tree 4a8d74accf9a67bab3da98729af214eb8469d01c
parent ade09b1190b89f7a9b85b2d757269ea1f616af87
author OCHyams <orlando.hyams@sony.com> 1603707650 +0000
committer Copybara-Service <copybara-worker@google.com> 1604018414 -0700

[DebugInfo][dexter] Add dexter tests for escaped locals

Recently there has been renewed interest in improving debug-info for variables
that (partially or otherwise) live on the stack in optimised code.

At the moment instcombine speculates that stack slots are probably going to be
promoted to registers, and prepares the debug-info accordingly. It runs a
function called LowerDbgDeclare which converts dbg.declares to a set of
dbg.values after loads, and before stores and calls. Sometimes the stack
location remains (e.g. for escaped locals). If any dbg.values become undef
where the stack location is still valid we end up unnecessarily reducing
variable location coverage due to our inability to track multiple locations
simultaneously. There is a flag to disable this feature
(-instcombine-lower-dbg-declare=0), which prevents this conversion at the cost
of sometimes providing incorrect location info in the face of DSE, DCE, GVN,
CSE etc.

This has been discussed fairly extensively on PR34136.

The idea of these tests is to provide examples of situations that we should
consider when designing a new system, to aid discussions and eventually help
evaluate the implementation.

Dexter isn't ideal for observing specific optimisation behaviour. Writing an
exaustive test suite would be difficult, and the resultant suite would be
fragile. However, I think having some concrete executable examples is useful
at least as a reference.

Differential Revision: https://reviews.llvm.org/D89543

GitOrigin-RevId: 66d03af88c2e4932ae53787e07f502b90ab220cc