Introducing nocapture, a parameter attribute for pointers to indicate that the

callee will not introduce any new aliases of that pointer.

The attributes had all bits allocated already, so I decided to collapse
alignment. Alignment was previously stored as a 16-bit integer from bits 16 to
32 of the attribute, but it was required to be a power of 2. Now it's stored in
log2 encoded form in five bits from 16 to 21. That gives us 11 more bits of
space.

You may have already noticed that you only need four bits to encode a 16-bit
power of two, so why five bits? Because the AsmParser accepted 32-bit
alignments, even though we couldn't store them (they were silently discarded).
Now we can store them in memory, but not in the bitcode.

The bitcode format was already storing these as 64-bit VBR integers. So, the
bitcode format stays the same, keeping the alignment values stored as 16 bit
raw values. There's some hideous code in the reader and writer that deals with
this, waiting to be ripped out the moment we run out of bits again and have to
replace the parameter attributes table encoding.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61019 91177308-0d34-0410-b5e6-96231b3b80d8

docs/AliasAnalysis.html

@@ -192,11 +192,11 @@ and returns MustAlias, MayAlias, or NoAlias as appropriate.
 
 <div class="doc_text">
 <p>The NoAlias response is used when the two pointers refer to distinct objects,
-even regardless of whether the pointers compare equal.  For example, freed
+regardless of whether the pointers compare equal.  For example, freed pointers
-pointers don't alias any pointers that were allocated afterwards.  As a
+don't alias any pointers that were allocated afterwards.  As a degenerate case,
-degenerate case, pointers returned by malloc(0) have no bytes for an object,
+pointers returned by malloc(0) have no bytes for an object, and are considered
-and are considered NoAlias even when malloc returns the same pointer.  The same
+NoAlias even when malloc returns the same pointer.  The same rule applies to
-rule applies to NULL pointers.</p>
+NULL pointers.</p>
 
 <p>The MayAlias response is used whenever the two pointers might refer to the
 same object.  If the two memory objects overlap, but do not start at the same

docs/LangRef.html

@@ -894,9 +894,15 @@ declare signext i8 @returns_signed_char()
     parameter.  The caller is responsible for ensuring that this is the
     case. On a function return value, <tt>noalias</tt> additionally indicates
     that the pointer does not alias any other pointers visible to the
-    caller. Note that this applies only to pointers that can be used to actually
+    caller. For further details, please see the discussion of the NoAlias
-    load/store a value: NULL, unique pointers from malloc(0), and freed pointers
+    response in
-    are considered to not alias anything.</dd>
+    <a href="http://llvm.org/docs/AliasAnalysis.html#MustMayNo">alias
+    analysis</a>.</dd>
+
+    <dt><tt>nocapture</tt></dt>
+    <dd>This indicates that the callee does not make any copies of the pointer
+    that outlive the callee itself. This is not a valid attribute for return
+    values.</dd>
 
     <dt><tt>nest</tt></dt>
     <dd>This indicates that the pointer parameter can be excised using the

include/llvm/Attributes.h

@@ -15,6 +15,7 @@
 #ifndef LLVM_ATTRIBUTES_H
 #define LLVM_ATTRIBUTES_H
 
+#include "llvm/Support/MathExtras.h"
 #include <cassert>
 #include <string>
 
@@ -23,7 +24,7 @@ class Type;
 
 /// Attributes - A bitset of attributes.
 typedef unsigned Attributes;
-  
+
 namespace Attribute {
 
