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2 Design and Usage of the InAlloca Attribute
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8 The :ref:`inalloca <attr_inalloca>` attribute is designed to allow
9 taking the address of an aggregate argument that is being passed by
10 value through memory. Primarily, this feature is required for
11 compatibility with the Microsoft C++ ABI. Under that ABI, class
12 instances that are passed by value are constructed directly into
13 argument stack memory. Prior to the addition of inalloca, calls in LLVM
14 were indivisible instructions. There was no way to perform intermediate
15 work, such as object construction, between the first stack adjustment
16 and the final control transfer. With inalloca, all arguments passed in
17 memory are modelled as a single alloca, which can be stored to prior to
18 the call. Unfortunately, this complicated feature comes with a large
19 set of restrictions designed to bound the lifetime of the argument
20 memory around the call.
22 For now, it is recommended that frontends and optimizers avoid producing
23 this construct, primarily because it forces the use of a base pointer.
24 This feature may grow in the future to allow general mid-level
25 optimization, but for now, it should be regarded as less efficient than
26 passing by value with a copy.
31 The example below is the intended LLVM IR lowering for some C++ code
32 that passes two default-constructed ``Foo`` objects to ``g`` in the
33 32-bit Microsoft C++ ABI.
37 // Foo is non-trivial.
38 struct Foo { int a, b; Foo(); ~Foo(); Foo(const Foo &); };
46 %struct.Foo = type { i32, i32 }
47 declare void @Foo_ctor(%struct.Foo* %this)
48 declare void @Foo_dtor(%struct.Foo* %this)
49 declare void @g(<{ %struct.Foo, %struct.Foo }>* inalloca %memargs)
53 %base = call i8* @llvm.stacksave()
54 %memargs = alloca <{ %struct.Foo, %struct.Foo }>
55 %b = getelementptr <{ %struct.Foo, %struct.Foo }>* %memargs, i32 1
56 call void @Foo_ctor(%struct.Foo* %b)
58 ; If a's ctor throws, we must destruct b.
59 %a = getelementptr <{ %struct.Foo, %struct.Foo }>* %memargs, i32 0
60 invoke void @Foo_ctor(%struct.Foo* %a)
61 to label %invoke.cont unwind %invoke.unwind
64 call void @g(<{ %struct.Foo, %struct.Foo }>* inalloca %memargs)
65 call void @llvm.stackrestore(i8* %base)
69 call void @Foo_dtor(%struct.Foo* %b)
70 call void @llvm.stackrestore(i8* %base)
74 To avoid stack leaks, the frontend saves the current stack pointer with
75 a call to :ref:`llvm.stacksave <int_stacksave>`. Then, it allocates the
76 argument stack space with alloca and calls the default constructor. The
77 default constructor could throw an exception, so the frontend has to
78 create a landing pad. The frontend has to destroy the already
79 constructed argument ``b`` before restoring the stack pointer. If the
80 constructor does not unwind, ``g`` is called. In the Microsoft C++ ABI,
81 ``g`` will destroy its arguments, and then the stack is restored in
90 The biggest design consideration for this feature is object lifetime.
91 We cannot model the arguments as static allocas in the entry block,
92 because all calls need to use the memory at the top of the stack to pass
93 arguments. We cannot vend pointers to that memory at function entry
94 because after code generation they will alias.
96 The rule against allocas between argument allocations and the call site
97 avoids this problem, but it creates a cleanup problem. Cleanup and
98 lifetime is handled explicitly with stack save and restore calls. In
99 the future, we may want to introduce a new construct such as ``freea``
100 or ``afree`` to make it clear that this stack adjusting cleanup is less
101 powerful than a full stack save and restore.
103 Nested Calls and Copy Elision
104 -----------------------------
106 We also want to be able to support copy elision into these argument
107 slots. This means we have to support multiple live argument
110 Consider the evaluation of:
114 // Foo is non-trivial.
115 struct Foo { int a; Foo(); Foo(const &Foo); ~Foo(); };
121 In this case, we want to be able to elide copies into ``bar``'s argument
122 slots. That means we need to have more than one set of argument frames
123 active at the same time. First, we need to allocate the frame for the
124 outer call so we can pass it in as the hidden struct return pointer to
125 the middle call. Then we do the same for the middle call, allocating a
126 frame and passing its address to ``Foo``'s default constructor. By
127 wrapping the evaluation of the inner ``bar`` with stack save and
128 restore, we can have multiple overlapping active call frames.
130 Callee-cleanup Calling Conventions
131 ----------------------------------
133 Another wrinkle is the existence of callee-cleanup conventions. On
134 Windows, all methods and many other functions adjust the stack to clear
135 the memory used to pass their arguments. In some sense, this means that
136 the allocas are automatically cleared by the call. However, LLVM
137 instead models this as a write of undef to all of the inalloca values
138 passed to the call instead of a stack adjustment. Frontends should
139 still restore the stack pointer to avoid a stack leak.
144 There is also the possibility of an exception. If argument evaluation
145 or copy construction throws an exception, the landing pad must do
146 cleanup, which includes adjusting the stack pointer to avoid a stack
147 leak. This means the cleanup of the stack memory cannot be tied to the
148 call itself. There needs to be a separate IR-level instruction that can
149 perform independent cleanup of arguments.
154 Eventually, it should be possible to generate efficient code for this
155 construct. In particular, using inalloca should not require a base
156 pointer. If the backend can prove that all points in the CFG only have
157 one possible stack level, then it can address the stack directly from
158 the stack pointer. While this is not yet implemented, the plan is that
159 the inalloca attribute should not change much, but the frontend IR
160 generation recommendations may change.