Commits on Sep 16, 2023

1. vm: restore the VM profiler …

   Summary
   =======
   
   Fix the compiler crashing when enabling the VM profiler (`--profilevm`)
   and fully restore the latter's original functionality. In addition, the
   profiler's overhead is significantly reduced.
   
   Details
   =======
   
   The VM profiler stopped working when the stack-frames moved from being
   `ref` based to being `seq` based
   (fb03691).
   
   This is due to the profiler expecting the frame index at the start of
   a measurement (`enter`) being valid at the end of a measurement
   (`leave`), but this is not true when executing, for example, a `Ret`
   instruction, which led to an out-of-bounds error.
   
   As a simple and low impact fix, instead of remembering the index,
   `leave` now takes the list of current stack frames (i.e., the stack) as
   input, treating the last item as the current frame. A minor side-effect
   of this is that a `Ret` instruction now counts for the caller, while a
   call instruction counts for the callee.
   
   There was also a bug in the rendering logic: `min` was used instead of
   `max`, meaning that the entry limiting didn't work.
   
   In addition, the documentation of the `vmprofiler` module is improved
   and the implementation is refactored:
   
   * the `data` table now uses a `PSym` as the key. The intention of the
     table was always to associate data with a procedure, but `TLineInfo`
     and later `SourceLinePosition` only approximated this. Using a `PSym`
     also makes it possible to render more information (such as the
     procedure's name) in the future
   * whether profiling is enabled is now a property of the `Profiler`
     instance, removing the dependency on a `ConfigRef`. This also slightly
     reduces the overhead of `enter`/`leave`
   * the `leaveImpl` procedure is improved. The time difference is only
     computed once, and instead of temporarily assigning each frame to a
     local (which incurred a very costly full copy), only `prc` is assigned
     to a local
   
   With access to the symbol, rendering now also renders the full file,
   line, and column of the profiled procedures again (instead of only the
   containing file).
   
   Finally, an integration test (`tprofiler.nim`) and a basic unit test
   (`tvmprofiler.nim`) are added in order to prevent future regressions.

   

   zerbina committed Sep 16, 2023
   Configuration menu

   • View commit details
   • Copy full SHA for 47a633b
   • Browse repository at this point

   Copy the full SHA

   47a633b View commit details

   Browse the repository at this point in the history