[mlir][LLVM] Model side effects of volatile and atomic load-store (#65730)

According to the LLVM language reference, both volatile memory
operations and atomic operations (except unordered) do not simply read
memory but also perform write operations on arbitrary memory[0][1].

In the case of volatile memory operations, this is the case due to the
read possibly having target specific properties. A common real-world
situation where this happens is reading memory mapped registers on an
MCU for example. Atomic operations are more special. They form a kind of
memory barrier which from the perspective of the optimizer/lang-ref
makes writes from other threads visible in the current thread. Any kind
of synchronization can therefore conservatively be modeled as a
write-effect.

This PR therefore adjusts the side effects of `llvm.load` and
`llvm.store` to add unknown global read and write effects if they are
either atomic or volatile.

Regarding testing: I am not sure how to best test this change for
`llvm.store` and the "globalness" of the effect that isn't just a unit
test checking that the output matches exactly. For the time being, I
added a test making sure that `llvm.load` does not get DCEd in
aforementioned cases.

Related logic in LLVM proper:

3398744a61/llvm/lib/IR/Instruction.cpp (L638-L676)


3398744a61/llvm/include/llvm/IR/Instructions.h (L258-L262)

[0] https://llvm.org/docs/LangRef.html#volatile-memory-accesses
[1] https://llvm.org/docs/Atomics.html#monotonic

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

@@ -326,9 +326,10 @@ def LLVM_GEPOp : LLVM_Op<"getelementptr", [Pure,
 }
 
 def LLVM_LoadOp : LLVM_MemAccessOpBase<"load",
-    [DeclareOpInterfaceMethods<PromotableMemOpInterface>,
+    [DeclareOpInterfaceMethods<MemoryEffectsOpInterface>,
+     DeclareOpInterfaceMethods<PromotableMemOpInterface>,
      DeclareOpInterfaceMethods<SafeMemorySlotAccessOpInterface>]> {
-  dag args = (ins Arg<LLVM_PointerTo<LLVM_LoadableType>, "", [MemRead]>:$addr,
+  dag args = (ins LLVM_PointerTo<LLVM_LoadableType>:$addr,
               OptionalAttr<I64Attr>:$alignment,
               UnitAttr:$volatile_,
               UnitAttr:$nontemporal,
@@ -399,10 +400,11 @@ def LLVM_LoadOp : LLVM_MemAccessOpBase<"load",
 }
 
 def LLVM_StoreOp : LLVM_MemAccessOpBase<"store",
-    [DeclareOpInterfaceMethods<PromotableMemOpInterface>,
+    [DeclareOpInterfaceMethods<MemoryEffectsOpInterface>,
+     DeclareOpInterfaceMethods<PromotableMemOpInterface>,
      DeclareOpInterfaceMethods<SafeMemorySlotAccessOpInterface>]> {
   dag args = (ins LLVM_LoadableType:$value,
-              Arg<LLVM_PointerTo<LLVM_LoadableType>,"",[MemWrite]>:$addr,
+              LLVM_PointerTo<LLVM_LoadableType>:$addr,
               OptionalAttr<I64Attr>:$alignment,
               UnitAttr:$volatile_,
               UnitAttr:$nontemporal,

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -812,6 +812,22 @@ Type GEPOp::getResultPtrElementType() {
 // LoadOp
 //===----------------------------------------------------------------------===//
 
+void LoadOp::getEffects(
+    SmallVectorImpl<SideEffects::EffectInstance<MemoryEffects::Effect>>
+        &effects) {
+  effects.emplace_back(MemoryEffects::Read::get(), getAddr());
+  // Volatile operations can have target-specific read-write effects on
+  // memory besides the one referred to by the pointer operand.
+  // Similarly, atomic operations that are monotonic or stricter cause
+  // synchronization that from a language point-of-view, are arbitrary
+  // read-writes into memory.
+  if (getVolatile_() || (getOrdering() != AtomicOrdering::not_atomic &&
+                         getOrdering() != AtomicOrdering::unordered)) {
+    effects.emplace_back(MemoryEffects::Write::get());
+    effects.emplace_back(MemoryEffects::Read::get());
+  }
+}
+
 /// Returns true if the given type is supported by atomic operations. All
 /// integer and float types with limited bit width are supported. Additionally,
 /// depending on the operation pointers may be supported as well.
@@ -932,6 +948,22 @@ static void printLoadType(OpAsmPrinter &printer, Operation *op, Type type,
 // StoreOp
 //===----------------------------------------------------------------------===//
 
+void StoreOp::getEffects(
+    SmallVectorImpl<SideEffects::EffectInstance<MemoryEffects::Effect>>
+        &effects) {
+  effects.emplace_back(MemoryEffects::Write::get(), getAddr());
+  // Volatile operations can have target-specific read-write effects on
+  // memory besides the one referred to by the pointer operand.
+  // Similarly, atomic operations that are monotonic or stricter cause
+  // synchronization that from a language point-of-view, are arbitrary
+  // read-writes into memory.
+  if (getVolatile_() || (getOrdering() != AtomicOrdering::not_atomic &&
+                         getOrdering() != AtomicOrdering::unordered)) {
+    effects.emplace_back(MemoryEffects::Write::get());
+    effects.emplace_back(MemoryEffects::Read::get());
+  }
+}
+
 LogicalResult StoreOp::verify() {
   Type valueType = getValue().getType();
   return verifyAtomicMemOp(*this, valueType,

mlir/test/Dialect/LLVMIR/canonicalize.mlir

@@ -191,3 +191,20 @@ llvm.func @alloca_dce() {
   %0 = llvm.alloca %c1_i64 x i32 : (i64) -> !llvm.ptr
   llvm.return
 }
+
+// -----
+
+// CHECK-LABEL: func @volatile_load
+llvm.func @volatile_load(%x : !llvm.ptr) {
+  // A volatile load may have side-effects such as a write operation to arbitrary memory.
+  // Make sure it is not removed.
+  // CHECK: llvm.load volatile
+  %0 = llvm.load volatile %x : !llvm.ptr -> i8
+  // Same with monotonic atomics and any stricter modes.
+  // CHECK: llvm.load %{{.*}} atomic monotonic
+  %2 = llvm.load %x atomic monotonic { alignment = 1 } : !llvm.ptr -> i8
+  // But not unordered!
+  // CHECK-NOT: llvm.load %{{.*}} atomic unordered
+  %3 = llvm.load %x  atomic unordered { alignment = 1 } : !llvm.ptr -> i8
+  llvm.return
+}