flang/docs/FIRArrayOperations.md - rust-lang/llvm-project - Git at Google

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 # Design: FIR Array operations

 ```eval_rst
 .. contents::
    :local:
 ```

 ## General

 The array operations in FIR model the copy-in/copy-out semantics over Fortran
 statements.

 Fortran language semantics sometimes require the compiler to make a temporary
 copy of an array or array slice. Situations where this can occur include:

 * Passing a non-contiguous array to a procedure that does not declare it as
   assumed-shape.
 * Array expressions, especially those involving `RESHAPE`, `PACK`, and `MERGE`.
 * Assignments of arrays where the array appears on both the left and right-hand
   sides of the assignment.
 * Assignments of `POINTER` arrays.

 There are currently the following operations:
 - `fir.array_load`
 - `fir.array_merge_store`
 - `fir.array_fetch`
 - `fir.array_update`
 - `fir.array_access`
 - `fir.array_amend`

 `array_load`(s) and `array_merge_store` are a pairing that brackets the lifetime
 of the array copies.

 `array_fetch` and `array_update` are defined to work as getter/setter pairs on
 values of elements from loaded array copies. These have "GEP-like" syntax and
 semantics.

 Fortran arrays are implicitly memory bound as are some other Fortran type/kind
 entities. For entities that can be atomically promoted to the value domain,
 we use `array_fetch` and `array_update`.

 `array_access` and `array_amend` are defined to work as getter/setter pairs on
 references to elements in loaded array copies. `array_access` has "GEP-like"
 syntax. `array_amend` annotates which loaded array copy is being written to.
 It is invalid to update an array copy without `array_amend`; doing so will
 result in undefined behavior.
 For those type/kinds that cannot be promoted to values, we must leave them in a
 memory reference domain, and we use `array_access` and `array_amend`.

 ## array_load

 This operation taken with `array_merge_store` captures Fortran's
 copy-in/copy-out semantics. One way to think of this is that array_load
 creates a snapshot copy of the entire array. This copy can then be used
 as the "original value" of the array while the array's new value is
 computed. The `array_merge_store` operation is the copy-out semantics, which
 merge the updates with the original array value to produce the final array
 result. This abstracts the copy operations as opposed to always creating
 copies or requiring dependence analysis be performed on the syntax trees
 and before lowering to the IR.

 Load an entire array as a single SSA value.

 ```fortran
   real :: a(o:n,p:m)
   ...
   ... = ... a ...
 ```

 One can use `fir.array_load` to produce an ssa-value that captures an
 immutable value of the entire array `a`, as in the Fortran array expression
 shown above. Subsequent changes to the memory containing the array do not
 alter its composite value. This operation lets one load an array as a
 value while applying a runtime shape, shift, or slice to the memory
 reference, and its semantics guarantee immutability.

 ```mlir
 %s = fir.shape_shift %lb1, %ex1, %lb2, %ex2 : (index, index, index, index) -> !fir.shapeshift<2>
 // load the entire array 'a'
 %v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shapeshift<2>) -> !fir.array<?x?xf32>
 // a fir.store here into array %a does not change %v
 ```

 # array_merge_store

 The `array_merge_store` operation stores a merged array value to memory.


 ```fortran
   real :: a(n,m)
   ...
   a = ...
 ```

 One can use `fir.array_merge_store` to merge/copy the value of `a` in an
 array expression as shown above.

 ```mlir
   %v = fir.array_load %a(%shape) : ...
   %r = fir.array_update %v, %f, %i, %j : (!fir.array<?x?xf32>, f32, index, index) -> !fir.array<?x?xf32>
   fir.array_merge_store %v, %r to %a : !fir.ref<!fir.array<?x?xf32>>
 ```

 This operation merges the original loaded array value, `%v`, with the
 chained updates, `%r`, and stores the result to the array at address, `%a`.

 This operation taken with `array_load`'s captures Fortran's
 copy-in/copy-out semantics. The first operands of `array_merge_store` is the
 result of the initial `array_load` operation. While this value could be
 retrieved by reference chasing through the different array operations it is
 useful to have it on hand directly for analysis passes since this directly
 defines the "bounds" of the Fortran statement represented by these operations.
 The intention is to allow copy-in/copy-out regions to be easily delineated,
 analyzed, and optimized.

 ## array_fetch

 The `array_fetch` operation fetches the value of an element in an array value.

 ```fortran
   real :: a(n,m)
   ...
   ... a ...
   ... a(r,s+1) ...
 ```

 One can use `fir.array_fetch` to fetch the (implied) value of `a(i,j)` in
 an array expression as shown above. It can also be used to extract the
 element `a(r,s+1)` in the second expression.

 ```mlir
   %s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
   // load the entire array 'a'
   %v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
   // fetch the value of one of the array value's elements
   %1 = fir.array_fetch %v, %i, %j : (!fir.array<?x?xf32>, index, index) -> f32
 ```

 It is only possible to use `array_fetch` on an `array_load` result value or a
 value that can be trace back transitively to an `array_load` as the dominating
 source. Other array operation such as `array_update` can be in between.

