Define a maximum supported alignment value for load, store, and

alloca instructions (constrained by their internal encoding),
and add error checking for it. Fix an instcombine bug which
generated huge alignment values (null is infinitely aligned).
This fixes undefined behavior noticed by John Regehr.

llvm-svn: 109643

llvm/include/llvm/Instructions.h

@@ -31,6 +31,10 @@ class APInt;
 class LLVMContext;
 class DominatorTree;
 
+/// MaximumAlignment - This is the greatest alignment value supported by
+/// load, store, and alloca instructions.
+static const unsigned MaximumAlignment = 1u << 29;
+
 //===----------------------------------------------------------------------===//
 //                                AllocaInst Class
 //===----------------------------------------------------------------------===//

llvm/lib/AsmParser/LLParser.cpp

@@ -1154,6 +1154,8 @@ bool LLParser::ParseOptionalAlignment(unsigned &Alignment) {
   if (ParseUInt32(Alignment)) return true;
   if (!isPowerOf2_32(Alignment))
     return Error(AlignLoc, "alignment is not a power of two");
+  if (Alignment > MaximumAlignment)
+    return Error(AlignLoc, "huge alignments are not supported yet");
   return false;
 }
 
@@ -1176,6 +1178,7 @@ bool LLParser::ParseOptionalCommaAlign(unsigned &Alignment,
     if (Lex.getKind() != lltok::kw_align)
       return Error(Lex.getLoc(), "expected metadata or 'align'");
     
+    LocTy AlignLoc = Lex.getLoc();
     if (ParseOptionalAlignment(Alignment)) return true;
   }
 

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -104,11 +104,15 @@ unsigned InstCombiner::GetOrEnforceKnownAlignment(Value *V,
   ComputeMaskedBits(V, Mask, KnownZero, KnownOne);
   unsigned TrailZ = KnownZero.countTrailingOnes();
 
-  // LLVM doesn't support alignments larger than this currently.
+  // Avoid trouble with rediculously large TrailZ values, such as
+  // those computed from a null pointer.
   TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1));
 
   unsigned Align = 1u << std::min(BitWidth - 1, TrailZ);
 
+  // LLVM doesn't support alignments larger than this currently.
+  Align = std::min(Align, MaximumAlignment);
+
   if (PrefAlign > Align)
     Align = EnforceKnownAlignment(V, Align, PrefAlign);
   

llvm/lib/VMCore/Instructions.cpp

@@ -891,6 +891,8 @@ AllocaInst::~AllocaInst() {
 
 void AllocaInst::setAlignment(unsigned Align) {
   assert((Align & (Align-1)) == 0 && "Alignment is not a power of 2!");
+  assert(Align <= MaximumAlignment &&
+         "Alignment is greater than MaximumAlignment!");
   setInstructionSubclassData(Log2_32(Align) + 1);
   assert(getAlignment() == Align && "Alignment representation error!");
 }
@@ -1026,8 +1028,11 @@ LoadInst::LoadInst(Value *Ptr, const char *Name, bool isVolatile,
 
 void LoadInst::setAlignment(unsigned Align) {
   assert((Align & (Align-1)) == 0 && "Alignment is not a power of 2!");
+  assert(Align <= MaximumAlignment &&
+         "Alignment is greater than MaximumAlignment!");
   setInstructionSubclassData((getSubclassDataFromInstruction() & 1) |
                              ((Log2_32(Align)+1)<<1));
+  assert(getAlignment() == Align && "Alignment representation error!");
 }
 
 //===----------------------------------------------------------------------===//
@@ -1122,8 +1127,11 @@ StoreInst::StoreInst(Value *val, Value *addr, bool isVolatile,
 
 void StoreInst::setAlignment(unsigned Align) {
   assert((Align & (Align-1)) == 0 && "Alignment is not a power of 2!");
+  assert(Align <= MaximumAlignment &&
+         "Alignment is greater than MaximumAlignment!");
   setInstructionSubclassData((getSubclassDataFromInstruction() & 1) |
                              ((Log2_32(Align)+1) << 1));
+  assert(getAlignment() == Align && "Alignment representation error!");
 }
 
 //===----------------------------------------------------------------------===//

llvm/test/Assembler/align-inst-alloca.ll

@@ -0,0 +1,6 @@
+; RUN: not llvm-as %s -o /dev/null 2>/dev/null
+
+define void @foo() {
+  %p = alloca i1, align 1073741824
+  ret void
+}

llvm/test/Assembler/align-inst-load.ll

@@ -0,0 +1,6 @@
+; RUN: not llvm-as %s -o /dev/null 2>/dev/null
+
+define void @foo() {
+  load i1* %p, align 1073741824
+  ret void
+}

llvm/test/Assembler/align-inst-store.ll

@@ -0,0 +1,6 @@
+; RUN: not llvm-as %s -o /dev/null 2>/dev/null
+
+define void @foo() {
+  store i1 false, i1* %p, align 1073741824
+  ret void
+}

llvm/test/Assembler/align-inst.ll

@@ -0,0 +1,8 @@
+; RUN: llvm-as %s -o /dev/null
+
+define void @foo() {
+  %p = alloca i1, align 536870912
+  load i1* %p, align 536870912
+  store i1 false, i1* %p, align 536870912
+  ret void
+}