This release is primarily about Windows. A critical virtual memory leak is resolved on all Windows platforms. The regression was present in all releases since 5.0.0.

Bug fixes:

• Fix a severe virtual memory leak on Windows. This regression was first released in 5.0.0. (@Ignition, @j0t, @frederik-h, @davidtgoldblatt, @interwq)
• Fix size 0 handling in posix_memalign(). This regression was first released in 5.2.0. (@interwq)
• Fix the prof_log unit test which may observe unexpected backtraces from compiler optimizations. The test was first added in 5.2.0. (@marxin, @gnzlbg, @interwq)
• Fix the declaration of the extent_avail tree. This regression was first released in 5.1.0. (@zoulasc)
• Fix an incorrect reference in jeprof. This functionality was first released in 3.0.0. (@prehistoric-penguin)
• Fix an assertion on the deallocation fast-path. This regression was first released in 5.2.0. (@yinan1048576)
• Fix the TLS_MODEL attribute in headers. This regression was first released in 5.0.0. (@zoulasc, @interwq)

Optimizations and refactors:

• Implement opt.retain on Windows and enable by default on 64-bit. (@interwq, @davidtgoldblatt)
• Optimize away a branch on the operator delete path. (@mgrice)
• Add format annotation to the format generator function. (@zoulasc)
• Refactor and improve the size class header generation. (@yinan1048576)
• Remove best fit. (@djwatson)
• Avoid blocking on background thread locks for stats. (@oranagra, @interwq)