[WebAssembly] Optimize away return instructions using fallthroughs.

This saves a small amount of code size, and is a first small step toward
passing values on the stack across block boundaries.

Differential Review: http://reviews.llvm.org/D20450

llvm-svn: 270294

6 files changed, 111 insertions, 10 deletions

diff --git a/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp b/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
index a450a24bd49..267d716dd1d 100644
--- a/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/InstPrinter/WebAssemblyInstPrinter.cpp
@@ -145,9 +145,9 @@ void WebAssemblyInstPrinter::printOperand(const MCInst *MI, unsigned OpNo,
     if (int(WAReg) >= 0)
       printRegName(O, WAReg);
     else if (OpNo >= MII.get(MI->getOpcode()).getNumDefs())
-      O << "$pop" << (WAReg & INT32_MAX);
+      O << "$pop" << WebAssemblyFunctionInfo::getWARegStackId(WAReg);
     else if (WAReg != WebAssemblyFunctionInfo::UnusedReg)
-      O << "$push" << (WAReg & INT32_MAX);
+      O << "$push" << WebAssemblyFunctionInfo::getWARegStackId(WAReg);
     else
       O << "$drop";
     // Add a '=' suffix if this is a def.
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
index d7a753d978b..64128bf031e 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -201,6 +201,30 @@ void WebAssemblyAsmPrinter::EmitInstruction(const MachineInstr *MI) {
     // These represent values which are live into the function entry, so there's
     // no instruction to emit.
     break;
+  case WebAssembly::FALLTHROUGH_RETURN_I32:
+  case WebAssembly::FALLTHROUGH_RETURN_I64:
+  case WebAssembly::FALLTHROUGH_RETURN_F32:
+  case WebAssembly::FALLTHROUGH_RETURN_F64: {
+    // These instructions represent the implicit return at the end of a
+    // function body. The operand is always a pop.
+    assert(MFI->isVRegStackified(MI->getOperand(0).getReg()));
+
+    if (isVerbose()) {
+      OutStreamer->AddComment("fallthrough-return: $pop" +
+                              utostr(MFI->getWARegStackId(
+                                  MFI->getWAReg(MI->getOperand(0).getReg()))));
+      OutStreamer->AddBlankLine();
+    }
+    break;
+  }
+  case WebAssembly::FALLTHROUGH_RETURN_VOID:
+    // This instruction represents the implicit return at the end of a
+    // function body with no return value.
+    if (isVerbose()) {
+      OutStreamer->AddComment("fallthrough-return");
+      OutStreamer->AddBlankLine();
+    }
+    break;
   default: {
     WebAssemblyMCInstLower MCInstLowering(OutContext, *this);
     MCInst TmpInst;
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
index ba4e1f989a9..444e275c6eb 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrControl.td
@@ -71,6 +71,10 @@ def END_LOOP  : I<(outs), (ins), [], "end_loop">;
 multiclass RETURN<WebAssemblyRegClass vt> {
   def RETURN_#vt : I<(outs), (ins vt:$val), [(WebAssemblyreturn vt:$val)],
                      "return  \t$val">;
+  // Equivalent to RETURN_#vt, for use at the end of a function when wasm
+  // semantics return by falling off the end of the block.
+  let isCodeGenOnly = 1 in
+  def FALLTHROUGH_RETURN_#vt : I<(outs), (ins vt:$val), []>;
 }
 
 let isTerminator = 1, hasCtrlDep = 1, isBarrier = 1 in {
@@ -80,6 +84,10 @@ let isReturn = 1 in {
   defm : RETURN<F32>;
   defm : RETURN<F64>;
   def RETURN_VOID : I<(outs), (ins), [(WebAssemblyreturn)], "return">;
+
+  // This is to RETURN_VOID what FALLTHROUGH_RETURN_#vt is to RETURN_#vt.
+  let isCodeGenOnly = 1 in
+  def FALLTHROUGH_RETURN_VOID : I<(outs), (ins), []>;
 } // isReturn = 1
   def UNREACHABLE : I<(outs), (ins), [(trap)], "unreachable">;
 } // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyMachineFunctionInfo.h b/llvm/lib/Target/WebAssembly/WebAssemblyMachineFunctionInfo.h
index 79950a6b257..c2f695db4a0 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyMachineFunctionInfo.h
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyMachineFunctionInfo.h
@@ -86,6 +86,12 @@ class WebAssemblyFunctionInfo final : public MachineFunctionInfo {
     assert(VReg = WARegs.size());
     WARegs.push_back(WAReg);
   }
+
+  // For a given stackified WAReg, return the id number to print with push/pop.
+  static unsigned getWARegStackId(unsigned Reg) {
+    assert(Reg & INT32_MIN);
+    return Reg & INT32_MAX;
+  }
 };
 
 } // end namespace llvm
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyPeephole.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyPeephole.cpp
index 19c227d2f94..56d44e6466e 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyPeephole.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyPeephole.cpp
@@ -12,17 +12,23 @@
 ///
 //===----------------------------------------------------------------------===//
 
-#include "WebAssembly.h"
 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
+#include "WebAssembly.h"
 #include "WebAssemblyMachineFunctionInfo.h"
 #include "WebAssemblySubtarget.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "wasm-peephole"
 
