https://github.com/andykaylor commented:
I have concerns about the CIR representation here. I think we should be
aligning our representation of complex operations with the MLIR complex
dialect. As such, we want `__real__` to be lowerable to `complex.re` but the
explicit representation of intermediate pointer loads and stores is going to
make that difficult.
The test case you have in this PR makes it difficult to reason about what I'm
interested in here because it is dealing so explicitly with pointers. I'd like
to consider something more like this:
```
double f(double _Complex a, double _Complex b) {
return __real__ a + __real__ b;
}
```
The incubator will generate the following CIR for this case:
```
cir.func dso_local @_Z1fCdS_(%arg0: !cir.complex<!cir.double>, %arg1:
!cir.complex<!cir.double> -> !cir.double extra(#fn_attr) {
%0 = cir.alloca !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>, ["a", init] {alignment = 8 : i64}
%1 = cir.alloca !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>, ["b", init] {alignment = 8 : i64}
%2 = cir.alloca !cir.double, !cir.ptr<!cir.double>, ["__retval"] {alignment
= 8 : i64}
cir.store %arg0, %0 : !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>
cir.store %arg1, %1 : !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>
%3 = cir.complex.real_ptr %0 : !cir.ptr<!cir.complex<!cir.double>> ->
!cir.ptr<!cir.double>
%4 = cir.load align(8) %3 : !cir.ptr<!cir.double>, !cir.double
%5 = cir.complex.real_ptr %1 : !cir.ptr<!cir.complex<!cir.double>> ->
!cir.ptr<!cir.double>
%6 = cir.load align(8) %5 : !cir.ptr<!cir.double>, !cir.double
%7 = cir.binop(add, %4, %6) : !cir.double
cir.store %7, %2 : !cir.double, !cir.ptr<!cir.double>
%8 = cir.load %2 : !cir.ptr<!cir.double>, !cir.double
cir.return %8 : !cir.double
}
```
That's not what I'd like to see. What I'd like to see is something more like
this:
```
cir.func dso_local @_Z1fCdS_(%arg0: !cir.complex<!cir.double>, %arg1:
!cir.complex<!cir.double> -> !cir.double extra(#fn_attr) {
%0 = cir.alloca !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>, ["a", init] {alignment = 8 : i64}
%1 = cir.alloca !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>, ["b", init] {alignment = 8 : i64}
%2 = cir.alloca !cir.double, !cir.ptr<!cir.double>, ["__retval"] {alignment
= 8 : i64}
cir.store %arg0, %0 : !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>
cir.store %arg1, %1 : !cir.complex<!cir.double>,
!cir.ptr<!cir.complex<!cir.double>>
%3 = cir.load align(8) %0 : !cir.ptr<!cir.complex<!cir.double>> ->
!cir.complex<!cir.double>
%4 = cir.complex.real %3 : !cir.complex<!cir.double> -> !cir.double
%5 = cir.load align(8) %1 : !cir.ptr<!cir.complex<!cir.double>> ->
!cir.complex<!cir.double>
%6 = cir.complex.real %5 : !cir.complex<!cir.double> -> !cir.double
%7 = cir.binop(add, %4, %6) : !cir.double
cir.store %7, %2 : !cir.double, !cir.ptr<!cir.double>
%8 = cir.load %2 : !cir.ptr<!cir.double>, !cir.double
cir.return %8 : !cir.double
}
```
The idea is that the `__real__` operation acts directly on the complex value
rather than manipulating its memory representation. We'll still need to go to
the memory representation when it gets lowered to LLVM IR, but I would like to
keep it higher level until then.
For your test case:
```
void foo10() {
double _Complex c;
double *realPtr = &__real__ c;
}
```
the expression `&__real__ c` is represented in the AST like this:
```
`-UnaryOperator <col:21, col:31> 'double *' prefix '&' cannot overflow
`-UnaryOperator <col:22, col:31> 'double' lvalue prefix '__real'
cannot overflow
`-DeclRefExpr <col:31> '_Complex double' lvalue Var 0x10ed3bd8 'c'
'_Complex double'
```
Hopefully, we could represent the `__real__` operator with a `cir.complex.real`
operation as I've proposed and then use the `&` operator to do something to get
a pointer. I don't know exactly how that would look.
This is, of course, something we'd want to change in the incubator first to
take advantage of the more comprehensive testing available there.
https://github.com/llvm/llvm-project/pull/144235
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