 /// Function parameters and results can have attributes to indicate how they 
@@ -44,16 +45,17 @@ const Attributes ByVal     = 1<<7;  ///< Pass structure by value
 const Attributes Nest      = 1<<8;  ///< Nested function static chain
 const Attributes ReadNone  = 1<<9;  ///< Function does not access memory
 const Attributes ReadOnly  = 1<<10; ///< Function only reads from memory
-const Attributes NoInline        = 1<<11; // inline=never 
+const Attributes NoInline        = 1<<11; ///< inline=never 
-const Attributes AlwaysInline    = 1<<12; // inline=always
+const Attributes AlwaysInline    = 1<<12; ///< inline=always
-const Attributes OptimizeForSize = 1<<13; // opt_size
+const Attributes OptimizeForSize = 1<<13; ///< opt_size
-const Attributes StackProtect    = 1<<14; // Stack protection.
+const Attributes StackProtect    = 1<<14; ///< Stack protection.
-const Attributes StackProtectReq = 1<<15; // Stack protection required.
+const Attributes StackProtectReq = 1<<15; ///< Stack protection required.
-const Attributes Alignment = 0xffff<<16; ///< Alignment of parameter (16 bits)
+const Attributes Alignment = 31<<16; ///< Alignment of parameter (5 bits)
-                                    // 0 = unknown, else in clear (not log)
+                                     // stored as log2 of alignment.
-                                    
+const Attributes NoCapture = 1<<21; ///< Function creates no aliases of pointer
+
 /// @brief Attributes that only apply to function parameters.
-const Attributes ParameterOnly = ByVal | Nest | StructRet;
+const Attributes ParameterOnly = ByVal | Nest | StructRet | NoCapture;
 
 /// @brief Attributes that only apply to function.
 const Attributes FunctionOnly = NoReturn | NoUnwind | ReadNone | ReadOnly | 
@@ -64,7 +66,7 @@ const Attributes VarArgsIncompatible = StructRet;
 
 /// @brief Attributes that are mutually incompatible.
 const Attributes MutuallyIncompatible[4] = {
-  ByVal | InReg | Nest  | StructRet,
+  ByVal | InReg | Nest | StructRet,
   ZExt  | SExt,
   ReadNone | ReadOnly,
   NoInline | AlwaysInline
@@ -76,7 +78,8 @@ Attributes typeIncompatible(const Type *Ty);
 /// This turns an int alignment (a power of 2, normally) into the
 /// form used internally in Attributes.
 inline Attributes constructAlignmentFromInt(unsigned i) {
-  return (i << 16);
+  assert(isPowerOf2_32(i) && "Alignment must be a power of two.");
+  return Log2_32(i) << 16;
 }
 
 /// The set of Attributes set in Attributes is converted to a

lib/AsmParser/LLLexer.cpp

@@ -491,6 +491,7 @@ int LLLexer::LexIdentifier() {
   KEYWORD("nounwind", NOUNWIND);
   KEYWORD("noreturn", NORETURN);
   KEYWORD("noalias", NOALIAS);
+  KEYWORD("nocapture", NOCAPTURE);
   KEYWORD("byval", BYVAL);
   KEYWORD("nest", NEST);
   KEYWORD("readnone", READNONE);

lib/AsmParser/llvmAsmParser.y

@@ -1136,8 +1136,8 @@ Module *llvm::RunVMAsmParser(llvm::MemoryBuffer *MB) {
 %token <OtherOpVal> EXTRACTVALUE INSERTVALUE
 
 // Function Attributes
-%token SIGNEXT ZEROEXT NORETURN INREG SRET NOUNWIND NOALIAS BYVAL NEST
+%token SIGNEXT ZEROEXT NORETURN INREG SRET NOUNWIND NOALIAS NOCAPTURE BYVAL
-%token READNONE READONLY GC OPTSIZE NOINLINE ALWAYSINLINE SSP SSPREQ
+%token READNONE READONLY GC OPTSIZE NOINLINE ALWAYSINLINE SSP SSPREQ NEST
 
 // Visibility Styles
 %token DEFAULT HIDDEN PROTECTED
@@ -1265,15 +1265,16 @@ OptCallingConv : /*empty*/          { $$ = CallingConv::C; } |
                   CHECK_FOR_ERROR
                  };
 
-Attribute     : ZEROEXT { $$ = Attribute::ZExt;      }
+Attribute     : ZEROEXT   { $$ = Attribute::ZExt;      }
-              | ZEXT    { $$ = Attribute::ZExt;      }
+              | ZEXT      { $$ = Attribute::ZExt;      }
-              | SIGNEXT { $$ = Attribute::SExt;      }
+              | SIGNEXT   { $$ = Attribute::SExt;      }
-              | SEXT    { $$ = Attribute::SExt;      }
+              | SEXT      { $$ = Attribute::SExt;      }
-              | INREG   { $$ = Attribute::InReg;     }
+              | INREG     { $$ = Attribute::InReg;     }
-              | SRET    { $$ = Attribute::StructRet; }
+              | SRET      { $$ = Attribute::StructRet; }
-              | NOALIAS { $$ = Attribute::NoAlias;   }
+              | NOALIAS   { $$ = Attribute::NoAlias;   }
-              | BYVAL   { $$ = Attribute::ByVal;     }
+              | NOCAPTURE { $$ = Attribute::NoCapture; }
-              | NEST    { $$ = Attribute::Nest;      }
+              | BYVAL     { $$ = Attribute::ByVal;     }
+              | NEST      { $$ = Attribute::Nest;      }
               | ALIGN EUINT64VAL { $$ =
                           Attribute::constructAlignmentFromInt($2);    }
               ;

