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[CIR][NFC] Add more examples for vtable-related ops (#1782)
This adds a few C++ examples to the documentation for the cir.vtable operations.
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‎clang/include/clang/CIR/Dialect/IR/CIROps.td‎

Lines changed: 169 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -2608,14 +2608,56 @@ def CIR_VTableAddrPointOp : CIR_Op<"vtable.address_point", [
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the vtable group (as specified by Itanium ABI), and `address_point.offset`
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(address point index) the actual address point within that vtable.
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The `name` argument to this operation must be the name of a C++ vtable
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object. The return value is the address of the virtual function pointer
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array within the vtable (the vptr). This value will be written to the
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vptr member of a dynamic class by the constructor of the class. Derived
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classes have their own vtable, which is used to obtain the vptr stored
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in instances of the derived class.
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26112618
The return type is always `!cir.vptr`.
26122619

2613-
Example:
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Examples:
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2622+
```C++
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struct Base {
2624+
Base();
2625+
virtual void f();
2626+
};
2627+
struct Derived : public Base {
2628+
Derived();
2629+
};
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```
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26142632
```mlir
2615-
cir.global linkonce_odr @_ZTV1B = ...
2633+
!rec_Base = !cir.record<struct "Base" {!cir.vptr}
2634+
!rec_Derived = !cir.record<struct "Derived" {!rec_Base}
2635+
...
2636+
// VTable for Base
2637+
cir.global linkonce_odr @_ZTV1Base = ...
26162638
...
2617-
%3 = cir.vtable.address_point(@_ZTV1B,
2639+
// Constructor for Base
2640+
cir.func dso_local @_ZN4BaseC2Ev ...
2641+
...
2642+
%2 = cir.vtable.address_point(@_ZTV1Base,
26182643
address_point = <index = 0, offset = 2>) : !cir.vptr
2644+
%3 = cir.vtable.get_vptr %1 : !cir.ptr<!rec_Base> -> !cir.ptr<!cir.vptr>
2645+
cir.store align(8) %2, %3 : !cir.vptr, !cir.ptr<!cir.vptr>
2646+
...
2647+
// VTable for Derived
2648+
cir.global linkonce_odr @_ZTV7Derived = ...
2649+
...
2650+
// Constructor for Derived
2651+
cir.func dso_local @_ZN7DerivedC2Ev ...
2652+
// Get the address of Base within this Derived instance
2653+
%2 = cir.base_class_addr %1 : !cir.ptr<!rec_Derived> nonnull [0]
2654+
cir.call @_ZN4BaseC2Ev(%2)
2655+
%3 = cir.vtable.address_point(@_ZTV7Derived,
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address_point = <index = 0, offset = 2>) : !cir.vptr
2657+
// The vptr is still at the start of the object in this case
2658+
%4 = cir.vtable.get_vptr %1 : !cir.ptr<!rec_Derived> -> !cir.ptr<!cir.vptr>
2659+
// This overwrites the vptr that was stored in the Base constructor call
2660+
cir.store align(8) %3, %4 : !cir.vptr, !cir.ptr<!cir.vptr>
26192661
```
26202662
}];
26212663

@@ -2650,9 +2692,41 @@ def CIR_VTableGetVPtrOp : CIR_Op<"vtable.get_vptr", [Pure]> {
26502692
The return type is always `!cir.ptr<!cir.vptr>`.
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26522694
Example:
2695+
```C++
2696+
struct S {
2697+
virtual void f1();
2698+
virtual void f2();
2699+
};
2700+
void f3(S *s) {
2701+
s->f2();
2702+
}
2703+
```
2704+
26532705
```mlir
2654-
%2 = cir.load %0 : !cir.ptr<!cir.ptr<!rec_C>>, !cir.ptr<!rec_C>
2655-
%3 = cir.vtable.get_vptr %2 : !cir.ptr<!rec_C> -> !cir.ptr<!cir.vptr>
2706+
// VTable for S
2707+
cir.global external @_ZTV1S = #cir.vtable<{
2708+
#cir.const_array<[
2709+
// Offset to the base object
2710+
#cir.ptr<null> : !cir.ptr<!u8i>,
2711+
// Type info for S
2712+
#cir.global_view<@_ZTI1S> : !cir.ptr<!u8i>,
2713+
// Pointer to S::f1
2714+
#cir.global_view<@_ZN1S2f1Ev> : !cir.ptr<!u8i>,
2715+
// Pointer to S::f2
2716+
#cir.global_view<@_ZN1S2f2Ev> : !cir.ptr<!u8i>
2717+
]> : !cir.array<!cir.ptr<!u8i> x 4>}> ...
