Allow targets to define minimum alignment for global variables

This patch adds a new common code feature that allows platform code to
request minimum alignment of global symbols.  The background for this is
that on SystemZ, the most efficient way to load addresses of global symbol
is the LOAD ADDRESS RELATIVE LONG (LARL) instruction.  This instruction
provides PC-relative addressing, but only to *even* addresses.  For this
reason, existing compilers will guarantee that global symbols are always
aligned to at least 2.  [ Since symbols would otherwise already use a
default alignment based on their type, this will usually only affect global
objects of character type or character arrays. ]  GCC also allows creating
symbols without that extra alignment by using explicit "aligned" attributes
(which then need to be used on both definition and each use of the symbol).

To enable support for this with Clang, this patch adds a
TargetInfo::MinGlobalAlign variable that provides a global minimum for the
alignment of every global object (unless overridden via explicit alignment
attribute), and adds code to respect this setting.  Within this patch, no
platform actually sets the value to anything but the default 1, resulting
in no change in behaviour on any existing target.

This version of the patch incorporates feedback from reviews by
Eric Christopher and John McCall.  Thanks to all reviewers!

Patch by Richard Sandiford.

llvm-svn: 181210

clang/include/clang/AST/ASTContext.h

@@ -1593,6 +1593,14 @@ public:
   /// beneficial for performance to overalign a data type.
   unsigned getPreferredTypeAlign(const Type *T) const;
 
+  /// \brief Return the alignment in bits that should be given to a
+  /// global variable with type \p T.
+  unsigned getAlignOfGlobalVar(QualType T) const;
+
+  /// \brief Return the alignment in characters that should be given to a
+  /// global variable with type \p T.
+  CharUnits getAlignOfGlobalVarInChars(QualType T) const;
+
   /// \brief Return a conservative estimate of the alignment of the specified
   /// decl \p D.
   ///

clang/include/clang/Basic/TargetInfo.h

@@ -66,6 +66,7 @@ protected:
   unsigned char LongWidth, LongAlign;
   unsigned char LongLongWidth, LongLongAlign;
   unsigned char SuitableAlign;
+  unsigned char MinGlobalAlign;
   unsigned char MaxAtomicPromoteWidth, MaxAtomicInlineWidth;
   unsigned short MaxVectorAlign;
   const char *DescriptionString;
@@ -266,6 +267,10 @@ public:
   /// object with a fundamental alignment requirement.
   unsigned getSuitableAlign() const { return SuitableAlign; }
 
+  /// getMinGlobalAlign - Return the minimum alignment of a global variable,
+  /// unless its alignment is explicitly reduced via attributes.
+  unsigned getMinGlobalAlign() const { return MinGlobalAlign; }
+
   /// getWCharWidth/Align - Return the size of 'wchar_t' for this target, in
   /// bits.
   unsigned getWCharWidth() const { return getTypeWidth(WCharType); }

clang/lib/AST/ASTContext.cpp

@@ -1283,6 +1283,10 @@ CharUnits ASTContext::getDeclAlign(const Decl *D, bool RefAsPointee) const {
         T = getBaseElementType(arrayType);
       }
       Align = std::max(Align, getPreferredTypeAlign(T.getTypePtr()));
+      if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
+        if (VD->hasGlobalStorage())
+          Align = std::max(Align, getTargetInfo().getMinGlobalAlign());
+      }
     }
 
     // Fields can be subject to extra alignment constraints, like if
@@ -1725,6 +1729,18 @@ unsigned ASTContext::getPreferredTypeAlign(const Type *T) const {
   return ABIAlign;
 }
 
