On Thu, 2022-06-30 at 08:44 -0700, Dave Hansen wrote:
> On 6/29/22 16:02, Kai Huang wrote:
> > On Wed, 2022-06-29 at 07:22 -0700, Dave Hansen wrote:
> > > On 6/24/22 04:21, Kai Huang wrote:
> > > What does that #ifdef get us? I suspect you're back to trying to
> > > silence compiler warnings with #ifdefs. The compiler *knows* that it's
> > > only used in this file. It's also used all of once. If you make it
> > > 'static inline', you'll likely get the same code generation, no
> > > warnings, and don't need an #ifdef.
> >
> > The purpose is not to avoid warning, but to make intel_cc_platform_has(enum
> > cc_attr attr) simple that when neither TDX host and TDX guest code is turned on,
> > it can be simple:
> >
> > static bool intel_cc_platform_has(enum cc_attr attr)
> > {
> > return false;
> > }
> >
> > So I don't need to depend on how internal functions are implemented in the
> > header files and I don't need to guess how does compiler generate code.
>
> I hate to break it to you, but you actually need to know how the
> compiler works for you to be able to write good code. Ignoring all the
> great stuff that the compiler does for you makes your code worse.
Agreed.
>
> > And also because I personally believe it doesn't hurt readability.
>
> Are you saying that you're ignoring long-established kernel coding style
> conventions because of your personal beliefs? That seem, um, like an
> approach that's unlikely to help your code get accepted.
Agreed. Will keep this in mind. Thanks.
>
> > > The other option is to totally lean on the compiler to figure things
> > > out. Compile this program, then disassemble it and see what main() does.
> > >
> > > static void func(void)
> > > {
> > > printf("I am func()\n");
> > > }
> > >
> > > void main(int argc, char **argv)
> > > {
> > > if (0)
> > > func();
> > > }
> > >
> > > Then, do:
> > >
> > > - if (0)
> > > + if (argc)
> > >
> > > and run it again. What changed in the disassembly?
> >
> > You mean compile it again? I have to confess I never tried and don't know.
> > I'll try when I got some spare time. Thanks for the info.
>
> Yes, compile it again and run it again.
>
> But, seriously, it's a quick exercise. I can help make you some spare
> time if you wish. Just let me know.
So I tried. Took me less than 5 mins:)
The
if (0)
func();
never generates the code to actually call the func():
0000000000401137 <main>:
401137: 55 push %rbp
401138: 48 89 e5 mov %rsp,%rbp
40113b: 89 7d fc mov %edi,-0x4(%rbp)
40113e: 48 89 75 f0 mov %rsi,-0x10(%rbp)
401142: 90 nop
401143: 5d pop %rbp
401144: c3 ret
While
if (argc)
func();
generates the code to check argc and call func():
0000000000401137 <main>:
401137: 55 push %rbp
401138: 48 89 e5 mov %rsp,%rbp
40113b: 48 83 ec 10 sub $0x10,%rsp
40113f: 89 7d fc mov %edi,-0x4(%rbp)
401142: 48 89 75 f0 mov %rsi,-0x10(%rbp)
401146: 83 7d fc 00 cmpl $0x0,-0x4(%rbp)
40114a: 74 05 je 401151 <main+0x1a>
40114c: e8 d5 ff ff ff call 401126 <func>
401151: 90 nop
401152: c9 leave
401153: c3 ret
This is kinda no surprise.
Were you trying to make point that
if (false)
func();
doesn't generate any additional code?
I get your point now. Thanks :)
