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[CIR][ABI][Lowering] Supports function pointers in the calling conven…
…tion lowering pass (#1003) This PR adds initial function pointers support for the calling convention lowering pass. This is a suggestion, so any other ideas are welcome. Several ideas was described in the #995 and basically what I'm trying to do is to generate a clean CIR code without additional `bitcast` operations for function pointers and without mix of lowered and initial function types. #### Problem Looks like we can not just lower the function type and cast the value since too many operations are involved. For instance, for the next simple code: ``` typedef struct { int a; } S; typedef int (*myfptr)(S); int foo(S s) { return 42 + s.a; } void bar() { myfptr a = foo; } ``` we get the next CIR for the function `bar` , before the calling convention lowering pass: ``` cir.func no_proto @bar() extra(#fn_attr) { %0 = cir.alloca !cir.ptr<!cir.func<!s32i (!ty_S)>>, !cir.ptr<!cir.ptr<!cir.func<!s32i (!ty_S)>>>, ["a", init] %1 = cir.get_global @foo : !cir.ptr<!cir.func<!s32i (!ty_S)>> cir.store %1, %0 : !cir.ptr<!cir.func<!s32i (!ty_S)>>, !cir.ptr<!cir.ptr<!cir.func<!s32i (!ty_S)>>> cir.return } ``` As one can see, first three operations depend on the function type. Once `foo` is lowered, we need to fix `GetGlobalOp`: otherwise the code will fail with the verification error since actual `foo` type (lowered) differs from the one currently expected by the `GetGlobalOp`. First idea would just rewrite only the `GetGlobalOp` and insert a bitcast after, so both `AllocaOp` and `StoreOp` would work witth proper types. Once the code will be more complex, we will need to take care about possible use cases, e.g. if we use arrays, we will need to track array accesses to it as well in order to insert this bitcast every time the array element is needed. One workaround I can think of: we fix the `GetGlobalOp` type and cast from the lowered type to the initial, and cast back before the actual call happens - but it doesn't sound as a good and clean approach (from my point of view, of course). So I suggest to use type converter and rewrite any operation that may deal with function pointers and make sure it has a proper type, and we don't have any unlowered function type in the program after the calling convention lowering pass. #### Implementation I added lowering for `AllocaOp`, `GetGlobalOp`, and split the lowering for `FuncOp` (former `CallConvLoweringPattern`) and lower `CallOp` separately. Frankly speaking, I tried to implement a pattern for each operation, but for some reasons the tests are not passed for windows and macOs in this case - something weird happens inside `applyPatternsAndFold` function. I suspect it's due to two different rewriters used - one in the `LoweringModule` and one in the mentioned function. So I decided to follow the same approach as it's done for the `LoweringPrepare` pass and don't involve this complex rewriting framework. Next I will add a type converter for the struct type, patterns for `ConstantOp` (for const arrays and `GlobalViewAttr`) In the end of the day we'll have (at least I hope so) a clean CIR code without any bitcasts for function pointers. cc @sitio-couto @bcardosolopes
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