Resubmit support for the 'nocapture' attribute.

The problematic part of this patch is that we were out of attribute bits,
requiring some fancy bit hacking to make it fit (by shrinking alignment)
without breaking existing users or the file format.

This change will require users to rebuild llvm-gcc to match llvm.

llvm-svn: 61239

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
-    load/store a value: NULL, unique pointers from malloc(0), and freed pointers
-    are considered to not alias anything.</dd>
+    caller. For further details, please see the discussion of the NoAlias
+    response in
+    <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 +45,18 @@ 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 AlwaysInline    = 1<<12; // inline=always
-const Attributes OptimizeForSize = 1<<13; // opt_size
-const Attributes StackProtect    = 1<<14; // Stack protection.
-const Attributes StackProtectReq = 1<<15; // Stack protection required.
-const Attributes Alignment = 0xffff<<16; ///< Alignment of parameter (16 bits)
-                                    // 0 = unknown, else in clear (not log)
-                                    
+const Attributes NoInline        = 1<<11; ///< inline=never 
+const Attributes AlwaysInline    = 1<<12; ///< inline=always
+const Attributes OptimizeForSize = 1<<13; ///< opt_size
+const Attributes StackProtect    = 1<<14; ///< Stack protection.
+const Attributes StackProtectReq = 1<<15; ///< Stack protection required.
+const Attributes Alignment = 31<<16; ///< Alignment of parameter (5 bits)
+                                     // stored as log2 of alignment with +1 bias
+                                     // 0 means unaligned different from align 1
+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 +67,7 @@ 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 +79,13 @@ 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);
+  // Default alignment, allow the target to define how to align it.
+  if (i == 0)
+    return 0;
+
+  assert(isPowerOf2_32(i) && "Alignment must be a power of two.");
+  assert(i <= 0x40000000 && "Alignment too large.");
+  return (Log2_32(i)+1) << 16;
 }
 
 /// The set of Attributes set in Attributes is converted to a
@@ -175,7 +184,11 @@ public:
   /// getParamAlignment - Return the alignment for the specified function
   /// parameter.
   unsigned getParamAlignment(unsigned Idx) const {
-    return (getAttributes(Idx) & Attribute::Alignment) >> 16;
+    Attributes Align = getAttributes(Idx) & Attribute::Alignment;
+    if (Align == 0)
+      return 0;
+
+    return 1ull << ((Align >> 16) - 1);
   }
   
   /// hasAttrSomewhere - Return true if the specified attribute is set for at

lib/Analysis/BasicAliasAnalysis.cpp

@@ -63,12 +63,23 @@ static bool AddressMightEscape(const Value *V) {
       // callees could modify it.
       break; // next use
     case Instruction::Call:
-      // If the call is to a few known safe intrinsics, we know that it does
-      // not escape.
-      // TODO: Eventually just check the 'nocapture' attribute.
-      if (!isa<MemIntrinsic>(I))
-        return true;
-      break;  // next use
+      // If the argument to the call has the nocapture attribute, then the call
+      // may store or load to the pointer, but it cannot escape.
+      if (cast<CallInst>(I)->paramHasAttr(UI.getOperandNo(), 
+                                          Attribute::NoCapture))
+        continue;
+
+      // FIXME: MemIntrinsics should have their operands marked nocapture!
+      if (isa<MemIntrinsic>(I))
+        continue;  // next use
+      return true;
+    case Instruction::Invoke:
+      // If the argument to the call has the nocapture attribute, then the call
+      // may store or load to the pointer, but it cannot escape.
+      if (cast<InvokeInst>(I)->paramHasAttr(UI.getOperandNo()-2,
+                                            Attribute::NoCapture))
+        continue;
+      return true;
     default:
       return true;
     }

test/Analysis/BasicAA/nocapture.ll

@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s | opt -basicaa -gvn -instcombine | llvm-dis | grep {ret i32 0}
+
+declare i32* @test(i32* nocapture)
+
+define i32 @test2() {
+       %P = alloca i32
+       %Q = call i32* @test(i32* %P)
+       %a = load i32* %P
+       store i32 4, i32* %Q   ;; cannot clobber P since it is nocapture.
+       %b = load i32* %P
+       %c = sub i32 %a, %b
+       ret i32 %c
+}
+