Split functions to create shrink wrapping opportunities

[tjablin]

Summary

Shrink wrapping functions can improve performance for functions
when the stack is unused. However, all functions where value
live-ranges contain function calls are ineligible for shrink
wrapping. Why?

1. To avoid saving and restoring values across call-sites, LLVM

allocates values with live-ranges that cross call sites to
callee-saved registers

2. If callee-saved registers are used, they must be

saved (usually in the function prolog)

3. ShrinkWrapping scans from the beginning of the function prolog

to the first use of the stack. If a callee-saved register is
saved to the stack in the function prolog, ShrinkWrapping will
give up early.

To increase the applicability of ShrinkWrapping, this pass
identifies instances where a live-range straddling a call-site
prevents ShrinkWrapping, and splits the original function into a
stackless ShrinkWrappable stub that potentially makes a tail call
to the remainder of the function.

This transformation harms debuggability and consequently is not
suitable for lower -O levels.

[hfinkel]

Before we move forward with this, an alternate strategy for this has seemingly presented itself (yesterday) which we should investigate: Enabling saving callee-saved registers by copying.

Please see http://reviews.llvm.org/D15340 (and http://reviews.llvm.org/D15341). Can we build on this somehow to avoid the need to split the functions early, but rather avoid the need for saving the registers in the prologue thus enabling us to shrink wrap the original function?

lib/Target/PowerPC/PPCEnableShrinkWrap.cpp

85

Debug intrinsics also need skipping.

103

There are many other things that use the stack, at least <= P7. int <-> fp conversions, for example, and vector insert/extracts (except with QPX).

lib/Target/PowerPC/PPCTargetMachine.cpp

306

Please don't reformat this whole section of code; please make a minimal change.

[gberry]

It doesn't seem like there is anything target specific in this transform pass. On the chance that this moves forward, would it make sense to move the transform into lib/Transforms and just have ppc add it for now? That way other targets could potentially take advantage of this pass too.

[tjablin]

Per Hal's comments, use a more conservative behavior in the case of Extract and Cast Instructions.

I explored the enabling saving callee-saved registers by copying as in http://reviews.llvm.org/D15340 and http://reviews.llvm.org/D15341, but I think this solution is less invasive and less likely to harm performance when tail calls optimization is enabled. In particular, I am worried that saving callee-saved registers by copying could harm performance in tight loops.

[majnemer]

lib/Target/PowerPC/PPCEnableShrinkWrap.cpp

35

Shouldn't this be sorted lower?

68

Please follow the LLVM coding standard for naming variables: http://llvm.org/docs/CodingStandards.html#name-types-functions-variables-and-enumerators-properly

75

llvm:: should be unnecessary.

[tjablin]

Fix style errors pointed out by David.

[tjablin]

Replaced with D17948, D17533, and D16984.

lib/Target/PowerPC/CMakeLists.txt

add_llvm_target(PowerPCCodeGen
   PPCAsmPrinter.cpp
   PPCBranchSelector.cpp
   PPCCTRLoops.cpp
   PPCHazardRecognizers.cpp
   PPCInstrInfo.cpp
   PPCISelDAGToDAG.cpp
   PPCISelLowering.cpp
   PPCEarlyReturn.cpp
+  PPCEnableShrinkWrap.cpp
   PPCFastISel.cpp
   PPCFrameLowering.cpp
   PPCLoopDataPrefetch.cpp
   PPCLoopPreIncPrep.cpp
   PPCMCInstLower.cpp
   PPCMachineFunctionInfo.cpp
   PPCMIPeephole.cpp
   PPCRegisterInfo.cpp

lib/Target/PowerPC/PPC.h

 #undef PPC
 
 namespace llvm {
   class PPCTargetMachine;
   class PassRegistry;
   class FunctionPass;
   class ImmutablePass;
   class MachineInstr;
+  class ModulePass;
   class AsmPrinter;
   class MCInst;
 
