[llvm] [AArch64] Cache repeated subtarget queries (NFC) (PR #222637)

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llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: Cullen Rhodes (c-rhodes)

<details>
<summary>Changes</summary>

Cache the last subtarget along with the function attributes used to select it. This allows reuse across both repeated queries for the same function and across functions with identical attributes.

Improves CTMark geomean -0.10% on aarch64-O3 and -0.17 on aarch64-O0-g.

https://llvm-compile-time-tracker.com/compare.php?from=87d9c3c24ff5941ac08799850a6549b561c98a81&to=544bb3d9ac28bc1a28248f6b03bcd95fa85b20e0&stat=instructions:u

Assisted-by: codex

---
Full diff: https://github.com/llvm/llvm-project/pull/222637.diff


2 Files Affected:

- (modified) llvm/lib/Target/AArch64/AArch64TargetMachine.cpp (+11-2) 
- (modified) llvm/lib/Target/AArch64/AArch64TargetMachine.h (+3) 


``````````diff
diff --git a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
index 8195ad04c3556..09a80980819c7 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -296,7 +296,10 @@ bool AArch64TargetMachine::isGlobalISelOptNone() const {
           !GlobalISelFlag);
 }
 
-void AArch64TargetMachine::reset() { SubtargetMap.clear(); }
+void AArch64TargetMachine::reset() {
+  SubtargetMap.clear();
+  LastSubtarget = nullptr;
+}
 
 //===----------------------------------------------------------------------===//
 // AArch64 Lowering public interface.
@@ -436,6 +439,10 @@ AArch64TargetMachine::~AArch64TargetMachine() = default;
 
 const AArch64Subtarget *
 AArch64TargetMachine::getSubtargetImpl(const Function &F) const {
+  AttributeSet FnAttrs = F.getAttributes().getFnAttrs();
+  if (LastSubtarget && LastSubtargetAttrs == FnAttrs)
+    return LastSubtarget;
+
   Attribute CPUAttr = F.getFnAttribute("target-cpu");
   Attribute TuneAttr = F.getFnAttribute("tune-cpu");
   Attribute FSAttr = F.getFnAttribute("target-features");
@@ -503,7 +510,9 @@ AArch64TargetMachine::getSubtargetImpl(const Function &F) const {
   if (IsStreaming && !I->hasSME())
     reportFatalUsageError("streaming SVE functions require SME");
 
-  return I.get();
+  LastSubtargetAttrs = FnAttrs;
+  LastSubtarget = I.get();
+  return LastSubtarget;
 }
 
 // Encourage placing FORM_TRANSPOSED_REG immediately before the instruction that
diff --git a/llvm/lib/Target/AArch64/AArch64TargetMachine.h b/llvm/lib/Target/AArch64/AArch64TargetMachine.h
index 98eaecbb6d2af..28b9a0144abfe 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetMachine.h
+++ b/llvm/lib/Target/AArch64/AArch64TargetMachine.h
@@ -16,6 +16,7 @@
 #include "AArch64InstrInfo.h"
 #include "AArch64Subtarget.h"
 #include "llvm/CodeGen/CodeGenTargetMachineImpl.h"
+#include "llvm/IR/Attributes.h"
 #include "llvm/IR/DataLayout.h"
 #include <optional>
 
@@ -25,6 +26,8 @@ class AArch64TargetMachine : public CodeGenTargetMachineImpl {
 protected:
   std::unique_ptr<TargetLoweringObjectFile> TLOF;
   mutable StringMap<std::unique_ptr<AArch64Subtarget>> SubtargetMap;
+  mutable AttributeSet LastSubtargetAttrs;
+  mutable const AArch64Subtarget *LastSubtarget = nullptr;
 
   /// Reset internal state.
   void reset() override;

``````````

</details>


https://github.com/llvm/llvm-project/pull/222637