[llvm] [IR] Reorder Value fields to put the SubclassID first (PR #75634)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

Author: Reid Kleckner (rnk)

<details>
<summary>Changes</summary>

Placing the class id at offset 0 should make `isa` and `dyn_cast` faster by eliminating the field offset (previously 0x10) from the memory operand, saving encoding space on x86, and, in theory, an add micro-op. You can see the load encodes one byte smaller here: https://godbolt.org/z/Whvz4can9

The compile time tracker shows some modestly positive results in the on the `cycle` metric and in the final clang binary size metric: https://llvm-compile-time-tracker.com/compare.php?from=33b54f01fe32030ff60d661a7a951e33360f82ee&to=2530347a57401744293c54f92f9781fbdae3d8c2&stat=cycles Clicking through to the per-library size breakdown shows that instcombine size reduces by 0.68%, which is meaningful, and I believe instcombine is known to be a hotspot.

It is, however, potentially noise. I still think we should do this, because notionally, the class id really acts as the vptr of the Value, and conventionally the vptr is always at offset 0.

---
Full diff: https://github.com/llvm/llvm-project/pull/75634.diff


2 Files Affected:

- (modified) llvm/include/llvm/IR/Value.h (+6-6) 
- (modified) llvm/lib/IR/Value.cpp (+3-3) 


``````````diff
diff --git a/llvm/include/llvm/IR/Value.h b/llvm/include/llvm/IR/Value.h
index 8fce56e12c5235..61f9d34eef3567 100644
--- a/llvm/include/llvm/IR/Value.h
+++ b/llvm/include/llvm/IR/Value.h
@@ -72,12 +72,6 @@ using ValueName = StringMapEntry<Value *>;
 /// objects that watch it and listen to RAUW and Destroy events.  See
 /// llvm/IR/ValueHandle.h for details.
 class Value {
-  Type *VTy;
-  Use *UseList;
-
-  friend class ValueAsMetadata; // Allow access to IsUsedByMD.
-  friend class ValueHandleBase;
-
   const unsigned char SubclassID;   // Subclass identifier (for isa/dyn_cast)
   unsigned char HasValueHandle : 1; // Has a ValueHandle pointing to this?
 
@@ -121,6 +115,12 @@ class Value {
   unsigned HasDescriptor : 1;
 
 private:
+  Type *VTy;
+  Use *UseList;
+
+  friend class ValueAsMetadata; // Allow access to IsUsedByMD.
+  friend class ValueHandleBase; // Allow access to HasValueHandle.
+
   template <typename UseT> // UseT == 'Use' or 'const Use'
   class use_iterator_impl {
     friend class Value;
diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index b6e25c46b514d8..94bb8ca6c6726a 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -51,9 +51,9 @@ static inline Type *checkType(Type *Ty) {
 }
 
 Value::Value(Type *ty, unsigned scid)
-    : VTy(checkType(ty)), UseList(nullptr), SubclassID(scid), HasValueHandle(0),
-      SubclassOptionalData(0), SubclassData(0), NumUserOperands(0),
-      IsUsedByMD(false), HasName(false), HasMetadata(false) {
+    : SubclassID(scid), HasValueHandle(0), SubclassOptionalData(0),
+      SubclassData(0), NumUserOperands(0), IsUsedByMD(false), HasName(false),
+      HasMetadata(false), VTy(checkType(ty)), UseList(nullptr) {
   static_assert(ConstantFirstVal == 0, "!(SubclassID < ConstantFirstVal)");
   // FIXME: Why isn't this in the subclass gunk??
   // Note, we cannot call isa<CallInst> before the CallInst has been

``````````

</details>


https://github.com/llvm/llvm-project/pull/75634