   FunctionPass *createPPCCTRLoops(PPCTargetMachine &TM);
 #ifndef NDEBUG
   FunctionPass *createPPCCTRLoopsVerify();
 #endif
   FunctionPass *createPPCLoopDataPrefetchPass();
   FunctionPass *createPPCLoopPreIncPrepPass(PPCTargetMachine &TM);
   FunctionPass *createPPCTOCRegDepsPass();
   FunctionPass *createPPCEarlyReturnPass();
   FunctionPass *createPPCVSXCopyPass();
   FunctionPass *createPPCVSXFMAMutatePass();
   FunctionPass *createPPCVSXSwapRemovalPass();
   FunctionPass *createPPCMIPeepholePass();
   FunctionPass *createPPCBranchSelectionPass();
   FunctionPass *createPPCISelDag(PPCTargetMachine &TM);
   FunctionPass *createPPCTLSDynamicCallPass();
   FunctionPass *createPPCBoolRetToIntPass();
+  ModulePass *createPPCEnableShrinkWrapPass();
   void LowerPPCMachineInstrToMCInst(const MachineInstr *MI, MCInst &OutMI,
                                     AsmPrinter &AP, bool isDarwin);
 
   void initializePPCVSXFMAMutatePass(PassRegistry&);
   void initializePPCBoolRetToIntPass(PassRegistry&);
+  void initializePPCEnableShrinkWrapPass(PassRegistry&);
   extern char &PPCVSXFMAMutateID;
 
   namespace PPCII {
-    
   /// Target Operand Flag enum.
   enum TOF {
     //===------------------------------------------------------------------===//
     // PPC Specific MachineOperand flags.
     MO_NO_FLAG,
-    
     /// MO_PLT_OR_STUB - On a symbol operand "FOO", this indicates that the
     /// reference is actually to the "FOO$stub" or "FOO@plt" symbol.  This is
     /// used for calls and jumps to external functions on Tiger and earlier, and
     /// for PIC calls on Linux and ELF systems.
     MO_PLT_OR_STUB = 1,
-    
     /// MO_PIC_FLAG - If this bit is set, the symbol reference is relative to
     /// the function's picbase, e.g. lo16(symbol-picbase).
     MO_PIC_FLAG = 2,
 
     /// MO_NLP_FLAG - If this bit is set, the symbol reference is actually to
     /// the non_lazy_ptr for the global, e.g. lo16(symbol$non_lazy_ptr-picbase).
     MO_NLP_FLAG = 4,
-    
     /// MO_NLP_HIDDEN_FLAG - If this bit is set, the symbol reference is to a
     /// symbol with hidden visibility.  This causes a different kind of
     /// non-lazy-pointer to be generated.
     MO_NLP_HIDDEN_FLAG = 8,
 
     /// The next are not flags but distinct values.
     MO_ACCESS_MASK = 0xf0,
 
enum TOF {
     MO_DTPREL_LO = 5 << 4,
     MO_TLSLD_LO  = 6 << 4,
     MO_TOC_LO    = 7 << 4,
 
     // Symbol for VK_PPC_TLS fixup attached to an ADD instruction
     MO_TLS       = 8 << 4
   };
   } // end namespace PPCII
-  
 } // end namespace llvm;
 