+static cl::opt<bool> DisableWebAssemblyFallthroughReturnOpt(
+    "disable-wasm-fallthrough-return-opt", cl::Hidden,
+    cl::desc("WebAssembly: Disable fallthrough-return optimizations."),
+    cl::init(false));
+
 namespace {
 class WebAssemblyPeephole final : public MachineFunctionPass {
   const char *getPassName() const override {
@@ -50,8 +56,7 @@ FunctionPass *llvm::createWebAssemblyPeephole() {
 
 /// If desirable, rewrite NewReg to a drop register.
 static bool MaybeRewriteToDrop(unsigned OldReg, unsigned NewReg,
-                               MachineOperand &MO,
-                               WebAssemblyFunctionInfo &MFI,
+                               MachineOperand &MO, WebAssemblyFunctionInfo &MFI,
                                MachineRegisterInfo &MRI) {
   bool Changed = false;
   if (OldReg == NewReg) {
@@ -60,19 +65,50 @@ static bool MaybeRewriteToDrop(unsigned OldReg, unsigned NewReg,
     MO.setReg(NewReg);
     MO.setIsDead();
     MFI.stackifyVReg(NewReg);
-    MFI.addWAReg(NewReg, WebAssemblyFunctionInfo::UnusedReg);
   }
   return Changed;
 }
 
+static bool MaybeRewriteToFallthrough(MachineInstr &MI, MachineBasicBlock &MBB,
+                                      const MachineFunction &MF,
+                                      WebAssemblyFunctionInfo &MFI,
+                                      MachineRegisterInfo &MRI,
+                                      const WebAssemblyInstrInfo &TII,
+                                      unsigned FallthroughOpc,
+                                      unsigned CopyLocalOpc) {
+  if (DisableWebAssemblyFallthroughReturnOpt)
+    return false;
+  if (&MBB != &MF.back())
+    return false;
+  if (&MI != &MBB.back())
+    return false;
+
+  // If the operand isn't stackified, insert a COPY_LOCAL to read the operand
+  // and stackify it.
+  MachineOperand &MO = MI.getOperand(0);
+  unsigned Reg = MO.getReg();
+  if (!MFI.isVRegStackified(Reg)) {
+    unsigned NewReg = MRI.createVirtualRegister(MRI.getRegClass(Reg));
+    BuildMI(MBB, MI, MI.getDebugLoc(), TII.get(CopyLocalOpc), NewReg)
+        .addReg(Reg);
+    MO.setReg(NewReg);
+    MFI.stackifyVReg(NewReg);
+  }
+
+  // Rewrite the return.
+  MI.setDesc(TII.get(FallthroughOpc));
+  return true;
+}
+
 bool WebAssemblyPeephole::runOnMachineFunction(MachineFunction &MF) {
   DEBUG({
-    dbgs() << "********** Store Results **********\n"
+    dbgs() << "********** Peephole **********\n"
            << "********** Function: " << MF.getName() << '\n';
   });
 
   MachineRegisterInfo &MRI = MF.getRegInfo();
   WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
+  const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
   const WebAssemblyTargetLowering &TLI =
       *MF.getSubtarget<WebAssemblySubtarget>().getTargetLowering();
   auto &LibInfo = getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
@@ -127,7 +163,34 @@ bool WebAssemblyPeephole::runOnMachineFunction(MachineFunction &MF) {
             }
           }
         }
+        break;
       }
+      // Optimize away an explicit void return at the end of the function.
+      case WebAssembly::RETURN_I32:
+        Changed |= MaybeRewriteToFallthrough(
+            MI, MBB, MF, MFI, MRI, TII, WebAssembly::FALLTHROUGH_RETURN_I32,
+            WebAssembly::COPY_LOCAL_I32);
+        break;
+      case WebAssembly::RETURN_I64:
+        Changed |= MaybeRewriteToFallthrough(
+            MI, MBB, MF, MFI, MRI, TII, WebAssembly::FALLTHROUGH_RETURN_I64,
+            WebAssembly::COPY_LOCAL_I64);
+        break;
+      case WebAssembly::RETURN_F32:
+        Changed |= MaybeRewriteToFallthrough(
+            MI, MBB, MF, MFI, MRI, TII, WebAssembly::FALLTHROUGH_RETURN_F32,
+            WebAssembly::COPY_LOCAL_F32);
+        break;
+      case WebAssembly::RETURN_F64:
+        Changed |= MaybeRewriteToFallthrough(
+            MI, MBB, MF, MFI, MRI, TII, WebAssembly::FALLTHROUGH_RETURN_F64,
+            WebAssembly::COPY_LOCAL_F64);
+        break;
+      case WebAssembly::RETURN_VOID:
+        if (!DisableWebAssemblyFallthroughReturnOpt &&
+            &MBB == &MF.back() && &MI == &MBB.back())
+          MI.setDesc(TII.get(WebAssembly::FALLTHROUGH_RETURN_VOID));
+        break;
       }
 
   return Changed;
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
index e4b049dd15d..32154af3c1c 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
@@ -225,10 +225,10 @@ void WebAssemblyPassConfig::addPreEmitPass() {
   // Lower br_unless into br_if.
   addPass(createWebAssemblyLowerBrUnless());
 
-  // Create a mapping from LLVM CodeGen virtual registers to wasm registers.
-  addPass(createWebAssemblyRegNumbering());
-
   // Perform the very last peephole optimizations on the code.
   if (getOptLevel() != CodeGenOpt::None)
     addPass(createWebAssemblyPeephole());
+
+  // Create a mapping from LLVM CodeGen virtual registers to wasm registers.
+  addPass(createWebAssemblyRegNumbering());
 }