[Bug 2064538] Re: Revert back frame pointers for s390x (remove -fno-omit-frame-pointer but use -mbackchain)

[Łukasz Zemczak]

Frank, can you help me in writing up some test cases for both of these
bugs? SO that we have formally a test case someone can follow or at
least understand. Thanks!

** Changed in: dpkg (Ubuntu Noble)
       Status: New => Fix Committed

** Tags added: verification-needed verification-needed-noble

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https://bugs.launchpad.net/bugs/2064538

Title:
  Revert back frame pointers for s390x (remove -fno-omit-frame-pointer
  but use -mbackchain)

Status in Ubuntu on IBM z Systems:
  New
Status in dpkg package in Ubuntu:
  Fix Released
Status in glibc package in Ubuntu:
  New
Status in linux package in Ubuntu:
  New
Status in dpkg source package in Noble:
  Fix Committed
Status in glibc source package in Noble:
  New
Status in linux source package in Noble:
  New
Status in dpkg source package in Oracular:
  Fix Released
Status in glibc source package in Oracular:
  New
Status in linux source package in Oracular:
  New

Bug description:
  The preferred way of doing stack unwinding on Linux on Z is via dwarf call frame information.
  In absence of a dwarf unwinder (as in the Linux kernel) a stack chain can be maintained at runtime in addition to the dwarf unwinding information.
  This allows for simple backtrace implementations, but imposes a small runtime overhead. For this to work, all code that might be part of backtrace must be built with the -mbackchain GCC option.

  The -fno-omit-framepointer switch is neither necessary nor helpful in this context.
  Having a (soft/simulated) frame pointer does not improve backtraces at all on IBM Z.
  However, forcing a frame pointer via the -fno-omit-frame-pointer option negatively affects performance for multiple reasons: extra prologue/epilogue overhead and fewer shrink-wrapping opportunities.
  Given -fno-omit-frame-pointer does not provide any improvements (backtraces or otherwise) and only reduces performance, -fno-omit-frame-pointers should not be used on IBM Z.

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