[ARM] Promote small global constants to constant pools

[jmolloy]

Summary

If a constant is unamed_addr and is only used within one function, we can save
on the code size and runtime cost of an indirection by changing the global's storage
to inside the constant pool. For example, instead of:

  ldr r0, .CPI0
  bl printf
  bx lr
.CPI0: &format_string
format_string: .asciz "hello, world!\n"

We can emit:

  adr r0, .CPI0
  bl printf
  bx lr
.CPI0: .asciz "hello, world!\n"

This can cause significant code size savings when many small strings are used in one
function (4 bytes per string).

[joerg]

Should PIC be factored in here as well? I think I would like to see testing for that at least.

[olista01]

lib/Target/ARM/ARMISelLowering.cpp

2890

Would it also be worth doing this for very small constants with multiple uses? If the constant is no larger than a pointer then this won't increase code size, and should help performance by removing a layer of indirection.

test/CodeGen/ARM/constantpool-promote.ll

43

Missing CHECK lines?

[jmolloy]

Hi Oliver and Joerg,

Thanks for the comments. I've fixed the issues you noticed Oliver, and Joerg I agree this should work for PIC modes so I've enabled that and tested it.

Cheers,

James

[olista01]

LGTM

[jmolloy]

Thanks Oliver, r281213.

lib/Target/ARM/ARMISelLowering.cpp

2890

Good point, I agree.

lib/Target/ARM/ARMAsmPrinter.h

   /// -1 if uninitialized, 0 if conflicting goals
   int OptimizationGoals;
 
+  /// List of globals that have had their storage promoted to a constant
+  /// pool. This lives between calls to runOnMachineFunction and collects
+  /// data from every MachineFunction. It is used during doFinalization
+  /// when all non-function globals are emitted.
+  SmallPtrSet<const GlobalVariable*,2> PromotedGlobals;
+
 public:
   explicit ARMAsmPrinter(TargetMachine &TM,
                          std::unique_ptr<MCStreamer> Streamer);

   void EmitStartOfAsmFile(Module &M) override;
   void EmitEndOfAsmFile(Module &M) override;
   void EmitXXStructor(const DataLayout &DL, const Constant *CV) override;
-
+  void EmitGlobalVariable(const GlobalVariable *GV) override;
+  
   // lowerOperand - Convert a MachineOperand into the equivalent MCOperand.
   bool lowerOperand(const MachineOperand &MO, MCOperand &MCOp);
 

lib/Target/ARM/ARMAsmPrinter.cpp

   OutStreamer->EmitValue(E, Size);
 }
 
+void ARMAsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
+  if (PromotedGlobals.count(GV))
+    // The global was promoted into a constant pool. It should not be emitted.
+    return;
+  AsmPrinter::EmitGlobalVariable(GV);
+}
+
 /// runOnMachineFunction - This uses the EmitInstruction()
 /// method to print assembly for each instruction.
 ///

   const Function* F = MF.getFunction();
   const TargetMachine& TM = MF.getTarget();
 
+  // Collect all globals that had their storage promoted to a constant pool.
+  // Functions are emitted before variables, so this accumulates promoted
+  // globals from all functions in PromotedGlobals.
+  for (auto *GV : AFI->getGlobalsPromotedToConstantPool())
+    PromotedGlobals.insert(GV);
+  
   // Calculate this function's optimization goal.
   unsigned OptimizationGoal;
   if (F->hasFnAttribute(Attribute::OptimizeNone))

lib/Target/ARM/ARMISelLowering.cpp

 STATISTIC(NumTailCalls, "Number of tail calls");
 STATISTIC(NumMovwMovt, "Number of GAs materialized with movw + movt");
 STATISTIC(NumLoopByVals, "Number of loops generated for byval arguments");
+STATISTIC(NumConstpoolPromoted,
+  "Number of constants with their storage promoted into constant pools");
 
 static cl::opt<bool>
 ARMInterworking("arm-interworking", cl::Hidden,
   cl::desc("Enable / disable ARM interworking (for debugging only)"),
   cl::init(true));
 
