[PATCH] D64162: Summary: [Attributor] Liveness analysis abstract attribute used to indicate which BasicBlocks are dead and can therefore be ignored.

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

I guess we need to implement a manifest to make testing easier.

I'd propose the following in the manifest method:
Split the block that is dead after the phi-nodes (see `splitBasicBlock`) and replace the unconditional jump with a `llvm_unreachable`.

On second thought, maybe manifest should go through the list of `noreturn` calls and place `llvm_unreachable` after them. Later, we will
have more "sources" in addition to `noreturn` and then we introduce the unreachable after all of them.



================
Comment at: llvm/include/llvm/Transforms/IPO/Attributor.h:723
+  /// Returns true if nounwind is known.
+  virtual bool isKnownDead() const = 0;
 };
----------------
I think these methods need to take a basic block and/or instruction.


================
Comment at: llvm/lib/Transforms/IPO/Attributor.cpp:1016
+
+    for (Instruction &I : instructions(&F)) {
+      ImmutableCallSite ICS(&I);
----------------
You need to explore the function in a different way:
Keep a list of "to be expored" paths defined by instructions.
Start with the first instruction in the entry block as a first "to be explored path".
Continue to the next instruction if it is not a `noreturn` call.
If it is the last in the block, add all successors instructions to the "to be explored" path list.
Make sure you never follow a path/instruction twice.

The functionality above should live in a separate function as it will be needed in the update method as well.
Basically, update is the same thing but the start points are all `noreturn` calls that are not `noreturn` anymore.


================
Comment at: llvm/lib/Transforms/IPO/Attributor.cpp:1026
+        /// If the block contains a noreturn call, it is assumed dead.
+        AssumedDeadBlocks.insert(I.getParent());
+      }
----------------
The code __after__ the `noreturn` call is assumed dead but not the block. 


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D64162