On Fri, May 01, 2026 at 10:13:15PM +0100, David Laight wrote:
> 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.
>
You're right, my earlier measurements included dependency rebuilds
after make clean. I re-measured using touch to isolate the actual
compile time of ring_buffer_benchmark.o on x86_64:
- Original ___STR: 1.757s
- v1 with __UNIQUE_ID (compiler.h): 1.836s
- sizeof __stringify (your suggestion): 1.781s
> >
> > 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
>
Yes, this works. I verified with objdump on the
samples/trace_printk module that all four cases branch correctly:
__trace_bputs, __trace_puts, __trace_bprintk, and __trace_printk.
I'll use this form in v3 since it's simpler than the compound literal.
> > 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
> > > > \
> > >
>
