Uploaded image for project: 'MariaDB Server'
  1. MariaDB Server
  2. MDEV-7942

InnoDB: abuse of UNIV_LIKELY()/UNIV_UNLIKELY()

Details

    Description

      UNIV_LIKELY()/UNIV_UNLIKELY() hints are supposed to improve branch prediction. They're defined in InnoDB as following:

      /* Tell the compiler that 'expr' probably evaluates to 'constant'. */
      		 
      # define UNIV_EXPECT(expr,constant) __builtin_expect(expr, constant)
      		 
      /* Tell the compiler that cond is likely to hold */
      		 
      #define UNIV_LIKELY(cond) UNIV_EXPECT(cond, TRUE)
      		 
      /* Tell the compiler that cond is unlikely to hold */
      		 
      #define UNIV_UNLIKELY(cond) UNIV_EXPECT(cond, FALSE)

      In other words they're expected to work only if cond evaluates to TRUE or FALSE,
      see https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html.

      However there're a few conditions that may evaluate to different values, e.g.:

      page/page0zip.cc:		if (UNIV_LIKELY(c_stream->avail_in)) {
      		 
      page/page0zip.cc:			if (UNIV_LIKELY(c_stream->avail_in)) {
      		 
      dict/dict0mem.cc:		if (UNIV_LIKELY(i) && UNIV_UNLIKELY(!table->col_names)) {

      Either fix condition or fix UNIV_LIKELY() to evaluate condition to bool (see likely() in my_global.h).
      Note: we'll gain little to nothing in terms of performance out of this. But at least we'll have correct use of these compiler hints.

      Attachments

        Activity

          People

            jplindst Jan Lindström (Inactive)
            svoj Sergey Vojtovich
            Votes:
            0 Vote for this issue
            Watchers:
            4 Start watching this issue

            Dates

              Created:
              Updated:
              Resolved:

              Git Integration

                Error rendering 'com.xiplink.jira.git.jira_git_plugin:git-issue-webpanel'. Please contact your Jira administrators.