+/// getAlignOfGlobalVar - Return the alignment in bits that should be given
+/// to a global variable of the specified type.
+unsigned ASTContext::getAlignOfGlobalVar(QualType T) const {
+  return std::max(getTypeAlign(T), getTargetInfo().getMinGlobalAlign());
+}
+
+/// getAlignOfGlobalVarInChars - Return the alignment in characters that
+/// should be given to a global variable of the specified type.
+CharUnits ASTContext::getAlignOfGlobalVarInChars(QualType T) const {
+  return toCharUnitsFromBits(getAlignOfGlobalVar(T));
+}
+
 /// DeepCollectObjCIvars -
 /// This routine first collects all declared, but not synthesized, ivars in
 /// super class and then collects all ivars, including those synthesized for

clang/lib/Basic/TargetInfo.cpp

@@ -37,6 +37,7 @@ TargetInfo::TargetInfo(const std::string &T) : TargetOpts(), Triple(T)
   LongWidth = LongAlign = 32;
   LongLongWidth = LongLongAlign = 64;
   SuitableAlign = 64;
+  MinGlobalAlign = 0;
   HalfWidth = 16;
   HalfAlign = 16;
   FloatWidth = 32;

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2339,6 +2339,8 @@ CodeGenModule::GetAddrOfConstantCFString(const StringLiteral *Literal) {
     new llvm::GlobalVariable(getModule(), C->getType(), /*isConstant=*/true,
                              Linkage, C, ".str");
   GV->setUnnamedAddr(true);
+  // Don't enforce the target's minimum global alignment, since the only use
+  // of the string is via this class initializer.
   if (isUTF16) {
     CharUnits Align = getContext().getTypeAlignInChars(getContext().ShortTy);
     GV->setAlignment(Align.getQuantity());
@@ -2472,6 +2474,8 @@ CodeGenModule::GetAddrOfConstantString(const StringLiteral *Literal) {
   new llvm::GlobalVariable(getModule(), C->getType(), isConstant, Linkage, C,
                            ".str");
   GV->setUnnamedAddr(true);
+  // Don't enforce the target's minimum global alignment, since the only use
+  // of the string is via this class initializer.
   CharUnits Align = getContext().getTypeAlignInChars(getContext().CharTy);
   GV->setAlignment(Align.getQuantity());
   Fields[1] = llvm::ConstantExpr::getGetElementPtr(GV, Zeros);
@@ -2576,7 +2580,7 @@ CodeGenModule::GetConstantArrayFromStringLiteral(const StringLiteral *E) {
 /// constant array for the given string literal.
 llvm::Constant *
 CodeGenModule::GetAddrOfConstantStringFromLiteral(const StringLiteral *S) {
-  CharUnits Align = getContext().getTypeAlignInChars(S->getType());
+  CharUnits Align = getContext().getAlignOfGlobalVarInChars(S->getType());
   if (S->isAscii() || S->isUTF8()) {
     SmallString<64> Str(S->getString());
     
@@ -2645,6 +2649,10 @@ llvm::Constant *CodeGenModule::GetAddrOfConstantString(StringRef Str,
   if (!GlobalName)
     GlobalName = ".str";
 
+  if (Alignment == 0)
+    Alignment = getContext().getAlignOfGlobalVarInChars(getContext().CharTy)
+      .getQuantity();
+
   // Don't share any string literals if strings aren't constant.
   if (LangOpts.WritableStrings)
     return GenerateStringLiteral(Str, false, *this, GlobalName, Alignment);

clang/lib/CodeGen/CodeGenModule.h

@@ -713,7 +713,7 @@ public:
   /// (if one is created).
   llvm::Constant *GetAddrOfConstantString(StringRef Str,
                                           const char *GlobalName=0,
-                                          unsigned Alignment=1);
+                                          unsigned Alignment=0);
 
   /// GetAddrOfConstantCString - Returns a pointer to a character array
   /// containing the literal and a terminating '\0' character. The result has
@@ -723,7 +723,7 @@ public:
   /// created).
   llvm::Constant *GetAddrOfConstantCString(const std::string &str,
                                            const char *GlobalName=0,
-                                           unsigned Alignment=1);
+                                           unsigned Alignment=0);
 
   /// GetAddrOfConstantCompoundLiteral - Returns a pointer to a constant global
   /// variable for the given file-scope compound literal expression.