[LangRef][Local] dereferenceable metadata violation is UB

I believe !dereferencable violation is immediate undefined behavior,
but this was not explicitly spelled out in LangRef. We already
assume that !dereferenceable is implicitly !noundef and cannot
return poison in isGuaranteedNotToBeUndefOrPoison().

The reason why we made dereferenceable implicitly noundef is that
the purpose of this metadata is to allow speculation, and that
would not be legal on a potential poison pointer.

Differential Revision: https://reviews.llvm.org/D148202

Author: nikic

llvm/docs/LangRef.rst

@@ -6656,7 +6656,8 @@ or switch that it is attached to is completely unpredictable.
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 The existence of the ``!dereferenceable`` metadata on the instruction
-tells the optimizer that the value loaded is known to be dereferenceable.
+tells the optimizer that the value loaded is known to be dereferenceable,
+otherwise the behavior is undefined.
 The number of bytes known to be dereferenceable is specified by the integer
 value in the metadata node. This is analogous to the ''dereferenceable''
 attribute on parameters and return values.
@@ -6668,7 +6669,7 @@ attribute on parameters and return values.
 
 The existence of the ``!dereferenceable_or_null`` metadata on the
 instruction tells the optimizer that the value loaded is known to be either
-dereferenceable or null.
+dereferenceable or null, otherwise the behavior is undefined.
 The number of bytes known to be dereferenceable is specified by the integer
 value in the metadata node. This is analogous to the ''dereferenceable_or_null''
 attribute on parameters and return values.

llvm/lib/Transforms/Utils/Local.cpp

@@ -2701,8 +2701,9 @@ void llvm::combineMetadata(Instruction *K, const Instruction *J,
         break;
       case LLVMContext::MD_dereferenceable:
       case LLVMContext::MD_dereferenceable_or_null:
-        K->setMetadata(Kind,
-          MDNode::getMostGenericAlignmentOrDereferenceable(JMD, KMD));
+        if (DoesKMove)
+          K->setMetadata(Kind,
+            MDNode::getMostGenericAlignmentOrDereferenceable(JMD, KMD));
         break;
       case LLVMContext::MD_preserve_access_index:
         // Preserve !preserve.access.index in K.

llvm/test/Transforms/GVN/metadata.ll

@@ -138,7 +138,7 @@ define i32 @load_load_noundef(ptr %p) {
 define void @load_dereferenceable_dominating(ptr %p) {
 ; CHECK-LABEL: define void @load_dereferenceable_dominating
 ; CHECK-SAME: (ptr [[P:%.*]]) {
-; CHECK-NEXT:    [[A:%.*]] = load ptr, ptr [[P]], align 8
+; CHECK-NEXT:    [[A:%.*]] = load ptr, ptr [[P]], align 8, !dereferenceable !7
 ; CHECK-NEXT:    call void @use.ptr(ptr [[A]])
 ; CHECK-NEXT:    call void @use.ptr(ptr [[A]])
 ; CHECK-NEXT:    ret void