[clang] [compiler-rt] [llvm] [TypeProf][InstrFDO]Implement more efficient comparison sequence for indirect-call-promotion with vtable profiles. (PR #81442)

[Teresa Johnson via cfe-commits]

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@@ -103,27 +112,226 @@ static cl::opt<bool>
     ICPDUMPAFTER("icp-dumpafter", cl::init(false), cl::Hidden,
                  cl::desc("Dump IR after transformation happens"));
 
+// Indirect call promotion pass will fall back to function-based comparison if
+// vtable-count / function-count is smaller than this threshold.
+static cl::opt<float> ICPVTablePercentageThreshold(
+    "icp-vtable-percentage-threshold", cl::init(0.99), cl::Hidden,
+    cl::desc("The percentage threshold of vtable-count / function-count for "
+             "cost-benefit analysis. "));
+
+// Although comparing vtables can save a vtable load, we may need to compare
+// vtable pointer with multiple vtable address points due to class inheritance.
+// Comparing with multiple vtables inserts additional instructions on hot code
+// path; and doing so for earlier candidate of one icall can affect later
+// function candidate in an undesired way. We allow multiple vtable comparison
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teresajohnson wrote:

The wording "earlier candidate of one icall can affect later function candidate in an undesired way" is a little confusing to me. I think what you mean is that doing so for an earlier candidate delays the comparisons for later candidates, but that for the last candidate, only the fallback path is affected? Do we expect to set this parameter above 1?

https://github.com/llvm/llvm-project/pull/81442