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

[Cullen Rhodes via llvm-commits]

https://github.com/c-rhodes updated https://github.com/llvm/llvm-project/pull/222637

>From 544bb3d9ac28bc1a28248f6b03bcd95fa85b20e0 Mon Sep 17 00:00:00 2001
From: Cullen Rhodes <cullen.rhodes at arm.com>
Date: Mon, 7 Sep 2026 21:39:49 +0000
Subject: [PATCH 1/2] [AArch64] Cache repeated subtarget queries (NFC)

CodeGen frequently queries the subtarget for the same function. Cache
the last result while invalidating it when function attributes change.

Assisted-by: codex
---
 llvm/lib/Target/AArch64/AArch64TargetMachine.cpp | 13 +++++++++++--
 llvm/lib/Target/AArch64/AArch64TargetMachine.h   |  3 +++
 2 files changed, 14 insertions(+), 2 deletions(-)

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;

>From 4898c7146f7ebebb57f503dbdf58717d72a94995 Mon Sep 17 00:00:00 2001
From: Cullen Rhodes <cullen.rhodes at arm.com>
Date: Fri, 11 Sep 2026 08:25:53 +0000
Subject: [PATCH 2/2] add comment

---
 llvm/lib/Target/AArch64/AArch64TargetMachine.cpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
index 09a80980819c7..2e833f6729737 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -439,6 +439,8 @@ AArch64TargetMachine::~AArch64TargetMachine() = default;
 
 const AArch64Subtarget *
 AArch64TargetMachine::getSubtargetImpl(const Function &F) const {
+  // Constructing the subtarget key is not cheap, avoid rebuilding it for
+  // repeated queries with the same function attributes.
   AttributeSet FnAttrs = F.getAttributes().getFnAttrs();
   if (LastSubtarget && LastSubtargetAttrs == FnAttrs)
     return LastSubtarget;