+static cl::opt<bool> EnableConstpoolPromotion(
+    "arm-promote-constant", cl::Hidden,
+    cl::desc("Enable / disable promotion of unnamed_addr constants into "
+             "constant pools"),
+    cl::init(true));
+static cl::opt<unsigned> ConstpoolPromotionMaxSize(
+    "arm-promote-constant-max-size", cl::Hidden,
+    cl::desc("Maximum size of constant to promote into a constant pool"),
+    cl::init(64));
+
 namespace {
   class ARMCCState : public CCState {
   public:

   llvm_unreachable("bogus TLS model");
 }
 
+/// Return true if all users of V are within function F, looking through
+/// ConstantExprs.
+static bool allUsersAreInFunction(const Value *V, const Function *F) {
+  SmallVector<const User*,4> Worklist;
+  for (auto *U : V->users())
+    Worklist.push_back(U);
+  while (!Worklist.empty()) {
+    auto *U = Worklist.pop_back_val();
+    if (isa<ConstantExpr>(U)) {
+      for (auto *UU : U->users())
+        Worklist.push_back(UU);
+      continue;
+    }
+
+    auto *I = dyn_cast<Instruction>(U);
+    if (!I || I->getParent()->getParent() != F)
+      return false;
+  }
+  return true;
+}
+
+static SDValue promoteToConstantPool(const GlobalValue *GV, SelectionDAG &DAG,
+                                     EVT PtrVT, SDLoc dl) {
+  // If we're creating a pool entry for a constant global with unnamed address,
+  // and the global is small enough, we can emit it inline into the constant pool
+  // to save ourselves an indirection.
+  //
+  // This is a win if the constant is only used in one function (so it doesn't
+  // need to be duplicated) or duplicating the constant wouldn't increase code
+  // size (implying the constant is no larger than 4 bytes).
+  const Function *F = DAG.getMachineFunction().getFunction();
+  auto *GVar = dyn_cast<GlobalVariable>(GV);
+  if (EnableConstpoolPromotion && GVar && GVar->hasInitializer() &&
+      GVar->isConstant() && GVar->hasGlobalUnnamedAddr()) {
+    // The constant islands pass can only really deal with alignment requests
+    // <= 4 bytes and cannot pad constants itself. Therefore we cannot promote
+    // any type wanting greater alignment requirements than 4 bytes. We also
+    // can only promote constants that are multiples of 4 bytes in size or
+    // are paddable to a multiple of 4. Currently we only try and pad constants
+    // that are strings for simplicity.
+    auto *Init = GVar->getInitializer();
+    auto *CDAInit = dyn_cast<ConstantDataArray>(Init);
+    unsigned Size = DAG.getDataLayout().getTypeAllocSize(Init->getType());
+    unsigned Align = DAG.getDataLayout().getABITypeAlignment(Init->getType());
+    unsigned RequiredPadding = 4 - (Size % 4);
+    bool PaddingPossible =
+        RequiredPadding == 4 || (CDAInit && CDAInit->isString());
+
+    if (PaddingPossible && Align <= 4 && Size <= ConstpoolPromotionMaxSize &&
+        (allUsersAreInFunction(GVar, F) || Size <= 4)) {
+      if (RequiredPadding != 4) {
+        StringRef S = CDAInit->getAsString();
+
+        SmallVector<uint8_t,16> V(S.size());
+        std::copy(S.bytes_begin(), S.bytes_end(), V.begin());
+        while (RequiredPadding--)
+          V.push_back(0);
+        Init = ConstantDataArray::get(*DAG.getContext(), V);
+      }
+      
+      SDValue CPAddr =
+        DAG.getTargetConstantPool(Init, PtrVT, Align);
+
+      MachineFunction &MF = DAG.getMachineFunction();
+      ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
+      AFI->markGlobalAsPromotedToConstantPool(GVar);
+      return DAG.getNode(ARMISD::Wrapper, dl, MVT::i32, CPAddr);
+    }
+  }
+  return SDValue();
+}
+
 SDValue ARMTargetLowering::LowerGlobalAddressELF(SDValue Op,
                                                  SelectionDAG &DAG) const {
   EVT PtrVT = getPointerTy(DAG.getDataLayout());

   bool IsRO =
       (isa<GlobalVariable>(GV) && cast<GlobalVariable>(GV)->isConstant()) ||
       isa<Function>(GV);
+  
+  if (TM.shouldAssumeDSOLocal(*GV->getParent(), GV))
+    if (SDValue V = promoteToConstantPool(GV, DAG, PtrVT, dl))
+      return V;
+
   if (isPositionIndependent()) {
     bool UseGOT_PREL = !TM.shouldAssumeDSOLocal(*GV->getParent(), GV);
 

lib/Target/ARM/ARMMachineFunctionInfo.h

   /// copies.
   bool IsSplitCSR;
 
+  /// Globals that have had their storage promoted into the constant pool.
+  SmallVector<const GlobalVariable*,2> PromotedGlobals;
+  
 public:
   ARMFunctionInfo() :
     isThumb(false),

