[meta] Make llvm passes debug info invariant

[vedantk]

Bugzilla Link	37728
Version	unspecified
OS	All
Depends On	#36588 #37063 llvm/llvm-bugzilla-archive#37727 #40314 #40456 #43204 #43767 #44225 #49035 #49044 #50454 #50525 #50539 #50540 #50911 #50912 #51880 #51881 #51882 #36654 #37061 #37062 llvm/llvm-bugzilla-archive#37726 #41483 #42636 #43103 #43309 #48790 #49326 #49924
CC	@bjope,@dstenb,@dwblaikie,@fredriss,@gregbedwell,@jmorse,@jryans,@pogo59,@rgal,@wjristow,@yuanfang-chen

Extended Description

This is a meta bug for tracking work related to making llvm passes debug info invariant. That means that the output of llvm passes should be the same, modulo debug info metadata & intrinsics, regardless of whether the input IR contains debug info. The motivation for this is that enabling debug info should not degrade optimizations.

There are at least two different approaches for detecting debug invariance bugs:

1. Compile a C program with/without -g, and check that the resulting text sections are identical.

2. Compile some LLVM IR using opt, with/without -debugify-each enabled, and check that the stripped output is identical. The -debugify-each mode applies/removes synthetic debug info to a Module before/after each pass in opt's pipeline.

The second approach might be a bit more flexible, seeing as it's possible to generate LLVM IR from a C program, and to then run a custom pipeline over the IR using opt.

In addition to finding debug invariance bugs, we should think about setting up infrastructure to detect regressions, and about adding utilities to the codebase which might make it easier to write debug info invariant passes.

[gregbedwell]

1. Compile a C program with/without -g, and check that the resulting text
   sections are identical.

For this approach we have:
https://llvm.org/devmtg/2015-04/slides/Verifying_code_gen_dash_g_final.pdf

We have this pipeline setup for some our Sony internal test-suites and it's generally very effective FWIW. We've not had the spare bandwidth to fix all of the existing bugs unfortunately, but we do tend to report/fix regressions as we spot them. Hopefully we'll be able to find some spare bandwidth to help with this effort :)

[pogo59]

Added a couple older bugs to the list. Post-RA Scheduler will likely be
pretty unpleasant, but it clearly has an effect and showed up fairly
often in our dash-g-no-change pipeline.

[llvmbot]

check_cfc test script
This is a shell script tool calling check_cfc.py to detect codegen difference with/without -g

Run as: ./check_cfc.sh 2>test_err.log

[llvmbot]

check_cfc test result
After run check_cfc.sh with llvm_test_suite as test code,
Codegen differences found in test_err.log

[llvmbot]

*** Bug llvm/llvm-bugzilla-archive#43757 has been marked as a duplicate of this bug. ***

[gregbedwell]

mentioned in issue #40314

[llvmbot]

mentioned in issue #40456

[llvmbot]

mentioned in issue #41483

[mikaelholmen]

mentioned in issue #42636

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#43757

[llvmbot]

mentioned in issue #43103

[llvmbot]

mentioned in issue #43204

[bjope]

mentioned in issue #43309

[llvmbot]

mentioned in issue #43767

[dstenb]

mentioned in issue #44225