[llvm] [RISCV][GISEL] Do not initialize GlobalISel objects unless needed (PR #98233)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland created https://github.com/llvm/llvm-project/pull/98233

Prior to this commit, we created the GlobalISel objects in the RISCVSubtarget constructor, even if we are not running GlobalISel. This patch moves creation of the GlobalISel objects into their getters, which ensures that we only create these objects if they are actually needed. This helps since the constructors of the GlobalISel objects have a significant amount of code.

We make the `unique_ptr`s `mutable` since GlobalISel passes only have access to `const TargetSubtargetInfo` through `MF.getSubtarget()`.

This patch is tested by the fact that all existing RISC-V GlobalISel tests remain passing.

>From ed09f6d1e44adf0e010963f9bea7f87d0c22fb99 Mon Sep 17 00:00:00 2001
From: Michael Maitland <michaeltmaitland at gmail.com>
Date: Tue, 9 Jul 2024 15:16:24 -0700
Subject: [PATCH] [RISCV][GISEL] Do not initialize GlobalISel objects unless
 needed

Prior to this commit, we created the GlobalISel objects in the
RISCVSubtarget constructor, even if we are not running GlobalISel. This
patch moves creation of the GlobalISel objects into their getters, which
ensures that we only create these objects if they are actually needed.
This helps since the constructors of the GlobalISel objects have a
significant amount of code.

We make the `unique_ptr`s `mutable` since GlobalISel passes only have
access to `const TargetSubtargetInfo` through `MF.getSubtarget()`.

This patch is tested by the fact that all existing RISC-V GlobalISel
tests remain passing.
---
 .../RISCV/GISel/RISCVInstructionSelector.cpp  |  2 +-
 llvm/lib/Target/RISCV/RISCV.h                 |  2 +-
 llvm/lib/Target/RISCV/RISCVSubtarget.cpp      | 23 +++++++++++--------
 llvm/lib/Target/RISCV/RISCVSubtarget.h        |  8 +++----
 4 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp b/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
index f511a20109803..d8ccab39c1f8a 100644
--- a/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
+++ b/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
@@ -1330,7 +1330,7 @@ void RISCVInstructionSelector::emitFence(AtomicOrdering FenceOrdering,
 namespace llvm {
 InstructionSelector *
 createRISCVInstructionSelector(const RISCVTargetMachine &TM,
-                               RISCVSubtarget &Subtarget,
+                               const RISCVSubtarget &Subtarget,
                                RISCVRegisterBankInfo &RBI) {
   return new RISCVInstructionSelector(TM, Subtarget, RBI);
 }
diff --git a/llvm/lib/Target/RISCV/RISCV.h b/llvm/lib/Target/RISCV/RISCV.h
index 8d2e1fc340c3e..f6ec6d9878e38 100644
--- a/llvm/lib/Target/RISCV/RISCV.h
+++ b/llvm/lib/Target/RISCV/RISCV.h
@@ -80,7 +80,7 @@ FunctionPass *createRISCVPushPopOptimizationPass();
 void initializeRISCVPushPopOptPass(PassRegistry &);
 
 InstructionSelector *createRISCVInstructionSelector(const RISCVTargetMachine &,
-                                                    RISCVSubtarget &,
+                                                    const RISCVSubtarget &,
                                                     RISCVRegisterBankInfo &);
 void initializeRISCVDAGToDAGISelLegacyPass(PassRegistry &);
 
diff --git a/llvm/lib/Target/RISCV/RISCVSubtarget.cpp b/llvm/lib/Target/RISCV/RISCVSubtarget.cpp
index e84ddc65e2b70..c06cd923834b7 100644
--- a/llvm/lib/Target/RISCV/RISCVSubtarget.cpp
+++ b/llvm/lib/Target/RISCV/RISCVSubtarget.cpp
@@ -97,29 +97,34 @@ RISCVSubtarget::RISCVSubtarget(const Triple &TT, StringRef CPU,
       RVVVectorBitsMin(RVVVectorBitsMin), RVVVectorBitsMax(RVVVectorBitsMax),
       FrameLowering(
           initializeSubtargetDependencies(TT, CPU, TuneCPU, FS, ABIName)),
-      InstrInfo(*this), RegInfo(getHwMode()), TLInfo(TM, *this) {
-  CallLoweringInfo.reset(new RISCVCallLowering(*getTargetLowering()));
-  Legalizer.reset(new RISCVLegalizerInfo(*this));
-
-  auto *RBI = new RISCVRegisterBankInfo(getHwMode());
-  RegBankInfo.reset(RBI);
-  InstSelector.reset(createRISCVInstructionSelector(
-      *static_cast<const RISCVTargetMachine *>(&TM), *this, *RBI));
-}
+      InstrInfo(*this), RegInfo(getHwMode()), TLInfo(TM, *this) {}
 
 const CallLowering *RISCVSubtarget::getCallLowering() const {
+  if (!CallLoweringInfo)
+    CallLoweringInfo.reset(new RISCVCallLowering(*getTargetLowering()));
   return CallLoweringInfo.get();
 }
 
 InstructionSelector *RISCVSubtarget::getInstructionSelector() const {
+  if (!InstSelector) {
+    if (!RegBankInfo)
+      RegBankInfo.reset(new RISCVRegisterBankInfo(getHwMode()));
+    InstSelector.reset(createRISCVInstructionSelector(
+        *static_cast<const RISCVTargetMachine *>(&TLInfo.getTargetMachine()),
+        *this, *static_cast<RISCVRegisterBankInfo *>(RegBankInfo.get())));
+  }
   return InstSelector.get();
 }
 
 const LegalizerInfo *RISCVSubtarget::getLegalizerInfo() const {
+  if (!Legalizer)
+    Legalizer.reset(new RISCVLegalizerInfo(*this));
   return Legalizer.get();
 }
 
 const RegisterBankInfo *RISCVSubtarget::getRegBankInfo() const {
+  if (!RegBankInfo)
+    RegBankInfo.reset(new RISCVRegisterBankInfo(getHwMode()));
   return RegBankInfo.get();
 }
 
diff --git a/llvm/lib/Target/RISCV/RISCVSubtarget.h b/llvm/lib/Target/RISCV/RISCVSubtarget.h
index 347c1bc3c278f..b146f48f81b72 100644
--- a/llvm/lib/Target/RISCV/RISCVSubtarget.h
+++ b/llvm/lib/Target/RISCV/RISCVSubtarget.h
@@ -245,10 +245,10 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
 
 protected:
   // GlobalISel related APIs.
-  std::unique_ptr<CallLowering> CallLoweringInfo;
-  std::unique_ptr<InstructionSelector> InstSelector;
-  std::unique_ptr<LegalizerInfo> Legalizer;
-  std::unique_ptr<RegisterBankInfo> RegBankInfo;
+  mutable std::unique_ptr<CallLowering> CallLoweringInfo;
+  mutable std::unique_ptr<InstructionSelector> InstSelector;
+  mutable std::unique_ptr<LegalizerInfo> Legalizer;
+  mutable std::unique_ptr<RegisterBankInfo> RegBankInfo;
 
   // Return the known range for the bit length of RVV data registers as set
   // at the command line. A value of 0 means nothing is known about that particular