[RISCV] Enable SeparateConstOffsetFromGEPPass for RISC-V

We see benefits in terms of performance.
CoreMark benchmarking demonstrates a 2.5% performance improvement.

Author: djtodoro

llvm/lib/Target/RISCV/RISCVTargetMachine.cpp

@@ -81,6 +81,10 @@ static cl::opt<bool> EnableRISCVCopyPropagation(
     "riscv-enable-copy-propagation",
     cl::desc("Enable the copy propagation with RISC-V copy instr"),
     cl::init(true), cl::Hidden);
+static cl::opt<bool>
+    EnableGEPOpt("riscv-enable-gep-opt", cl::Hidden,
+                 cl::desc("Enable optimizations on complex GEPs"),
+                 cl::init(false));
 
 static cl::opt<bool> EnableRISCVDeadRegisterElimination(
     "riscv-enable-dead-defs", cl::Hidden,
@@ -373,7 +377,6 @@ class RISCVPassConfig : public TargetPassConfig {
     if (!ST.getMacroFusions().empty()) {
       DAG = DAG ? DAG : createGenericSchedLive(C);
 
-      const RISCVSubtarget &ST = C->MF->getSubtarget<RISCVSubtarget>();
       if (ST.useLoadStorePairs()) {
         DAG->addMutation(createLoadClusterDAGMutation(DAG->TII, DAG->TRI));
         DAG->addMutation(createStoreClusterDAGMutation(DAG->TII, DAG->TRI));
@@ -466,6 +469,16 @@ void RISCVPassConfig::addIRPasses() {
   addPass(createAtomicExpandLegacyPass());
   addPass(createRISCVZacasABIFixPass());
 
+  if (TM->getOptLevel() == CodeGenOptLevel::Aggressive && EnableGEPOpt) {
+    addPass(createSeparateConstOffsetFromGEPPass(false));
+    // Call EarlyCSE pass to find and remove subexpressions in the lowered
+    // result.
+    addPass(createEarlyCSEPass());
+    // Do loop invariant code motion in case part of the lowered result is
+    // invariant.
+    addPass(createLICMPass());
+  }
+
   if (getOptLevel() != CodeGenOptLevel::None) {
     if (EnableLoopDataPrefetch)
       addPass(createLoopDataPrefetchPass());

llvm/test/CodeGen/RISCV/riscv-enable-gep-opt.ll

@@ -0,0 +1,10 @@
+; RUN: llc -mtriple=riscv32 -O3 -riscv-enable-gep-opt=true -debug-pass=Structure < %s -o /dev/null 2>&1 | \
+; RUN:   grep -v "Verify generated machine code" | \
+; RUN:   FileCheck %s --check-prefixes=CHECK
+
+
+; REQUIRES: asserts
+
+; CHECK-LABEL: Pass Arguments:
+; CHECK: Split GEPs to a variadic base and a constant offset for better CSE
+