[PATCH] D112852: [GlobalISel] Allow DBG_VALUE to use invalid vregs throughout pipeline

Sat Nov 20 10:29:29 PST 2021

jackoalan added a comment.

In D112852#3098095 <https://reviews.llvm.org/D112852#3098095>, @MatzeB wrote:

> Before we weaken the verifier

The weakening conditions are now much more targeted to this specific case.

> do we have an idea how much benefit we get from allowing these half-initialize vregs?

In terms of benefits, `D109750` mentions a 0.5% codegen time reduction, but its effects are limited to the legalizer. This goes further by replacing all calls to `eraseFromParentAndMarkDBGValuesForRemoval` with `eraseFromParent`.

> Is searching for these `DBG_VALUE` at the end of GlobalISel and remove them there an option?

The issue here is determining where the barrier for disallowing these undefined vregs should be. `LiveDebugVariables` naturally discards these particular uses, so this is sufficient based on my knowledge.

Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D112852/new/

https://reviews.llvm.org/D112852