[BOLT] Make safe ICF work with relative vtable (#170629)

To support relative vtable, we divide each vtable entry address by 4
instead of 8 when tracking addresses covered by vtable using bitvector.
This guarantees accuracy, saves the trouble of checking for each vtable
whether it is relative, and is also necessary for libraries for which
some contributing compilation units are compiled with relative vtable
while others are not.

Author: yozhu

bolt/include/bolt/Passes/IdenticalCodeFolding.h

@@ -37,17 +37,19 @@ class IdenticalCodeFolding : public BinaryFunctionPass {
   Error runOnFunctions(BinaryContext &BC) override;
 
 private:
+  static constexpr uint64_t VTableAddressGranularity = 4;
+
   /// Bit vector of memory addresses of vtables.
   llvm::SparseBitVector<> VTableBitVector;
 
   /// Return true if the memory address is in a vtable.
   bool isAddressInVTable(uint64_t Address) const {
-    return VTableBitVector.test(Address / 8);
+    return VTableBitVector.test(Address / VTableAddressGranularity);
   }
 
   /// Mark memory address of a vtable as used.
   void setAddressUsedInVTable(uint64_t Address) {
-    VTableBitVector.set(Address / 8);
+    VTableBitVector.set(Address / VTableAddressGranularity);
   }
 
   /// Scan symbol table and mark memory addresses of

bolt/lib/Passes/IdenticalCodeFolding.cpp

@@ -380,7 +380,8 @@ void IdenticalCodeFolding::initVTableReferences(const BinaryContext &BC) {
     if (!Data->getName().starts_with("_ZTV") && // vtable
         !Data->getName().starts_with("_ZTCN"))  // construction vtable
       continue;
-    for (uint64_t I = Address, End = I + Data->getSize(); I < End; I += 8)
+    for (uint64_t I = Address, End = I + Data->getSize(); I < End;
+         I += VTableAddressGranularity)
       setAddressUsedInVTable(I);
   }
 }

bolt/test/safe-icf-relative-vtable.cpp

@@ -0,0 +1,26 @@
+// Test safe ICF works with binaries that contain relative vtable.
+
+// REQUIRES: system-linux,asserts
+
+// RUN: %clang %cxxflags -o %t.so %s -Wl,-q -fno-rtti
+// RUN: llvm-bolt %t.so -o %t.bolt --no-threads --icf=safe \
+// RUN:   --debug-only=bolt-icf 2>&1 | FileCheck %s
+
+// RUN: %clang %cxxflags -o %t.so %s -Wl,-q -fno-rtti \
+// RUN:   -fexperimental-relative-c++-abi-vtables
+// RUN: llvm-bolt %t.so -o %t.bolt --no-threads --icf=safe \
+// RUN:   --debug-only=bolt-icf 2>&1 | FileCheck %s
+
+// CHECK: folding {{.*bar.*}} into {{.*foo.*}}
+// CHECK-NOT: skipping function with reference taken {{.*bar.*}}
+
+class TT {
+public:
+  virtual int foo(int a) { return ++a; }
+  virtual int bar(int a) { return ++a; }
+};
+
+int main() {
+  TT T;
+  return T.foo(0) + T.bar(1);
+}