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -362,6 +362,20 @@ bool BitcodeReader::ParseAttributeBlock() {
       Attributes RetAttribute = Attribute::None;
       Attributes FnAttribute = Attribute::None;
       for (unsigned i = 0, e = Record.size(); i != e; i += 2) {
+        // FIXME: remove in LLVM 3.0
+        // The alignment is stored as a 16-bit raw value from bits 31--16.
+        // We shift the bits above 31 down by 11 bits.
+
+        unsigned Alignment = (Record[i+1] & (0xffffull << 16)) >> 16;
+        if (Alignment && !isPowerOf2_32(Alignment))
+          return Error("Alignment is not a power of two.");
+
+        Attributes ReconstitutedAttr = Record[i+1] & 0xffff;
+        if (Alignment)
+          ReconstitutedAttr |= Attribute::constructAlignmentFromInt(Alignment);
+        ReconstitutedAttr |= (Record[i+1] & (0xffffull << 32)) >> 11;
+        Record[i+1] = ReconstitutedAttr;
+
         if (Record[i] == 0)
           RetAttribute = Record[i+1];
         else if (Record[i] == ~0U)

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -122,7 +122,16 @@ static void WriteAttributeTable(const ValueEnumerator &VE,
     for (unsigned i = 0, e = A.getNumSlots(); i != e; ++i) {
       const AttributeWithIndex &PAWI = A.getSlot(i);
       Record.push_back(PAWI.Index);
-      Record.push_back(PAWI.Attrs);
+
+      // FIXME: remove in LLVM 3.0
+      // Store the alignment in the bitcode as a 16-bit raw value instead of a
+      // 5-bit log2 encoded value. Shift the bits above the alignment up by
+      // 11 bits.
+      uint64_t FauxAttr = PAWI.Attrs & 0xffff;
+      FauxAttr |= (1ull<<16)<<((PAWI.Attrs & Attribute::Alignment) >> 16);
+      FauxAttr |= (PAWI.Attrs & (0x3FFull << 21)) << 11;
+
+      Record.push_back(FauxAttr);
     }
     
     Stream.EmitRecord(bitc::PARAMATTR_CODE_ENTRY, Record);

lib/VMCore/Attributes.cpp

@@ -37,6 +37,8 @@ std::string Attribute::getAsString(Attributes Attrs) {
     Result += "inreg ";
   if (Attrs & Attribute::NoAlias)
     Result += "noalias ";
+  if (Attrs & Attribute::NoCapture)
+    Result += "nocapture ";
   if (Attrs & Attribute::StructRet)
     Result += "sret ";  
   if (Attrs & Attribute::ByVal)
@@ -59,10 +61,11 @@ std::string Attribute::getAsString(Attributes Attrs) {
     Result += "sspreq ";
   if (Attrs & Attribute::Alignment) {
     Result += "align ";
-    Result += utostr((Attrs & Attribute::Alignment) >> 16);
+    Result += utostr(1ull << ((Attrs & Attribute::Alignment)>>16));
     Result += " ";
   }
   // Trim the trailing space.
+  assert(!Result.empty() && "Unknown attribute!");
   Result.erase(Result.end()-1);
   return Result;
 }
@@ -76,7 +79,7 @@ Attributes Attribute::typeIncompatible(const Type *Ty) {
   
   if (!isa<PointerType>(Ty))
     // Attributes that only apply to pointers.
-    Incompatible |= ByVal | Nest | NoAlias | StructRet;
+    Incompatible |= ByVal | Nest | NoAlias | StructRet | NoCapture;
   
   return Incompatible;
 }