Remove WebAssemblyISD::MEMORY_COPY and add comments.

Author: sunfishcode

llvm/lib/Target/WebAssembly/WebAssemblyISD.def

@@ -42,8 +42,6 @@ HANDLE_NODETYPE(PROMOTE_LOW)
 HANDLE_NODETYPE(TRUNC_SAT_ZERO_S)
 HANDLE_NODETYPE(TRUNC_SAT_ZERO_U)
 HANDLE_NODETYPE(DEMOTE_ZERO)
-HANDLE_NODETYPE(MEMORY_COPY)
-HANDLE_NODETYPE(MEMORY_FILL)
 HANDLE_NODETYPE(I64_ADD128)
 HANDLE_NODETYPE(I64_SUB128)
 HANDLE_NODETYPE(I64_MUL_WIDE_S)
@@ -55,6 +53,9 @@ HANDLE_MEM_NODETYPE(GLOBAL_SET)
 HANDLE_MEM_NODETYPE(TABLE_GET)
 HANDLE_MEM_NODETYPE(TABLE_SET)
 
-// Bulk memory instructions that require branching to handle empty ranges.
+// Bulk memory instructions. These follow LLVM's expected semantics of
+// supporting out-of-bounds pointers if the length is zero, by insertig
+// a branch around Wasm's `memory.copy` and `memory.fill`, which would
+// otherwise trap.
 HANDLE_NODETYPE(MEMCPY)
 HANDLE_NODETYPE(MEMSET)

llvm/lib/Target/WebAssembly/WebAssemblyInstrBulkMemory.td

@@ -29,19 +29,13 @@ def wasm_memset_t : SDTypeProfile<0, 4,
   [SDTCisInt<0>, SDTCisPtrTy<1>, SDTCisInt<2>, SDTCisInt<3>]
 >;
 
-// memory.copy (may trap on empty ranges)
-def wasm_memory_copy : SDNode<"WebAssemblyISD::MEMORY_COPY", wasm_memcpy_t,
-                              [SDNPHasChain, SDNPMayLoad, SDNPMayStore]>;
-
-// memory.copy with a branch to avoid trapping
+// memory.copy with a branch to avoid trapping in the case of out-of-bounds
+// pointers with empty ranges.
 def wasm_memcpy : SDNode<"WebAssemblyISD::MEMCPY", wasm_memcpy_t,
                          [SDNPHasChain, SDNPMayLoad, SDNPMayStore]>;
 
-// memory.fill (may trap on empty ranges)
-def wasm_memory_fill : SDNode<"WebAssemblyISD::MEMORY_FILL", wasm_memset_t,
-                              [SDNPHasChain, SDNPMayStore]>;
-
-// memory.fill with a branch to avoid trapping
+// memory.fill with a branch to avoid trapping in the case of out-of-bounds
+// pointers with empty ranges.
 def wasm_memset : SDNode<"WebAssemblyISD::MEMSET", wasm_memset_t,
                          [SDNPHasChain, SDNPMayStore]>;