Apply suggestions from code review

Co-authored-by: Ralf Jung <[email protected]>

Author: scottmcm

Diff for: compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -1164,6 +1164,7 @@ fn codegen_regular_intrinsic_call<'tcx>(
             let params = vec![AbiParam::new(fx.pointer_type); 3];
             let returns = vec![AbiParam::new(types::I32)];
             let args = &[lhs_ptr, rhs_ptr, bytes_val];
+            // Here we assume that the `memcmp` provided by the target is a NOP for size 0.
             let cmp = fx.lib_call("memcmp", params, returns, args)[0];
             ret.write_cvalue(fx, CValue::by_val(cmp, ret.layout()));
         }

Diff for: compiler/rustc_codegen_gcc/src/intrinsic/mod.rs

@@ -311,6 +311,7 @@ impl<'a, 'gcc, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                     let a_ptr = self.bitcast(a, void_ptr_type);
                     let b_ptr = self.bitcast(b, void_ptr_type);
 
+                    // Here we assume that the `memcmp` provided by the target is a NOP for size 0.
                     let builtin = self.context.get_builtin_function("memcmp");
                     let cmp = self.context.new_call(None, builtin, &[a_ptr, b_ptr, n]);
                     self.sext(cmp, self.type_ix(32))

Diff for: compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -330,6 +330,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
             }
 
             sym::compare_bytes => {
+                // Here we assume that the `memcmp` provided by the target is a NOP for size 0.
                 let cmp = self.call_intrinsic(
                     "memcmp",
                     &[args[0].immediate(), args[1].immediate(), args[2].immediate()],