[LangRef] Comment on validity of volatile ops on null (#139803)

Some hardware (for example, certain AVR chips) have peripheral registers
mapped to the data space address 0. Although a volatile load/store on
`ptr null` already generates expected code, the wording in the LangRef
makes operations on null seem like undefined behavior in all cases. This
commit adds a comment that, for volatile operations, it may be defined
behavior to access the address null, if the architecture permits it. The
intended use case is MMIO registers with hard-coded addresses that
include bit-value 0. A simple CodeGen test is included for AVR, as an
architecture known to have this quirk, that does `load volatile` and
`store volatile` to `ptr null`, expecting to generate `lds <reg>, 0` and
`sts 0, <reg>`.

See [this
thread](https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/Adding.20the.20possibility.20of.20volatile.20access.20to.20address.200)
and [the
RFC](https://discourse.llvm.org/t/rfc-volatile-access-to-non-dereferenceable-memory-may-be-well-defined/86303)
for discussion and context.

Author: LuigiPiucco

llvm/docs/LangRef.rst

@@ -3573,7 +3573,8 @@ can read and/or modify state which is not accessible via a regular load
 or store in this module. Volatile operations may use addresses which do
 not point to memory (like MMIO registers). This means the compiler may
 not use a volatile operation to prove a non-volatile access to that
-address has defined behavior.
+address has defined behavior. This includes addresses typically forbidden,
+such as the pointer with bit-value 0.
 
 The allowed side-effects for volatile accesses are limited.  If a
 non-volatile store to a given address would be legal, a volatile
@@ -4292,7 +4293,10 @@ The semantics of non-zero address spaces are target-specific. Memory
 access through a non-dereferenceable pointer is undefined behavior in
 any address space. Pointers with the bit-value 0 are only assumed to
 be non-dereferenceable in address space 0, unless the function is
-marked with the ``null_pointer_is_valid`` attribute.
+marked with the ``null_pointer_is_valid`` attribute. However, *volatile*
+access to any non-dereferenceable address may have defined behavior
+(according to the target), and in this case the attribute is not needed
+even for address 0.
 
 If an object can be proven accessible through a pointer with a
 different address space, the access may be modified to use that

llvm/test/CodeGen/AVR/volatile-null.ll

@@ -0,0 +1,15 @@
+; RUN: llc < %s -mtriple=avr | FileCheck %s
+
+define i8 @load_volatile_null() {
+; CHECK-LABEL: load_volatile_null:
+; CHECK: lds r24, 0
+    %result = load volatile i8, ptr null
+    ret i8 %result
+}
+
+define void @store_volatile_null(i8 %a) {
+; CHECK-LABEL: store_volatile_null:
+; CHECK: sts 0, r24
+    store volatile i8 %a, ptr null
+    ret void
+}