 #endif

lib/Target/PowerPC/PPCEnableShrinkWrap.cpp

+//===- PPCEnableShrinkWrap.cpp - split functions to create shrink wrapping
+// opportunities ==/
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Shrink wrapping functions can improve performance for functions when the
+// stack is unused. However, all functions where value live-ranges contain
+// function calls are ineligible for shrink wrapping. Why?
+//
+// 1. To avoid saving and restoring values across call-sites, LLVM allocates
+// values with live-ranges that cross call sites to callee-saved registers
+//
+// 2. If callee-saved registers are used, they must be saved (usually in the
+// function prolog)
+//
+// 3. ShrinkWrapping scans from the beginning of the function prolog to the
+// first use of the stack. If a callee-saved register is saved to the stack in
+// the function prolog, ShrinkWrapping will give up early.
+//
+// To increase the applicability of ShrinkWrapping, this pass identifies
+// instances where a live-range straddling a call-site prevents ShrinkWrapping,
+// and splits the original function into a stackless ShrinkWrappable stub that
+// potentially makes a tail call to the remainder of the function.
+//
+// This transformation harms debuggability and consequently is not suitable for
+// lower -O levels.
+//
+//===----------------------------------------------------------------------===//
+#include "PPC.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Pass.h"
+
+using namespace llvm;
+
+static cl::
+opt<bool> DisablePreIncPrep("disable-ppc-enable-shrink-wrap", cl::Hidden,
+                            cl::desc("Disable PPC enable shrink wrap pass"));
+
+class PPCEnableShrinkWrap : public ModulePass {
+
+  static BasicBlock *getUniqueExitBlock(Function &F) {
+    BasicBlock *ExitBlock = nullptr;
+    for (auto &BB : F) {
+      if (isa<ReturnInst>(BB.getTerminator())) {
+        if (ExitBlock)
+          return nullptr;
+        else
+          ExitBlock = &BB;
+      }
+    }
+    return ExitBlock;
+  }
+
+  static bool branchesTo(TerminatorInst *term, BasicBlock *succ) {
+    for (unsigned i = 0; i < term->getNumSuccessors(); ++i)
+      if (term->getSuccessor(i) == succ)
+        return true;
+    return false;
+  }
+
+  static bool canBeComputedWithoutStack(llvm::Value *V) {
+    if (isa<Constant>(V))
+      return true;
+    if (isa<Argument>(V))
+      return true;
+    if (isa<PHINode>(V))
+      return true;
+    if (isa<BasicBlock>(V))
+      return true;
+    if (isa<AllocaInst>(V))
+      return false;
+    if (IntrinsicInst *I = dyn_cast<IntrinsicInst>(V))
+      return
+        I->getIntrinsicID() == Intrinsic::lifetime_end ||
+        I->getIntrinsicID() == Intrinsic::lifetime_start;
+    if (isa<DbgInfoIntrinsic>(V))
+      return true;
+    if (isa<CallInst>(V))
+      return false;
+    // Some casts and extracts require a stack slot for proc <= P7
+    // Conservatively assume all casts and extracts use the stack, since there
+    // is no easy way to query this information from the Target
+    if (isa<CastInst>(V) || isa<ExtractValueInst>(V))
+      return false;
+
+    Instruction *I = dyn_cast<Instruction>(V);
+    if (!I)
+      return false;
+    if (I->mayHaveSideEffects())
+      return false;
+    if (I->mayReadFromMemory())
+      return false;
+
+    for (unsigned i = 0; i < I->getNumOperands(); ++i)
+      if (!canBeComputedWithoutStack(I->getOperand(i)))
+        return false;
+
+    return true;
+  }
+
+  static bool canBeComputedWithoutStack(BasicBlock *exitBlock,
+                                        BasicBlock &entry) {
+    for (auto &I : *exitBlock) {
+      if (PHINode *Phi = dyn_cast<PHINode>(&I)) {
+        if (!canBeComputedWithoutStack(Phi->getIncomingValueForBlock(&entry)))
+          return false;
+      } else if (!canBeComputedWithoutStack(&I)) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  static bool hasCallInst(Function &F) {
+    for (auto &BB : F)
+      for (auto &I : BB)
+        if (isa<CallInst>(I) && !isa<IntrinsicInst>(I))
+          return true;
+    return false;
+  }
+
+  static bool hasVariadicCall(Function &F) {
+    for (auto &BB : F)
+      for (auto &I : BB)
+        if (CallInst *CI = dyn_cast<CallInst>(&I))
+          if (CI->getFunctionType()->isVarArg())
+            return true;
+    return false;
+  }
+
+  static bool mayHaveSideEffects(BasicBlock &BB) {
+    for (auto &I : BB)
+      if (I.mayHaveSideEffects())
