Hi,
I'm not sure if this is a problem with gcc or merely a lack of
understanding on my part correctly understanding inline assembler
constraints. Consider the following C++ test case:
int main(char* argv, int argc)
{
unsigned char result;
void *eflags;
setIfCondition_Zero(result,eflags);
}
void setIfCondition_Zero(unsigned char &condStatus, void *eflags)
{
__asm__ volatile
(
"pushf \n\t" //pushfdl on 32 bit mode
"push %1 \n\t"
"popf \n\t" //popdfl on 32 bit mode
"setz %0 \n\t"
"popf \n\t"
: "=r"(condStatus): "r"(eflags): "%rsp");
}
And the generated code for the setIfConditionZero function:
000000000040059c <_Z19setIfCondition_ZeroRhPv>:
40059c: 55 push %rbp
40059d: 48 89 e5 mov %rsp,%rbp
4005a0: 48 89 7d f8 mov %rdi,0xfffffffffffffff8(%rbp)
4005a4: 48 89 75 f0 mov %rsi,0xfffffffffffffff0(%rbp)
4005a8: 48 8b 45 f0 mov 0xfffffffffffffff0(%rbp),%rax
4005ac: 9c pushfq
4005ad: 50 push %rax
4005ae: 9d popfq
4005af: 0f 94 c2 sete %dl
4005b2: 9d popfq
4005b3: 48 8b 45 f8 mov 0xfffffffffffffff8(%rbp),%rax
4005b7: 88 10 mov %dl,(%rax)
4005b9: c9 leaveq
4005ba: c3 retq
The problem should be obvious. The temporary store of the reference gets
stamped on by the stack manipulation. In larger functions containing
inline assembler and calls to subroutines I have seen gcc generate a
stack frame somewhat like:
push %rbp
mov %rsp,%rbp
push %r12
push %rbx
==> sub $0x30,%rsp
mov %rdi,0xffffffffffffffe8(%rbp)
mov %sil,0xffffffffffffffe7(%rbp)
..
..
Making sure that the temporary space isn't stepped on.
I have worked around the problem for the time being by adding a "sub
0x10, %%rsp" to the inline assembler on the assumption the final leave
will clean everything up. However shouldn't there be a constraint that I
can use to tell gcc the program will use n bytes of stack? I've read the
manual but the section on constraints reads a little too cryptically for
me.
BTW, "g++ --version" == g++ (GCC) 3.3.5
Regards,
--
Alex, homepage: http://www.bennee.com/~alex/
"Nuclear war would really set back cable." -- Ted Turner
