On Fri, 1 May 2026 22:40:17 +0800
Qian-Yu Lin <[email protected]> wrote:
...
> Yes. I measured compile time of kernel/trace/ring_buffer_benchmark.o
> after make clean on an x86_64 machine running Ubuntu 24.04 LTS:
>
> - Original _______STR: 49.8s
> - v1 with __UNIQUE_ID (compiler.h): 53.5s
> - compound literal (no extra include): 33.2s
That difference looks far to big to me.
And the times are far too large to be measuring the actual compile time.
>
> I propose using a compound literal in v2, which eliminates the local
> variable entirely and requires no extra include:
>
> #define trace_printk(fmt, ...) \
> do { \
> if (sizeof((char[]) \
> {__stringify((__VA_ARGS__))}) > 3) \
> do_trace_printk(fmt, ##__VA_ARGS__); \
There has to be a better way to align that code.
Although you should be able to use:
if (sizeof __stringify((__VA_ARGS__)) > 3)
(I've omitted one set of parenthesis for clarity)
You could change __stringify() to work with __VA_ARGS__ the you don't need
the extra (); this works fine:
#define _x(...) #__VA_ARGS__
#define x(...) _x(__VA_ARGS__)
#define z abcd
int a = sizeof x(z, v); /* 8 */
See: https://godbolt.org/z/zo4h4nr9b
-- David
> else \
> trace_puts(fmt); \
> } while (0)
>
> This fully eliminates the shadowing risk without any compile overhead.
>
> Qian-Yu
>
> >
> >
> > > #include <linux/compiler_attributes.h>
> > > #include <linux/instruction_pointer.h>
> > > #include <linux/stddef.h>
> > > @@ -84,15 +85,18 @@ do {
> > > \
> > > * let gcc optimize the rest.
> > > */
> > >
> > > -#define trace_printk(fmt, ...) \
> > > +#define ___trace_printk(fmt, str, ...) \
> > > do { \
> > > - char _______STR[] = __stringify((__VA_ARGS__)); \
> > > - if (sizeof(_______STR) > 3) \
> > > + char str[] = __stringify((__VA_ARGS__)); \
> > > + if (sizeof(str) > 3) \
> > > do_trace_printk(fmt, ##__VA_ARGS__); \
> > > else \
> > > trace_puts(fmt); \
> > > } while (0)
> > >
> > > +#define trace_printk(fmt, ...) \
> > > + ___trace_printk(fmt, __UNIQUE_ID(str), ##__VA_ARGS__)
> > > +
> > > #define do_trace_printk(fmt, args...)
> > > \
> > > do {
> > > \
> > > static const char *trace_printk_fmt __used \
> >