+        return true;
+    return false;
+  }
+
+public:
+  static char ID;
+  PPCEnableShrinkWrap() : ModulePass(ID) {
+    initializePPCEnableShrinkWrapPass(*PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const {
+    ModulePass::getAnalysisUsage(AU);
+  }
+
+  bool runOnModule(Module &M) {
+    bool Changed = false;
+    if (DisablePreIncPrep) return Changed;
+    SmallVector<Function *, 16> Funcs;
+    for (auto &F : M)
+      Funcs.emplace_back(&F);
+    for (auto &F : Funcs)
+      if (!F->isDeclaration())
+        Changed |= runOnFunction(*F);
+    return Changed;
+  }
+
+  static bool runOnFunction(Function &F) {
+    // Presently, only implemented for non-variadic functions that don't call
+    // variadic functions and that contain at least one callsite.
+    if (F.isVarArg())
+      return false;
+    if (!hasCallInst(F))
+      return false;
+    if (hasVariadicCall(F))
+      return false;
+
+    // Only unique exit blocks are handled
+    BasicBlock *ExitBlock = getUniqueExitBlock(F);
+    if (!ExitBlock)
+      return false;
+
+    // Look for the pattern of entry blocks that immediately branch to exit
+    // where the entry block and the exit block can be computed without the
+    // stack.
+    BasicBlock &Entry = F.getEntryBlock();
+    BranchInst *EntryTerm = dyn_cast<BranchInst>(Entry.getTerminator());
+    if (!EntryTerm)
+      return false;
+    if (EntryTerm->isUnconditional())
+      return false;
+    if (isa<Constant>(EntryTerm->getCondition()))
+      return false;
+    if (!branchesTo(EntryTerm, ExitBlock))
+      return false;
+    if (!canBeComputedWithoutStack(EntryTerm))
+      return false;
+    if (!canBeComputedWithoutStack(ExitBlock, Entry))
+      return false;
+    if (mayHaveSideEffects(Entry))
+      return false;
+
+    // Clone the original function
+    ValueToValueMapTy VMap;
+    ClonedCodeInfo CodeInfo;
+    Function *CloneF = CloneFunction(&F, VMap, true, &CodeInfo);
+    CloneF->setName(F.getName() + ".cont");
+    CloneF->setLinkage(GlobalValue::InternalLinkage);
+    F.getParent()->getFunctionList().push_back(CloneF);
+
+    // Replace the non-exit block successor of the entry block that makes a tail
+    // call to the cloned version of the function.
+    LLVMContext &C = F.getContext();
+    BasicBlock *TailCallBB = BasicBlock::Create(C, "tailCallBB", &F);
+    for (unsigned i = 0; i < EntryTerm->getNumSuccessors(); ++i) {
+      BasicBlock *Succ = EntryTerm->getSuccessor(i);
+      if (Succ != ExitBlock) {
+        for (auto I = Succ->begin(); &*I != Succ->getFirstNonPHI(); ++I) {
+          PHINode *Phi = cast<PHINode>(I);
+          Phi->removeIncomingValue(&Entry);
+        }
+        EntryTerm->setSuccessor(i, TailCallBB);
+      }
+    }
+    // Create the tail call
+    SmallVector<Value *, 16> Args;
+    for (auto &Arg : F.args())
+      Args.emplace_back(&Arg);
+    CallInst *TailCall = CallInst::Create(CloneF, Args, "", TailCallBB);
+    TailCall->setTailCall(true);
+    TailCall->setCallingConv(CloneF->getCallingConv());
+    // Return the result of the tail call if necessary.
+    if (F.getReturnType()->isVoidTy())
+      ReturnInst::Create(C, TailCallBB);
+    else
+      ReturnInst::Create(C, TailCall, TailCallBB);
+
+    // In the clone function, we know the non-early exit path will be taken
+    BranchInst *CloneEntryTerm = cast<BranchInst>(VMap[EntryTerm]);
+    Value *Cond = CloneEntryTerm->getSuccessor(0) == VMap[ExitBlock]
+                      ? ConstantInt::getFalse(C)
+                      : ConstantInt::getTrue(C);
+    CloneEntryTerm->setCondition(Cond);
+    // Add a hint to favor the early exit path. This improves performance when
+    // the early exit path is common and does not harm performance much if the
+    // early exit path is rare and the non-early exit path is relatively long
+    // running.
+    ExitBlock->moveAfter(&Entry);
+    SmallVector<uint32_t, 2> Weights = {
+        EntryTerm->getSuccessor(0) == ExitBlock ? 100u : 1u,
+        EntryTerm->getSuccessor(0) == ExitBlock ? 1u : 100u};
+    EntryTerm->setMetadata(LLVMContext::MD_prof,
+                           MDBuilder(C).createBranchWeights(Weights));
+    return true;
+  }
+};
+
+char PPCEnableShrinkWrap::ID = 0;
+INITIALIZE_PASS(PPCEnableShrinkWrap, "create-wrapper-opps",
+                "Find opportunities for wrapping", false, false)
+
+ModulePass *llvm::createPPCEnableShrinkWrapPass() {
+  return new PPCEnableShrinkWrap();
+}

