[PATCH] D97244: [SimplifyCFG] Update passingValueIsAlwaysUndefined to check more attributes

[Guozhi Wei via Phabricator via llvm-commits]

Carrot added inline comments.


================
Comment at: llvm/include/llvm/IR/InstrTypes.h:1663
+           paramHasAttr(ArgNo, Attribute::Dereferenceable) ||
+           // dereferenceable implies noundef, and null is a well-defined value.
+           paramHasAttr(ArgNo, Attribute::DereferenceableOrNull);
----------------
aqjune wrote:
> Carrot wrote:
> > It is not true for "this" parameter. It always has dereferenceable and nonnull attribute, if "this" is not used in the function, passing in undef is no problem.
> > 
> > It causes wrong code generated for our application.
> > passing in undef is no problem
> 
> This property holds for `nonnull` and `align`, but not for the three attributes listed here. Passing undef to these is UB because LangRef says violation of these is UB. `dereferenceable` paragraph says:
> 
> ```
> The pointer should be well defined, otherwise it is undefined behavior
> ```
> 
> Is your program being transformed from passing valid `this` to passing `undef` by some pass? Then, the pass is the buggy one.
Since "this" is not used in the function, DeadArgumentElimination pass changes all callers' valid "this" to undef.

@spatel, what's your opinion on where to fix it?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D97244/new/

https://reviews.llvm.org/D97244