Commits on Oct 31, 2024

1. Hoist do concurrent nest bounds/steps outside the nest (llvm#114020) …

   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.

   

   ergawy committed Oct 31, 2024
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