r338697 | dim | 2018-09-15 23:22:50 +0200 (Sat, 15 Sep 2018) | 21 lines

Pull in r325478 from upstream clang trunk (by Ivan A. Kosarev):

  [CodeGen] Initialize large arrays by copying from a global

  Currently, clang compiles explicit initializers for array elements
  into series of store instructions. For large arrays of built-in types
  this results in bloated output code and significant amount of time
  spent on the instruction selection phase. This patch fixes the issue
  by initializing such arrays with global constants that store the
  binary image of the initializer.

  Differential Revision: https://reviews.llvm.org/D43181

This should fix a compiler hang (and excessive memory usage) while
building the science/rmg port.

Approved by:	re (kib)
Reported by:	yuri@tsoft.com
See also:	https://bugs.llvm.org/show_bug.cgi?id=38798
MFC after:	3 days

Index: tools/clang/lib/CodeGen/CGExprAgg.cpp
===================================================================
--- tools/clang/lib/CodeGen/CGExprAgg.cpp	(revision 338696)
+++ tools/clang/lib/CodeGen/CGExprAgg.cpp	(revision 338697)
@@ -14,6 +14,7 @@
 #include "CodeGenFunction.h"
 #include "CGObjCRuntime.h"
 #include "CodeGenModule.h"
+#include "ConstantEmitter.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/DeclTemplate.h"
@@ -85,7 +86,7 @@ class AggExprEmitter : public StmtVisitor<AggExprE
   void EmitMoveFromReturnSlot(const Expr *E, RValue Src);
 
   void EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
-                     QualType elementType, InitListExpr *E);
+                     QualType ArrayQTy, InitListExpr *E);
 
   AggValueSlot::NeedsGCBarriers_t needsGC(QualType T) {
     if (CGF.getLangOpts().getGC() && TypeRequiresGCollection(T))
@@ -392,12 +393,15 @@ static bool isTrivialFiller(Expr *E) {
 
 /// \brief Emit initialization of an array from an initializer list.
 void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
-                                   QualType elementType, InitListExpr *E) {
+                                   QualType ArrayQTy, InitListExpr *E) {
   uint64_t NumInitElements = E->getNumInits();
 
   uint64_t NumArrayElements = AType->getNumElements();
   assert(NumInitElements <= NumArrayElements);
 
+  QualType elementType =
+      CGF.getContext().getAsArrayType(ArrayQTy)->getElementType();
+
   // DestPtr is an array*.  Construct an elementType* by drilling
   // down a level.
   llvm::Value *zero = llvm::ConstantInt::get(CGF.SizeTy, 0);
@@ -409,6 +413,29 @@ void AggExprEmitter::EmitArrayInit(Address DestPtr
   CharUnits elementAlign =
     DestPtr.getAlignment().alignmentOfArrayElement(elementSize);
 
+  // Consider initializing the array by copying from a global. For this to be
+  // more efficient than per-element initialization, the size of the elements
+  // with explicit initializers should be large enough.
+  if (NumInitElements * elementSize.getQuantity() > 16 &&
+      elementType.isTriviallyCopyableType(CGF.getContext())) {
+    CodeGen::CodeGenModule &CGM = CGF.CGM;
+    ConstantEmitter Emitter(CGM);
+    LangAS AS = ArrayQTy.getAddressSpace();
+    if (llvm::Constant *C = Emitter.tryEmitForInitializer(E, AS, ArrayQTy)) {
+      auto GV = new llvm::GlobalVariable(
+          CGM.getModule(), C->getType(),
+          CGM.isTypeConstant(ArrayQTy, /* ExcludeCtorDtor= */ true),
+          llvm::GlobalValue::PrivateLinkage, C, "constinit",
+          /* InsertBefore= */ nullptr, llvm::GlobalVariable::NotThreadLocal,
+          CGM.getContext().getTargetAddressSpace(AS));
+      Emitter.finalize(GV);
+      CharUnits Align = CGM.getContext().getTypeAlignInChars(ArrayQTy);
+      GV->setAlignment(Align.getQuantity());
+      EmitFinalDestCopy(ArrayQTy, CGF.MakeAddrLValue(GV, ArrayQTy, Align));
+      return;
+    }
+  }
+
   // Exception safety requires us to destroy all the
   // already-constructed members if an initializer throws.
   // For that, we'll need an EH cleanup.
@@ -1156,11 +1183,8 @@ void AggExprEmitter::VisitInitListExpr(InitListExp
 
   // Handle initialization of an array.
   if (E->getType()->isArrayType()) {
-    QualType elementType =
-        CGF.getContext().getAsArrayType(E->getType())->getElementType();
-
     auto AType = cast<llvm::ArrayType>(Dest.getAddress().getElementType());
-    EmitArrayInit(Dest.getAddress(), AType, elementType, E);
+    EmitArrayInit(Dest.getAddress(), AType, E->getType(), E);
     return;
   }