     }
     return It;
   }
+
+  /// Indicate to the backend that \c GV has had its storage changed to inside
+  /// a constant pool. This means it no longer needs to be emitted as a
+  /// global variable.
+  void markGlobalAsPromotedToConstantPool(const GlobalVariable *GV) {
+    PromotedGlobals.push_back(GV);
+  }
+  ArrayRef<const GlobalVariable*> getGlobalsPromotedToConstantPool() {
+    return PromotedGlobals;
+  }
 };
 } // End llvm namespace
 

test/CodeGen/ARM/constantpool-promote.ll

+; RUN: llc -relocation-model=static < %s | FileCheck %s
+; RUN: llc -relocation-model=pic < %s | FileCheck %s
+; RUN: llc -relocation-model=ropi < %s | FileCheck %s
+; RUN: llc -relocation-model=rwpi < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-n32-S64"
+target triple = "armv7--linux-gnueabihf"
+
+@.str = private unnamed_addr constant [2 x i8] c"s\00", align 1
+@.str1 = private unnamed_addr constant [69 x i8] c"this string is far too long to fit in a literal pool by far and away\00", align 1
+@.str2 = private unnamed_addr constant [27 x i8] c"this string is just right!\00", align 1
+@.str3 = private unnamed_addr constant [26 x i8] c"this string is used twice\00", align 1
+@.str4 = private unnamed_addr constant [29 x i8] c"same string in two functions\00", align 1
+@.arr1 = private unnamed_addr constant [2 x i16] [i16 3, i16 4], align 2
+
+; CHECK-LABEL: @test1
+; CHECK: adr r0, [[x:.*]]
+; CHECK: [[x]]:
+; CHECK: .asciz "s\000\000"
+define void @test1() #0 {
+  tail call void @a(i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str, i32 0, i32 0)) #2
+  ret void
+}
+
+declare void @a(i8*) #1
+
+; CHECK-LABEL: @test2
+; CHECK-NOT: .asci
+; CHECK: .fnend
+define void @test2() #0 {
+  tail call void @a(i8* getelementptr inbounds ([69 x i8], [69 x i8]* @.str1, i32 0, i32 0)) #2
+  ret void
+}
+
+; CHECK-LABEL: @test3
+; CHECK: adr r0, [[x:.*]]
+; CHECK: [[x]]:
+; CHECK: .asciz "this string is just right!\000"
+define void @test3() #0 {
+  tail call void @a(i8* getelementptr inbounds ([27 x i8], [27 x i8]* @.str2, i32 0, i32 0)) #2
+  ret void
+}
+
+
+; CHECK-LABEL: @test4
+; CHECK: adr r{{.*}}, [[x:.*]]
+; CHECK: [[x]]:
+; CHECK: .asciz "this string is used twice\000\000"
+define void @test4() #0 {
+  tail call void @a(i8* getelementptr inbounds ([26 x i8], [26 x i8]* @.str3, i32 0, i32 0)) #2
+  tail call void @a(i8* getelementptr inbounds ([26 x i8], [26 x i8]* @.str3, i32 0, i32 0)) #2
+  ret void
+}
+
+; CHECK-LABEL: @test5a
+; CHECK-NOT: adr
+define void @test5a() #0 {
+  tail call void @a(i8* getelementptr inbounds ([29 x i8], [29 x i8]* @.str4, i32 0, i32 0)) #2
+  ret void
+}
+
+define void @test5b() #0 {
+  tail call void @b(i8* getelementptr inbounds ([29 x i8], [29 x i8]* @.str4, i32 0, i32 0)) #2
+  ret void
+}
+
+; CHECK-LABEL: @test6a
+; CHECK: adr r0, [[x:.*]]
+; CHECK: [[x]]:
+; CHECK: .short 3
+; CHECK: .short 4
+define void @test6a() #0 {
+  tail call void @c(i16* getelementptr inbounds ([2 x i16], [2 x i16]* @.arr1, i32 0, i32 0)) #2
+  ret void
+}
+
+; CHECK-LABEL: @test6b
+; CHECK: adr r0, [[x:.*]]
+; CHECK: [[x]]:
+; CHECK: .short 3
+; CHECK: .short 4
+define void @test6b() #0 {
+  tail call void @c(i16* getelementptr inbounds ([2 x i16], [2 x i16]* @.arr1, i32 0, i32 0)) #2
+  ret void
+}
+
+declare void @b(i8*) #1
+declare void @c(i16*) #1
+
+attributes #0 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #1 = { "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
+attributes #2 = { nounwind }
+
+!llvm.module.flags = !{!0, !1}
+!llvm.ident = !{!2}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{i32 1, !"min_enum_size", i32 4}
+!2 = !{!"Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)"}