[llvm] [ObjectSizeOffsetVisitor] Add a max recursion depth (PR #67479)

[Nikita Popov via llvm-commits]

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@@ -50,6 +51,12 @@ using namespace llvm;
 
 #define DEBUG_TYPE "memory-builtins"
 
+static cl::opt<unsigned> ObjectSizeOffsetVisitorMaxRecurseDepth(
+    "object-size-offset-visitor-max-recurse-depth",
+    cl::desc(
+        "Maximum number of PHIs for ObjectSizeOffsetVisitor to look through"),
+    cl::init(100));
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nikic wrote:

> I think it's unlikely that we hit huge fanout in real world code, it's typically pretty linear at least with large autogenerated code.

I guess the fact that values don't get revisited does avoid large fan-out in practice.

> In the case I was looking at, this dropped DSE time from >30s to <1s.

Huh, that sounds like a bug. We usually don't query objectsize on non-root objects, and DSE probably shouldn't do that either.

Somewhat expensive objectsize analysis is okay for actual llvm.objectsize intrinsics, but not for generic passes.

https://github.com/llvm/llvm-project/pull/67479