[PATCH] D155773: [llvm][MemoryBuiltins] Add alloca support to getInitialValueOfAllocation

[John McIver via Phabricator via cfe-commits]

jmciver added a comment.

@nikic as per my GSoC project I am trying, under the guidance of @nlopes, a load attribute based approach to migrating uninitialized load to poison. The first attribute that I am working on is !freeze which effectively inserts a freeze poison if the load is uninitialized. Obviously this can only be done in
optimizations that allow instruction creation. In the future I am looking to add a load instruction parameter to getInitialValueOfAllocation to allow modification of the returned constant based on the allocation function/instruction and the presence of a load attribute.

So why pack the alloca test into getInitialValueOfAllocation? The reasoning is that attribute is on a per-load instruction, but allocation could be alloca or function. I agree if the application of the load attribute were universal passing in the default value to mem2reg would work, but as we are modulating the returned constant based on individual load instruction attributes this would not work. Hence the query based approach.

As for the TLI I agree that making it required is probably not ideal. @nlopes can vouch that I thought about this :-). Two options off the top of my head are:

1. Make the TLI parameter of getInitialValueOfAllocation a pointer with defaults to nullptr.

2. Add a separated query function for alloca instructions.

The advantage to option 1 is all allocation initial state queries are handled in one place. The detractor is we have a parameter that has a default nullptr.

The advantage to option 2 is allocation functions vs alloca instruction are handled by functions that have only the required parameterizations exposed. The detractor is there will be some combinatorial overlap between the two functions.

Thoughts?


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