[LSR] Split the -lsr-term-fold transformation into it's own pass (#104234)

This transformation doesn't actually use any of the internal state of
LSR and recomputes all information from SCEV.  Splitting it out makes
it easier to test.
    
Note that long term I would like to write a version of this transform
which *is* integrated with LSR's solver, but if that happens, we'll
just delete the extra pass.
    
Integration wise, I switched from using TTI to using a pass configuration
variable.  This seems slightly more idiomatic, and means we don't run
the extra logic on any target other than RISCV.

Author: preames

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -739,11 +739,6 @@ class TargetTransformInfo {
   /// cost should return false, otherwise return true.
   bool isNumRegsMajorCostOfLSR() const;
 
-  /// Return true if LSR should attempts to replace a use of an otherwise dead
-  /// primary IV in the latch condition with another IV available in the loop.
-  /// When successful, makes the primary IV dead.
-  bool shouldFoldTerminatingConditionAfterLSR() const;
-
   /// Return true if LSR should drop a found solution if it's calculated to be
   /// less profitable than the baseline.
   bool shouldDropLSRSolutionIfLessProfitable() const;
@@ -1888,7 +1883,6 @@ class TargetTransformInfo::Concept {
   virtual bool isLSRCostLess(const TargetTransformInfo::LSRCost &C1,
                              const TargetTransformInfo::LSRCost &C2) = 0;
   virtual bool isNumRegsMajorCostOfLSR() = 0;
-  virtual bool shouldFoldTerminatingConditionAfterLSR() const = 0;
   virtual bool shouldDropLSRSolutionIfLessProfitable() const = 0;
   virtual bool isProfitableLSRChainElement(Instruction *I) = 0;
   virtual bool canMacroFuseCmp() = 0;
@@ -2367,9 +2361,6 @@ class TargetTransformInfo::Model final : public TargetTransformInfo::Concept {
   bool isNumRegsMajorCostOfLSR() override {
     return Impl.isNumRegsMajorCostOfLSR();
   }
-  bool shouldFoldTerminatingConditionAfterLSR() const override {
-    return Impl.shouldFoldTerminatingConditionAfterLSR();
-  }
   bool shouldDropLSRSolutionIfLessProfitable() const override {
     return Impl.shouldDropLSRSolutionIfLessProfitable();
   }

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

@@ -244,8 +244,6 @@ class TargetTransformInfoImplBase {
 
   bool isNumRegsMajorCostOfLSR() const { return true; }
 
-  bool shouldFoldTerminatingConditionAfterLSR() const { return false; }
-
   bool shouldDropLSRSolutionIfLessProfitable() const { return false; }
 
   bool isProfitableLSRChainElement(Instruction *I) const { return false; }

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -394,11 +394,6 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
     return TargetTransformInfoImplBase::isNumRegsMajorCostOfLSR();
   }
 
-  bool shouldFoldTerminatingConditionAfterLSR() const {
-    return TargetTransformInfoImplBase::
-        shouldFoldTerminatingConditionAfterLSR();
-  }
-
   bool shouldDropLSRSolutionIfLessProfitable() const {
     return TargetTransformInfoImplBase::shouldDropLSRSolutionIfLessProfitable();
   }

llvm/include/llvm/CodeGen/TargetPassConfig.h

@@ -140,6 +140,9 @@ class TargetPassConfig : public ImmutablePass {
   /// callers.
   bool RequireCodeGenSCCOrder = false;
 
+  /// Enable LoopTermFold immediately after LSR
+  bool EnableLoopTermFold = false;
+
   /// Add the actual instruction selection passes. This does not include
   /// preparation passes on IR.
   bool addCoreISelPasses();

llvm/include/llvm/InitializePasses.h

@@ -169,6 +169,7 @@ void initializeLoopInfoWrapperPassPass(PassRegistry &);
 void initializeLoopPassPass(PassRegistry &);
 void initializeLoopSimplifyPass(PassRegistry &);
 void initializeLoopStrengthReducePass(PassRegistry &);
+void initializeLoopTermFoldPass(PassRegistry &);
 void initializeLoopUnrollPass(PassRegistry &);
 void initializeLowerAtomicLegacyPassPass(PassRegistry &);
 void initializeLowerConstantIntrinsicsPass(PassRegistry &);

llvm/include/llvm/LinkAllPasses.h

@@ -90,6 +90,7 @@ struct ForcePassLinking {
     (void)llvm::createLoopExtractorPass();
     (void)llvm::createLoopSimplifyPass();
     (void)llvm::createLoopStrengthReducePass();
+    (void)llvm::createLoopTermFoldPass();
     (void)llvm::createLoopUnrollPass();
     (void)llvm::createLowerGlobalDtorsLegacyPass();
     (void)llvm::createLowerInvokePass();

llvm/include/llvm/Passes/MachinePassRegistry.def

@@ -79,6 +79,7 @@ FUNCTION_PASS("win-eh-prepare", WinEHPreparePass())
 #define LOOP_PASS(NAME, CREATE_PASS)
 #endif
 LOOP_PASS("loop-reduce", LoopStrengthReducePass())
+LOOP_PASS("loop-term-fold", LoopTermFoldPass())
 #undef LOOP_PASS
 
 #ifndef MACHINE_MODULE_PASS

llvm/include/llvm/Transforms/Scalar.h

@@ -51,6 +51,14 @@ Pass *createLICMPass();
 //
 Pass *createLoopStrengthReducePass();
 
+//===----------------------------------------------------------------------===//
+//
+// LoopTermFold -  This pass attempts to eliminate the last use of an IV in
+// a loop terminator instruction by rewriting it in terms of another IV.
+// Expected to be run immediately after LSR.
+//
+Pass *createLoopTermFoldPass();
+
 //===----------------------------------------------------------------------===//
 //
 // LoopUnroll - This pass is a simple loop unrolling pass.

llvm/include/llvm/Transforms/Scalar/LoopTermFold.h

@@ -0,0 +1,30 @@
+//===- LoopTermFold.h - Loop Term Fold Pass ---------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_SCALAR_LOOPTERMFOLD_H
+#define LLVM_TRANSFORMS_SCALAR_LOOPTERMFOLD_H
+
+#include "llvm/Analysis/LoopAnalysisManager.h"
+#include "llvm/IR/PassManager.h"
+
+namespace llvm {
+
+class Loop;
+class LPMUpdater;
+
+class LoopTermFoldPass : public PassInfoMixin<LoopTermFoldPass> {
+public:
+  PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
+                        LoopStandardAnalysisResults &AR, LPMUpdater &U);
+};
+
+} // end namespace llvm
+
+#endif // LLVM_TRANSFORMS_SCALAR_LOOPTERMFOLD_H

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -427,10 +427,6 @@ bool TargetTransformInfo::isNumRegsMajorCostOfLSR() const {
   return TTIImpl->isNumRegsMajorCostOfLSR();
 }
 
-bool TargetTransformInfo::shouldFoldTerminatingConditionAfterLSR() const {
-  return TTIImpl->shouldFoldTerminatingConditionAfterLSR();
-}
-
 bool TargetTransformInfo::shouldDropLSRSolutionIfLessProfitable() const {
   return TTIImpl->shouldDropLSRSolutionIfLessProfitable();
 }