If you have the following multi-range `do concurrent` loop:
```fortran
do concurrent(i=1:n, j=1:bar(n*m, n/m))
a(i) = n
end do
```
Currently, flang generates the following IR:
```mlir
fir.do_loop %arg1 = %42 to %44 step %c1 unordered {
...
%53:3 = hlfir.associate %49 {adapt.valuebyref} : (i32) -> (!fir.ref<i32>, !fir.ref<i32>, i1)
%54:3 = hlfir.associate %52 {adapt.valuebyref} : (i32) -> (!fir.ref<i32>, !fir.ref<i32>, i1)
%55 = fir.call @_QFPbar(%53#1, %54#1) fastmath<contract> : (!fir.ref<i32>, !fir.ref<i32>) -> i32
hlfir.end_associate %53#1, %53#2 : !fir.ref<i32>, i1
hlfir.end_associate %54#1, %54#2 : !fir.ref<i32>, i1
%56 = fir.convert %55 : (i32) -> index
...
fir.do_loop %arg2 = %46 to %56 step %c1_4 unordered {
...
}
}
```
However, if `bar` is impure, then we have a direct violation of the
standard:
```
C1143 A reference to an impure procedure shall not appear within a DO CONCURRENT construct.
```
Moreover, the standard describes the execution of `do concurrent`
construct in multiple stages:
```
11.1.7.4 Execution of a DO construct
...
11.1.7.4.2 DO CONCURRENT loop control
The concurrent-limit and concurrent-step expressions in the concurrent-control-list are evaluated. ...
11.1.7.4.3 The execution cycle
...
The block of a DO CONCURRENT construct is executed for every active combination of the index-name values.
Each execution of the block is an iteration. The executions may occur in any order.
```
From the above 2 points, it seems to me that execution is divided in
multiple consecutive stages: 11.1.7.4.2 is the stage where we evaluate
all control expressions including the step and then 11.1.7.4.3 is the
stage to execute the block of the concurrent loop itself using the
combination of possible iteration values.
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