2718+
// f3()
2719+
cir.func dso_local @_Z2f3P1S(%s: !cir.ptr<!rec_S>) {
2720+
// Get the vptr -- This points to offset 2 in the vtable.
2721+
%1 = cir.vtable.get_vptr %s : !cir.ptr<!rec_S> -> !cir.ptr<!cir.vptr>
2722+
%2 = cir.load align(8) %1 : !cir.ptr<!cir.vptr>, !cir.vptr
2723+
// Get the address of b->f2() -- may be Base::f2() or Derived::f2()
2724+
%3 = cir.vtable.get_virtual_fn_addr %2[1] : !cir.vptr
2725+
-> !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_Base>)>>>
2726+
%4 = cir.load align(8) %3
2727+
: !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_Base>)>>>,
2728+
!cir.ptr<!cir.func<(!cir.ptr<!rec_Base>)>>
2729+
cir.call %4(%b)
26562730
```
26572731
}];
26582732

@@ -2689,19 +2763,98 @@ def CIR_VTableGetVirtualFnAddrOp : CIR_Op<"vtable.get_virtual_fn_addr", [
26892763

26902764
The return type is a pointer-to-pointer to the function type.
26912765

2692-
Example:
2766+
Example 1:
2767+
Suppose we have two classes, Base and Derived, where Derived overrides
2768+
virtual functions that were defined in Base. When a pointer to a Base
2769+
object is used to call one of these function, we may not know at compile
2770+
time whether it points to an instance of Base or an instance of Derived.
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The compiler does not need to know. It will load the vptr from the object
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and use that to get the address of the correct function to call. The
2773+
vptr will have been initialized in the object's constructor to point to
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the correct vtable for the object being instantiated.
2775+
```C++
2776+
// In this example, when f3 is called, we don't know at compile-time
2777+
// whether
2778+
struct Base {
2779+
virtual void f1();
2780+
virtual void f2();
2781+
};
2782+
struct Derived : public Base {
2783+
void f1() override;
2784+
void f2() override;
2785+
};
2786+
void f3(Base *b) {
2787+
b->f2();
2788+
}
2789+
```
2790+
26932791
```mlir
2694-
%2 = cir.load %0 : !cir.ptr<!cir.ptr<!rec_C>>, !cir.ptr<!rec_C>
2695-
%3 = cir.vtable.get_vptr %2 : !cir.ptr<!rec_C> -> !cir.ptr<!cir.vptr>
2696-
%4 = cir.load %3 : !cir.ptr<!cir.vptr>, !cir.vptr
2697-
%5 = cir.vtable.get_virtual_fn_addr %4[2] : !cir.vptr
2698-
-> !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_C>) -> !s32i>>>
2699-
%6 = cir.load align(8) %5 : !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_C>)
2700-
-> !s32i>>>,
2701-
!cir.ptr<!cir.func<(!cir.ptr<!rec_C>) -> !s32i>>
2702-
%7 = cir.call %6(%2) : (!cir.ptr<!cir.func<(!cir.ptr<!rec_C>) -> !s32i>>,
2703-
!cir.ptr<!rec_C>) -> !s32i
2792+
// VTable for Base
2793+
cir.global external @_ZTV4Base = #cir.vtable<{
2794+
#cir.const_array<[
2795+
#cir.ptr<null> : !cir.ptr<!u8i>,
2796+
#cir.global_view<@_ZTI4Base> : !cir.ptr<!u8i>,
2797+
#cir.global_view<@_ZN4Base2f1Ev> : !cir.ptr<!u8i>,
2798+
#cir.global_view<@_ZN4Base2f2Ev> : !cir.ptr<!u8i>
2799+
]> : !cir.array<!cir.ptr<!u8i> x 4>}> ...
