[CIR] Add cir.global_addr attribute #1248
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This patch adds a new attribute `#cir.global_addr`` to the CIR dialect. This attribute is quite similar to `#cir.global_view`, except that the new attribute represents the address of the global variable as an integer instead of a pointer. And the new attribute does not have the "indecies" stuff. CIRGen would not generate this attribute. I'm adding this new attribute because it could be useful during ABI lowering. For example, when doing ABI lowering for a member function pointer constant, ItaniumABI needs to lower the constant into a `#cir.const_struct` with two fields, first of which is an integer that represents the address of a function. This is where we need this attribute.
| @@ -198,4 +205,17 @@ module { | |||
| } | |||
| // MLIR: %0 = llvm.mlir.addressof @zero_array | |||
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| cir.func @const_global_addr() -> !u64i { | |||
| %0 = cir.const #cir.global_addr<@".str"> : !u64i | |||
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Sounds like this is equivalent to a global view with zero index? Why can't this be modeled ash cir.const #cir.global_view<@".str"> ?
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cir.global_addr is of integer types, and I'm not sure if it's appropriate to make cir.global_view an integer type. If it's OK I could just update cir.global_view instead of introducing a new one.
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My take is that we want here is a constant pointer coming out of cir.const, followed up by a cast to integer?
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My take is that we want here is a constant pointer coming out of cir.const, followed up by a cast to integer?
In this case, yes. But I'm actually proposing this attribute for the following scenario where you don't have space for additional operations:
%0 = cir.const #cir.const_struct {
#cir.global_addr @aaa,
#cir.global_addr @bbb
}And I'm looking at this scenario because recently I'm working on the LLVM lowering of member function pointers. Upon ABI lowering, a member function pointer is lowered to a struct with two fields of type ptrdiff_t. When the member function pointer represents a non-virtual member function, the first field stores the address of the target function as an integer. Thus to represent constant member function pointers I need an attribute that works like #cir.global_addr.
This PR adds a new attribute
#cir.global_addrto the CIR dialect. This attribute is quite similar to#cir.global_view, except that the new attribute represents the address of the global variable as an integer instead of a pointer. And the new attribute does not have the "indecies" stuff.CIRGen would not generate this attribute. I'm adding this new attribute because it could be useful during ABI lowering. For example, when doing ABI lowering for a member function pointer constant, ItaniumABI needs to lower the constant into a
#cir.const_structwith two fields, first of which is an integer that represents the address of a function. This is where we need this attribute.