lib/Target/PowerPC/PPCTargetMachine.cpp

 extern "C" void LLVMInitializePowerPCTarget() {
   // Register the targets
   RegisterTargetMachine<PPC32TargetMachine> A(ThePPC32Target);
   RegisterTargetMachine<PPC64TargetMachine> B(ThePPC64Target);
   RegisterTargetMachine<PPC64TargetMachine> C(ThePPC64LETarget);
 
   PassRegistry &PR = *PassRegistry::getPassRegistry();
   initializePPCBoolRetToIntPass(PR);
+  initializePPCEnableShrinkWrapPass(PR);
 }
 
 /// Return the datalayout string of a subtarget.
 static std::string getDataLayoutString(const Triple &T) {
   bool is64Bit = T.getArch() == Triple::ppc64 || T.getArch() == Triple::ppc64le;
   std::string Ret;
 
   // Most PPC* platforms are big endian, PPC64LE is little endian.

 };
 } // namespace
 
 TargetPassConfig *PPCTargetMachine::createPassConfig(PassManagerBase &PM) {
   return new PPCPassConfig(this, PM);
 }
 
 void PPCPassConfig::addIRPasses() {
   if (TM->getOptLevel() != CodeGenOpt::None)
     addPass(createPPCBoolRetToIntPass());
+  if (TM->getOptLevel() > CodeGenOpt::Default)
+    addPass(createPPCEnableShrinkWrapPass());
   addPass(createAtomicExpandPass(&getPPCTargetMachine()));
 
   // For the BG/Q (or if explicitly requested), add explicit data prefetch
   // intrinsics.
   bool UsePrefetching = TM->getTargetTriple().getVendor() == Triple::BGQ &&
                         getOptLevel() != CodeGenOpt::None;
   if (EnablePrefetch.getNumOccurrences() > 0)
     UsePrefetching = EnablePrefetch;

test/CodeGen/PowerPC/EnableShrinkWrapTest.ll

+; RUN: opt -create-wrapper-opps -S -o - < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-n32:64"
+target triple = "powerpc64le-unknown-linux-gnu"
+
+; CHECK-LABEL: testEnableShrinkWrap
+define signext i32 @testEnableShrinkWrap(i32 signext %i) {
+entry:
+  %tobool = icmp eq i32 %i, 0
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+; CHECK call i32 @testEnableStrinkWrap.cont
+  tail call void @doSomething()
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  ret i32 %i
+}
+
+; CHECK-LABEL: doSomething
+declare void @doSomething()
+
+; CHECK-LABEL: testEnableShrinkWrapVariadic
+define signext i32 @testEnableShrinkWrapVariadic(i32 signext %i, ...) {
+entry:
+  %tobool = icmp eq i32 %i, 0
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+; CHECK: call void @doSomething
+tail call void @doSomething()
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  ret i32 %i
+}
+
+; CHECK-LABEL: callVariadic
+define signext i32 @callVariadic(i32 signext %i) {
+entry:
+  %tobool = icmp eq i32 %i, 0
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+; CHECK: call void (i32, ...) @doSomethingVariadic
+  tail call void (i32, ...) @doSomethingVariadic(i32 signext %i, i32 signext 7)
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  ret i32 %i
+}
+
+; CHECK-LABEL: declare void @doSomethingVariadic
+declare void @doSomethingVariadic(i32 signext, ...)
+
+; CHECK-LABEL: tooComplicatedControlFlow
+define signext i32 @tooComplicatedControlFlow(i32 zeroext %i, i32 zeroext %j) {
+entry:
+  br label %do.body
+
+do.body:                                          ; preds = %do.body, %entry
+  %i.addr.0 = phi i32 [ %i, %entry ], [ %dec, %do.body ]
+; CHECK: call void @doSomething
+  tail call void @doSomething()
+  %dec = add i32 %i.addr.0, -1
+  %tobool = icmp eq i32 %dec, 0
+  br i1 %tobool, label %do.body.1.preheader, label %do.body
+
+do.body.1.preheader:                              ; preds = %do.body
+  br label %do.body.1
+
+do.body.1:                                        ; preds = %do.body.1.preheader, %do.body.1
+  %j.addr.0 = phi i32 [ %dec3, %do.body.1 ], [ %j, %do.body.1.preheader ]
+; CHECK: call void @doSomething
+  tail call void @doSomething()
+  %dec3 = add i32 %j.addr.0, -1
+  %tobool4 = icmp eq i32 %dec3, 0
+  br i1 %tobool4, label %do.end.5, label %do.body.1
+
+do.end.5:                                         ; preds = %do.body.1
+  ret i32 7
+}
+
+; CHECK-LABEL: notSideEffectFree
+define signext i32 @notSideEffectFree(i32* nocapture readonly %i) {
+entry:
+  %incdec.ptr = getelementptr inbounds i32, i32* %i, i64 1
+  %0 = load i32, i32* %i, align 4
+  %tobool = icmp eq i32 %0, 0
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:                                          ; preds = %entry
+; CHECK: call void @doSomething
+  tail call void @doSomething()
+  br label %if.end
+
+if.end:                                           ; preds = %entry, %if.then
+  %1 = load i32, i32* %incdec.ptr, align 4
+  ret i32 %1
+}
+
+; CHECK-LABEL: testEnableShrinkWrap.cont
+; CHECK: call void @doSomething