[CGSCC] Add statistic on largest SCC visited (#128073)

To help debugging long compile times.

Author: aeubanks

llvm/lib/Analysis/CGSCCPassManager.cpp

@@ -13,6 +13,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Analysis/LazyCallGraph.h"
 #include "llvm/IR/Constant.h"
@@ -33,6 +34,8 @@
 
 using namespace llvm;
 
+STATISTIC(LargestCGSCC, "Number of functions in the largest SCC");
+
 // Explicit template instantiations and specialization definitions for core
 // template typedefs.
 namespace llvm {
@@ -82,6 +85,8 @@ PassManager<LazyCallGraph::SCC, CGSCCAnalysisManager, LazyCallGraph &,
     if (!PI.runBeforePass(*Pass, *C))
       continue;
 
+    LargestCGSCC.updateMax(C->size());
+
     PreservedAnalyses PassPA = Pass->run(*C, AM, G, UR);
 
     // Update the SCC if necessary.

llvm/test/Other/largest-scc-stat.ll

@@ -0,0 +1,18 @@
+; REQUIRES: asserts
+; RUN: opt -passes='no-op-cgscc' -disable-output -stats < %s 2>&1 | FileCheck %s --check-prefix=ONE
+; RUN: opt -passes='cgscc(instcombine)' -disable-output -stats < %s 2>&1 | FileCheck %s --check-prefix=TWO
+
+; ONE: 1 cgscc - Number of functions in the largest SCC
+; TWO: 2 cgscc - Number of functions in the largest SCC
+
+define void @f1() {
+  %f = bitcast ptr @f2 to ptr
+  call void %f()
+  ret void
+}
+
+define void @f2() {
+  %f = bitcast ptr @f1 to ptr
+  call void %f()
+  ret void
+}