[PATCH] D97108: [Statepoint Lowering] Allow dead gc pointer from deopt section to be on register.

[Philip Reames via Phabricator via llvm-commits]

reames added a comment.

In D97108#2590224 <https://reviews.llvm.org/D97108#2590224>, @skatkov wrote:

> From my point of view it looks better in terms that we explicitly process deopt gc pointers as other gc values.
> The disadvantages is that now we will have a lot of dead tied-def registers and deopt value will be listed twice in uses.

This comment confused me greatly, and caused me to take a closer look at the current patch.  After doing so, I'm glad I did!  This version is unsound.

For the gc pointer to remain valid until deoptimization, we must update it during any GC cycles during the lifetime of the call and before the deopt point.  To do that, we need to model the call as modifying the pointer.  Otherwise, the register allocator is free to place it in a read only location (such as an argument slot) or combine the updated and non-updated copies of value into a single register.  (Consider the case where a null check on the original unrelocated value was reordered after the safepoint.)  The fact this one doesn't cause a tied def is a symptom of a bug, not a code quality improvement.


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  https://reviews.llvm.org/D97108/new/

https://reviews.llvm.org/D97108