp
0x0000000000400572 <main+34>: retq=20=20=20
0x0000000000400573 <main+35>: nop=20=20=20=20
End of assembler dump.
Dump of assembler code for function main:
0x0000000000400550 <main+0>: push %rbp
0x0000000000400551 <main+1>: mov %rsp,%rbp
0x0000000000400554 <main+4>: movb $0x3,0xffffffffffffffff(%rbp)
0x0000000000400558 <main+8>: xor %eax,%eax
0x000000000040055a <main+10>: pop %rbp
0x000000000040055b <main+11>: retq=20=20=20
End of assembler dump.
Dump of assembler code for function main:
0x0000000000400550 <main+0>: push %rbp
0x0000000000400551 <main+1>: mov %rsp,%rbp
0x0000000000400554 <main+4>: movb $0x3,0xffffffffffffffff(%rbp)
0x0000000000400558 <main+8>: xor %eax,%eax
0x000000000040055a <main+10>: pop %rbp
0x000000000040055b <main+11>: retq=20=20=20
End of assembler dump.
DES
--=20
Dag-Erling Sm=C3=B8rgrav - des@des.no
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