2800+
// VTable for Derived
2801+
cir.global external @_ZTV7Derived = #cir.vtable<{
2802+
#cir.const_array<[
2803+
#cir.ptr<null> : !cir.ptr<!u8i>,
2804+
#cir.global_view<@_ZTI7Derived> : !cir.ptr<!u8i>,
2805+
#cir.global_view<@_ZN7Derived2f1Ev> : !cir.ptr<!u8i>,
2806+
#cir.global_view<@_ZN7Derived2f2Ev> : !cir.ptr<!u8i>
2807+
]> : !cir.array<!cir.ptr<!u8i> x 4>}> ...
2808+
// f3()
2809+
cir.func dso_local @_Z2f3P4Base(%b: !cir.ptr<!rec_Base>)
2810+
// Get the vptr
2811+
%1 = cir.vtable.get_vptr %b : !cir.ptr<!rec_Base> -> !cir.ptr<!cir.vptr>
2812+
%2 = cir.load align(8) %1 : !cir.ptr<!cir.vptr>, !cir.vptr
2813+
// Get the address of b->f2() -- may be Base::f2() or Derived::f2()
2814+
%3 = cir.vtable.get_virtual_fn_addr %2[1] : !cir.vptr
2815+
-> !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_Base>)>>>
2816+
%4 = cir.load align(8) %3
2817+
: !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_Base>)>>>,
2818+
!cir.ptr<!cir.func<(!cir.ptr<!rec_Base>)>>
2819+
cir.call %4(%b)
27042820
```
2821+
2822+
Example 2:
2823+
Consider the case of multiple inheritance, where Base1 and Base2 both
2824+
provide virtual functions and a third class, Derived, inherits from both
2825+
bases. When a pointer to a Derived is used to call a virtual function in
2826+
Base2, we must retrieve a pointer to the Base2 portion of the Derived object
2827+
and use that pointer to get the vptr for Base2 as a base class.
2828+
```C++
2829+
struct Base1 {
2830+
virtual void f1();
2831+
};
2832+
struct Base2 {
2833+
virtual void f2();
2834+
};
2835+
struct Derived : public Base1, Base2 { };
2836+
void f3(Derived *d) {
2837+
d->f2();
2838+
}
2839+
```
2840+
2841+
```mlir
2842+
!rec_Base1 = !cir.record<struct "Base1" {!cir.vptr}
2843+
!rec_Base2 = !cir.record<struct "Base2" {!cir.vptr}
2844+
!rec_Derived = !cir.record<struct "Derived" {!rec_Base1, !rec_Base2}
2845+
cir.func dso_local @_Z2f3P7Derived(%d: !cir.ptr<!rec_Derived>)
2846+
%2 = cir.base_class_addr %d : !cir.ptr<!rec_Derived> nonnull [8]
2847+
-> !cir.ptr<!rec_Base2>
2848+
%3 = cir.vtable.get_vptr %2 : !cir.ptr<!rec_Base2> -> !cir.ptr<!cir.vptr>
2849+
%4 = cir.load align(8) %3 : !cir.ptr<!cir.vptr>, !cir.vptr
2850+
%5 = cir.vtable.get_virtual_fn_addr %4[0] : !cir.vptr
2851+
-> !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_Base2>)>>>
2852+
%6 = cir.load align(8) %5
2853+
: !cir.ptr<!cir.ptr<!cir.func<(!cir.ptr<!rec_Base2>)>>>,
2854+
!cir.ptr<!cir.func<(!cir.ptr<!rec_Base2>)>>
2855+
cir.call %6(%2) : (!cir.ptr<!cir.func<(!cir.ptr<!rec_Base2>)>>,
2856+
!cir.ptr<!rec_Base2>) -> ()
2857+
```
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}];
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let arguments = (ins

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