 ## array_update

 The `array_update` operation is used to update the value of an element in an
 array value. A new array value is returned where all element values of the input
 array are identical except for the selected element which is the value passed in
 the update.

 ```fortran
   real :: a(n,m)
   ...
   a = ...
 ```

 One can use `fir.array_update` to update the (implied) value of `a(i,j)`
 in an array expression as shown above.

 ```mlir
   %s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
   // load the entire array 'a'
   %v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
   // update the value of one of the array value's elements
   // %r_{ij} = %f  if (i,j) = (%i,%j),   %v_{ij} otherwise
   %r = fir.array_update %v, %f, %i, %j : (!fir.array<?x?xf32>, f32, index, index) -> !fir.array<?x?xf32>
   fir.array_merge_store %v, %r to %a : !fir.ref<!fir.array<?x?xf32>>
 ```

 An array value update behaves as if a mapping function from the indices
 to the new value has been added, replacing the previous mapping. These
 mappings can be added to the ssa-value, but will not be materialized in
 memory until the `fir.array_merge_store` is performed.
 `fir.array_update` can be seen as an array access with a notion that the array
 will be changed at the accessed position when `fir.array_merge_store` is
 performed.

 ## array_access

 The `array_access` provides a reference to a single element from an array value.
 This is *not* a view in the immutable array, otherwise it couldn't be stored to.
 It can be see as a logical copy of the element and its position in the array.
 Tis reference can be written to and modified withoiut changing the original
 array.

 The `array_access` operation is used to fetch the memory reference of an element
 in an array value.

 ```fortran
   real :: a(n,m)
   ...
   ... a ...
   ... a(r,s+1) ...
 ```

 One can use `fir.array_access` to recover the implied memory reference to
 the element `a(i,j)` in an array expression `a` as shown above. It can also
 be used to recover the reference element `a(r,s+1)` in the second
 expression.

 ```mlir
   %s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
   // load the entire array 'a'
   %v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
   // fetch the value of one of the array value's elements
   %1 = fir.array_access %v, %i, %j : (!fir.array<?x?xf32>, index, index) -> !fir.ref<f32>
 ```

 It is only possible to use `array_access` on an `array_load` result value or a
 value that can be trace back transitively to an `array_load` as the dominating
 source. Other array operation such as `array_amend` can be in between.

 `array_access` if mainly used with `character`'s arrays and arrays of derived
 types where because they might have a non-compile time sizes that would be
 useless too load entirely or too big to load.

 Here is a simple example with a `character` array assignment.

 Fortran
 ```
 subroutine foo(c1, c2, n)
   integer(8) :: n
   character(n) :: c1(:), c2(:)
   c1 = c2
 end subroutine
 ```

 It results in this cleaned-up FIR:
 ```
 func @_QPfoo(%arg0: !fir.box<!fir.array<?x!fir.char<1,?>>>, %arg1: !fir.box<!fir.array<?x!fir.char<1,?>>>, %arg2: !fir.ref<i64>) {
     %0 = fir.load %arg2 : !fir.ref<i64>
     %c0 = arith.constant 0 : index
     %1:3 = fir.box_dims %arg0, %c0 : (!fir.box<!fir.array<?x!fir.char<1,?>>>, index) -> (index, index, index)
     %2 = fir.array_load %arg0 : (!fir.box<!fir.array<?x!fir.char<1,?>>>) -> !fir.array<?x!fir.char<1,?>>
     %3 = fir.array_load %arg1 : (!fir.box<!fir.array<?x!fir.char<1,?>>>) -> !fir.array<?x!fir.char<1,?>>
     %c1 = arith.constant 1 : index
     %4 = arith.subi %1#1, %c1 : index
     %5 = fir.do_loop %arg3 = %c0 to %4 step %c1 unordered iter_args(%arg4 = %2) -> (!fir.array<?x!fir.char<1,?>>) {
       %6 = fir.array_access %3, %arg3 : (!fir.array<?x!fir.char<1,?>>, index) -> !fir.ref<!fir.char<1,?>>
       %7 = fir.array_access %arg4, %arg3 : (!fir.array<?x!fir.char<1,?>>, index) -> !fir.ref<!fir.char<1,?>>
       %false = arith.constant false
       %8 = fir.convert %7 : (!fir.ref<!fir.char<1,?>>) -> !fir.ref<i8>
       %9 = fir.convert %6 : (!fir.ref<!fir.char<1,?>>) -> !fir.ref<i8>
       fir.call @llvm.memmove.p0i8.p0i8.i64(%8, %9, %0, %false) : (!fir.ref<i8>, !fir.ref<i8>, i64, i1) -> ()
       %10 = fir.array_amend %arg4, %7 : (!fir.array<?x!fir.char<1,?>>, !fir.ref<!fir.char<1,?>>) -> !fir.array<?x!fir.char<1,?>>
       fir.result %10 : !fir.array<?x!fir.char<1,?>>
     }
     fir.array_merge_store %2, %5 to %arg0 : !fir.array<?x!fir.char<1,?>>, !fir.array<?x!fir.char<1,?>>, !fir.box<!fir.array<?x!fir.char<1,?>>>
     return
   }
   func private @llvm.memmove.p0i8.p0i8.i64(!fir.ref<i8>, !fir.ref<i8>, i64, i1)
 }
 ```

