[RISCV] Add VLOptimizer pass

The purpose of this optimization is to make the VL argument, for instructions
that have a VL argument, as small as possible. This is implemented by
visiting each instruction in reverse order and checking that if it has a VL
argument, whether the VL can be reduced.

This is done before vsetvli insertion to reduce the number of generated
vsetvlis. It can also reduce the number of vsetvli instructions that toggle
the VL (the vtype may still need to get set).

The list of supported instructions is currently whitelisted for
safety. In the future, we could add more instructions to isSupportedInstr
to support even more VL optimization.

Co-authored-by: Craig Topper <[email protected]>
Co-authored-by: Kito Cheng <[email protected]>

Author: 3 people

llvm/lib/Target/RISCV/CMakeLists.txt

@@ -61,6 +61,7 @@ add_llvm_target(RISCVCodeGen
   RISCVTargetObjectFile.cpp
   RISCVTargetTransformInfo.cpp
   RISCVVectorPeephole.cpp
+  RISCVVLOptimizer.cpp
   GISel/RISCVCallLowering.cpp
   GISel/RISCVInstructionSelector.cpp
   GISel/RISCVLegalizerInfo.cpp

llvm/lib/Target/RISCV/RISCV.h

@@ -102,6 +102,9 @@ void initializeRISCVPreLegalizerCombinerPass(PassRegistry &);
 
 FunctionPass *createRISCVPostLegalizerLowering();
 void initializeRISCVPostLegalizerLoweringPass(PassRegistry &);
+
+FunctionPass *createRISCVVLOptimizerPass();
+void initializeRISCVVLOptimizerPass(PassRegistry &);
 } // namespace llvm
 
 #endif

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -103,6 +103,10 @@ static cl::opt<bool> EnableVSETVLIAfterRVVRegAlloc(
     cl::desc("Insert vsetvls after vector register allocation"),
     cl::init(true));
 
+static cl::opt<bool> EnableVLOptimizer("riscv-enable-vloptimizer",
+                                       cl::desc("Enable the VL Optimizer pass"),
+                                       cl::init(true), cl::Hidden);
+
 extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeRISCVTarget() {
   RegisterTargetMachine<RISCVTargetMachine> X(getTheRISCV32Target());
   RegisterTargetMachine<RISCVTargetMachine> Y(getTheRISCV64Target());
@@ -552,8 +556,11 @@ void RISCVPassConfig::addMachineSSAOptimization() {
 
 void RISCVPassConfig::addPreRegAlloc() {
   addPass(createRISCVPreRAExpandPseudoPass());
-  if (TM->getOptLevel() != CodeGenOptLevel::None)
+  if (TM->getOptLevel() != CodeGenOptLevel::None) {
     addPass(createRISCVMergeBaseOffsetOptPass());
+    if (EnableVLOptimizer)
+      addPass(createRISCVVLOptimizerPass());
+  }
 
   addPass(createRISCVInsertReadWriteCSRPass());
   addPass(createRISCVInsertWriteVXRMPass());