On 08/19/2010 09:10 AM, Job Noorman wrote:
> Andrew Haley <aph at redhat dot com> wrote:
>
>> Pretty obviously, replacing "movl $_ZN3Foo6foobarEv, %eax; push %rax"
>> with "push $_ZN3Foo6foobarEv" is an optimization.
>
> Yes, it would be if GCC does it without my help (which, by the way, it does
> not, no matter which optimizations used). However, since I'm explicitly
> telling GCC to emit "push $_ZN3Foo6foobarEv", it is an optimization from my
> side and should have nothing to do with any optimizations from GCC's side.
"should" statement with missing "because": syntax error. :-)
>> And nothing that a gcc developer says will convince you otherwise?
>
> Nothing a GCC developer has already said has convinced me. Of course
> I can be convinced.
>> If I were you, I'd use
>>
>> void test()
>> {
>> asm("push %0;"
>> :
>> : "g"((plain_foobar_t)&Foo::foobar));
>> }
>
> That will output the exact same assembly code as when using the "r"
> constraint. I really want to use the "i" constraint to skip the extra "mov".
Works for me:
-O0:
movl $_ZN3Foo6foobarEv, %eax
#APP
# 13 "ttt.cc" 1
push %rax;
# 0 "" 2
#NO_APP
-O2:
#APP
# 13 "ttt.cc" 1
push $_ZN3Foo6foobarEv;
# 0 "" 2
#NO_APP
> And, sorry if I sound like a broken record, I still think this should be
> possible. Consider the following (which was pointed out to me in a comment on
> my bug report):
>
> void test()
> {
> asm("push %0;"
> :
> : "i"((void*)(plain_foobar_t)&Foo::foobar));
> }
>
> This always works! Now, if you could give me a good explanation of how it
> could be that "(void*)expr" is accepted as a constant expression when "expr"
> is not,
I already did, I think: some things get constant folded regardless of
optimization level, but this is just luck.
> I might be convinced that this is not a bug.
But gcc doesn't support the "i" constraint with anything other than a
real compile-time constant (in the sense of __builtin_constant_p) so
it's not a bug.
Andrew.
