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

[Steve Langasek]

** Also affects: dpkg (Ubuntu)
   Importance: Undecided
       Status: New

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

Status in Ubuntu on IBM z Systems:
  New
Status in dpkg package in Ubuntu:
  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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