Add this new structure to apidefs.h:
typedef struct TSFastDbgCntl_
{
const char * const tag; // nul-terminated string
const volatile char * const on; // pointer to 1-byte flag
}
TSFastDbgCntl;
Add this new API function, that returns a pointer to an instance of the
above structure:
TSFastDbgCntl * TSCreateFastDbgCntl(const char *tag);
Add this macro, which would be used in place of direct use of TSDebug().
#define TSFastDbg(_FD_CNTL, ...) \
do { \
if ((_FD_CNTL)->on) \
TSDebug((_FD_CNTL)->tag, __VA_ARGS__); \
} while(0)
The first parameter to TSFastDbg() is a pointer that was returned by
TSCreateFastDbgCntl(). The remaining parameters are printf parameters
(format string and values matching format specifiers). The core would be
responsible for changing the value of the 'on' fields in the TSFastDbgCntl
instances. For direct calls to TSDebug(), even if the tag is disabled, all
the parameters have to be prepared and pushed onto the stack. As was
discussed in IRC, you have to feel guilty about doing converting a
string_view to a string and outputting it with c_str(). Because the code
to do this will be executed even if the debug tag is disabled.
Furthermore, for each call to TSDebug, it's necessary to do an associative
lookup on the tag string to determine that output for the tag is disabled.
We would want to put equivalent capabilities in Diags.h in the core for
debug output. The implementation is non-trivial, but I think it would take
at most a week including the Au test additions.
I looked at making TSFastDbg() an inline function rather than a macro:
https://godbolt.org/z/IfVbBk
The opitimization works well if you compile the code with the inline
function as C, but the optimization is poor if you compile it as C++. This
difference exists for both gcc and clang. It's weird.
