[CallPromotionUtil] See through function alias when devirtualizing a virtual call on an alloca.

[minglotus-6]

• Extract utility function from DevirtModule::tryFindVirtualCallTargets, which sees through an alias to a function. Call this utility function in the WPD callsite.
• For type profiling work, this helper function will be used by indirect-call-promotion pass to find the function pointer at a specified vtable offset (an example in this line)

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Mingming Liu (minglotus-6)

Changes

• The utility function is extracted from the implementation of DevirtModule::tryFindVirtualCallTargets

---

Full diff: https://github.com/llvm/llvm-project/pull/80736.diff

4 Files Affected:

• (modified) llvm/include/llvm/Analysis/TypeMetadataUtils.h (+5)
• (modified) llvm/lib/Analysis/TypeMetadataUtils.cpp (+17)
• (modified) llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp (+4-6)
• (modified) llvm/lib/Transforms/Utils/CallPromotionUtils.cpp (+4-7)

diff --git a/llvm/include/llvm/Analysis/TypeMetadataUtils.h b/llvm/include/llvm/Analysis/TypeMetadataUtils.h
index dab67aad1ab0e..9f8c364b49375 100644
--- a/llvm/include/llvm/Analysis/TypeMetadataUtils.h
+++ b/llvm/include/llvm/Analysis/TypeMetadataUtils.h
@@ -14,6 +14,7 @@
 #ifndef LLVM_ANALYSIS_TYPEMETADATAUTILS_H
 #define LLVM_ANALYSIS_TYPEMETADATAUTILS_H
 
+#include "llvm/IR/GlobalVariable.h"
 #include <cstdint>
 
 namespace llvm {
@@ -77,6 +78,10 @@ void findDevirtualizableCallsForTypeCheckedLoad(
 Constant *getPointerAtOffset(Constant *I, uint64_t Offset, Module &M,
                              Constant *TopLevelGlobal = nullptr);
 
+// Given a vtable, returns the function pointer specified by Offset.
+std::pair<Function *, Constant *>
+getFunctionAtVTableOffset(GlobalVariable *GV, uint64_t Offset, Module &M);
+
 /// Finds the same "relative pointer" pattern as described above, where the
 /// target is `F`, and replaces the entire pattern with a constant zero.
 void replaceRelativePointerUsersWithZero(Function *F);
diff --git a/llvm/lib/Analysis/TypeMetadataUtils.cpp b/llvm/lib/Analysis/TypeMetadataUtils.cpp
index bbaee06ed8a55..0e6859f622a7d 100644
--- a/llvm/lib/Analysis/TypeMetadataUtils.cpp
+++ b/llvm/lib/Analysis/TypeMetadataUtils.cpp
@@ -201,6 +201,23 @@ Constant *llvm::getPointerAtOffset(Constant *I, uint64_t Offset, Module &M,
   return nullptr;
 }
 
+std::pair<Function *, Constant *>
+llvm::getFunctionAtVTableOffset(GlobalVariable *GV, uint64_t Offset,
+                                Module &M) {
+  Constant *Ptr = getPointerAtOffset(GV->getInitializer(), Offset, M, GV);
+  if (!Ptr)
+    return std::pair<Function *, Constant *>(nullptr, nullptr);
+
+  auto C = Ptr->stripPointerCasts();
+  // Make sure this is a function or alias to a function.
+  auto Fn = dyn_cast<Function>(C);
+  auto A = dyn_cast<GlobalAlias>(C);
+  if (!Fn && A)
+    Fn = dyn_cast<Function>(A->getAliasee());
+
+  return std::pair<Function *, Constant *>(Fn, C);
+}
+
 void llvm::replaceRelativePointerUsersWithZero(Function *F) {
   for (auto *U : F->users()) {
     auto *PtrExpr = dyn_cast<ConstantExpr>(U);
diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 01aba47cdbfff..154ff876a53cd 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1071,12 +1071,10 @@ bool DevirtModule::tryFindVirtualCallTargets(
     if (!Ptr)
       return false;
 
-    auto C = Ptr->stripPointerCasts();
-    // Make sure this is a function or alias to a function.
-    auto Fn = dyn_cast<Function>(C);
-    auto A = dyn_cast<GlobalAlias>(C);
-    if (!Fn && A)
-      Fn = dyn_cast<Function>(A->getAliasee());
+    Function *Fn = nullptr;
+    Constant *C = nullptr;
+    std::tie(Fn, C) =
+        getFunctionAtVTableOffset(TM.Bits->GV, TM.Offset + ByteOffset, M);
 
     if (!Fn)
       return false;
diff --git a/llvm/lib/Transforms/Utils/CallPromotionUtils.cpp b/llvm/lib/Transforms/Utils/CallPromotionUtils.cpp
index e42cdab64446e..4e84927f1cfc9 100644
--- a/llvm/lib/Transforms/Utils/CallPromotionUtils.cpp
+++ b/llvm/lib/Transforms/Utils/CallPromotionUtils.cpp
@@ -597,16 +597,13 @@ bool llvm::tryPromoteCall(CallBase &CB) {
     // Not in the form of a global constant variable with an initializer.
     return false;
 
-  Constant *VTableGVInitializer = GV->getInitializer();
   APInt VTableGVOffset = VTableOffsetGVBase + VTableOffset;
   if (!(VTableGVOffset.getActiveBits() <= 64))
     return false; // Out of range.
-  Constant *Ptr = getPointerAtOffset(VTableGVInitializer,
-                                     VTableGVOffset.getZExtValue(),
-                                     *M);
-  if (!Ptr)
-    return false; // No constant (function) pointer found.
-  Function *DirectCallee = dyn_cast<Function>(Ptr->stripPointerCasts());
+
+  Function *DirectCallee = nullptr;
+  std::tie(DirectCallee, std::ignore) =
+      getFunctionAtVTableOffset(GV, VTableGVOffset.getZExtValue(), *M);
   if (!DirectCallee)
     return false; // No function pointer found.
 

[teresajohnson]

Suggest making the PR title shorter and moving some of the details there currently to the description. Also, I think this can be marked NFC?

[teresajohnson]

Can you instead just use a forward declaration for GlobalVariable, like we do for other types below?

[minglotus-6]

done (along with #include <utility>).

TIL that forward declaration is almost always better,thanks!

[teresajohnson]

Shouldn't this earlier call to getPointerAtOffset be removed now?

[minglotus-6]

ah good catch. done.

[teresajohnson]

Document the pair contents.

[minglotus-6]

done. Also updated function impl to return a pair of nullptr when the not-null pointer cannot be casted to function and is not an alias to a function.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[minglotus-6]

Suggest making the PR title shorter and moving some of the details there currently to the description. Also, I think this can be marked NFC?

Makes sense to make PR title shorter and move implementation details. It's not strictly an NFC (not stated in a simple way in prior title), rephrased the title.