Document the maximum LLVM IR alignment, which is 1 << 29 or 0.5 GiB

Add verifier checks.  We already check these in the assembly parser, but
a frontend producing IR in memory wouldn't hit those checks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213027 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LangRef.rst

@@ -582,7 +582,7 @@ to over-align the global if the global has an assigned section. In this
 case, the extra alignment could be observable: for example, code could
 assume that the globals are densely packed in their section and try to
 iterate over them as an array, alignment padding would break this
-iteration.
+iteration. The maximum alignment is ``1 << 29``.
 
 Globals can also have a :ref:`DLL storage class <dllstorageclass>`.
 
@@ -4925,9 +4925,10 @@ bytes of memory on the runtime stack, returning a pointer of the
 appropriate type to the program. If "NumElements" is specified, it is
 the number of elements allocated, otherwise "NumElements" is defaulted
 to be one. If a constant alignment is specified, the value result of the
-allocation is guaranteed to be aligned to at least that boundary. If not
+allocation is guaranteed to be aligned to at least that boundary. The
-specified, or if zero, the target can choose to align the allocation on
+alignment may not be greater than ``1 << 29``. If not specified, or if
-any convenient boundary compatible with the type.
+zero, the target can choose to align the allocation on any convenient
+boundary compatible with the type.
 
 '``type``' may be any sized type.
 
@@ -5001,7 +5002,8 @@ or an omitted ``align`` argument means that the operation has the ABI
 alignment for the target. It is the responsibility of the code emitter
 to ensure that the alignment information is correct. Overestimating the
 alignment results in undefined behavior. Underestimating the alignment
-may produce less efficient code. An alignment of 1 is always safe.
+may produce less efficient code. An alignment of 1 is always safe. The
+maximum possible alignment is ``1 << 29``.
 
 The optional ``!nontemporal`` metadata must reference a single
 metadata name ``<index>`` corresponding to a metadata node with one
@@ -5087,7 +5089,7 @@ alignment for the target. It is the responsibility of the code emitter
 to ensure that the alignment information is correct. Overestimating the
 alignment results in undefined behavior. Underestimating the
 alignment may produce less efficient code. An alignment of 1 is always
-safe.
+safe. The maximum possible alignment is ``1 << 29``.
 
 The optional ``!nontemporal`` metadata must reference a single metadata
 name ``<index>`` corresponding to a metadata node with one ``i32`` entry of

lib/IR/Verifier.cpp

@@ -380,6 +380,8 @@ void Verifier::visitGlobalValue(const GlobalValue &GV) {
           "Global is external, but doesn't have external or weak linkage!",
           &GV);
 
+  Assert1(GV.getAlignment() <= Value::MaximumAlignment,
+          "huge alignment values are unsupported", &GV);
   Assert1(!GV.hasAppendingLinkage() || isa<GlobalVariable>(GV),
           "Only global variables can have appending linkage!", &GV);
 
@@ -1891,6 +1893,8 @@ void Verifier::visitLoadInst(LoadInst &LI) {
   Type *ElTy = PTy->getElementType();
   Assert2(ElTy == LI.getType(),
           "Load result type does not match pointer operand type!", &LI, ElTy);
+  Assert1(LI.getAlignment() <= Value::MaximumAlignment,
+          "huge alignment values are unsupported", &LI);
   if (LI.isAtomic()) {
     Assert1(LI.getOrdering() != Release && LI.getOrdering() != AcquireRelease,
             "Load cannot have Release ordering", &LI);
@@ -1966,6 +1970,8 @@ void Verifier::visitStoreInst(StoreInst &SI) {
   Assert2(ElTy == SI.getOperand(0)->getType(),
           "Stored value type does not match pointer operand type!",
           &SI, ElTy);
+  Assert1(SI.getAlignment() <= Value::MaximumAlignment,
+          "huge alignment values are unsupported", &SI);
   if (SI.isAtomic()) {
     Assert1(SI.getOrdering() != Acquire && SI.getOrdering() != AcquireRelease,
             "Store cannot have Acquire ordering", &SI);
@@ -1997,6 +2003,8 @@ void Verifier::visitAllocaInst(AllocaInst &AI) {
           &AI);
   Assert1(AI.getArraySize()->getType()->isIntegerTy(),
           "Alloca array size must have integer type", &AI);
+  Assert1(AI.getAlignment() <= Value::MaximumAlignment,
+          "huge alignment values are unsupported", &AI);
 
   visitInstruction(AI);
 }