 `fir.array_access` and `fir.array_amend` split the two purposes of
 `fir.array_update` into two distinct operations to work on type/kind that must
 reside in the memory reference domain. `fir.array_access` captures the array
 access semantics and `fir.array_amend` denotes which `fir.array_access` is the
 lhs.

 We do not want to start loading the entire `!fir.ref<!fir.char<1,?>>` here since
 it has dynamic length, and even if constant, could be too long to do so.

 ## array_amend

 The `array_amend` operation marks an array value as having been changed via a
 reference obtain by an `array_access`. It acts as a logical transaction log
 that is used to merge the final result back with an `array_merge_store`
 operation.

 ```mlir
   // fetch the value of one of the array value's elements
   %1 = fir.array_access %v, %i, %j : (!fir.array<?x?xT>, index, index) -> !fir.ref<T>
   // modify the element by storing data using %1 as a reference
   %2 = ... %1 ...
   // mark the array value
   %new_v = fir.array_amend %v, %2 : (!fir.array<?x?xT>, !fir.ref<T>) -> !fir.array<?x?xT>
 ```

 ## Example

 Here is an example of a FIR code using several array operations together. The
 example below is a simplified version of the FIR code comiing from the
 following Fortran code snippet.

 ```fortran
 subroutine s(a,l,u)
   type t
     integer m
   end type t
   type(t) :: a(:)
   integer :: l, u
   forall (i=l:u)
     a(i) = a(u-i+1)
   end forall
 end
 ```

 ```
 func @_QPs(%arg0: !fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>, %arg1: !fir.ref<i32>, %arg2: !fir.ref<i32>) {
   %l = fir.load %arg1 : !fir.ref<i32>
   %l_index = fir.convert %l : (i32) -> index
   %u = fir.load %arg2 : !fir.ref<i32>
   %u_index = fir.convert %u : (i32) -> index
   %c1 = arith.constant 1 : index
   // This is the "copy-in" array used on the RHS of the expression. It will be indexed into and loaded at each iteration.
   %array_a_src = fir.array_load %arg0 : (!fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>) -> !fir.array<?x!fir.type<_QFsTt{m:i32}>>

   // This is the "seed" for the "copy-out" array on the LHS. It'll flow from iteration to iteration and gets
   // updated at each iteration.
   %array_a_dest_init = fir.array_load %arg0 : (!fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>) -> !fir.array<?x!fir.type<_QFsTt{m:i32}>>

   %array_a_final = fir.do_loop %i = %l_index to %u_index step %c1 unordered iter_args(%array_a_dest = %array_a_dest_init) -> (!fir.array<?x!fir.type<_QFsTt{m:i32}>>) {
     // Compute indexing for the RHS and array the element.
     %u_minus_i = arith.subi %u_index, %i : index // u-i
     %u_minus_i_plus_one = arith.addi %u_minus_i, %c1: index // u-i+1
     %a_src_ref = fir.array_access %array_a_src, %u_minus_i_plus_one {Fortran.offsets} : (!fir.array<?x!fir.type<_QFsTt{m:i32}>>, index) -> !fir.ref<!fir.type<_QFsTt{m:i32}>>
     %a_src_elt = fir.load %a_src_ref : !fir.ref<!fir.type<_QFsTt{m:i32}>>

     // Get the reference to the element in the array on the LHS
     %a_dst_ref = fir.array_access %array_a_dest, %i {Fortran.offsets} : (!fir.array<?x!fir.type<_QFsTt{m:i32}>>, index) -> !fir.ref<!fir.type<_QFsTt{m:i32}>>

     // Store the value, and update the array
     fir.store %a_src_elt to %a_dst_ref : !fir.ref<!fir.type<_QFsTt{m:i32}>>
     %updated_array_a = fir.array_amend %array_a_dest, %a_dst_ref : (!fir.array<?x!fir.type<_QFsTt{m:i32}>>, !fir.ref<!fir.type<_QFsTt{m:i32}>>) -> !fir.array<?x!fir.type<_QFsTt{m:i32}>>

     // Forward the current updated array to the next iteration.
     fir.result %updated_array_a : !fir.array<?x!fir.type<_QFsTt{m:i32}>>
   }
   // Store back the result by merging the initial value loaded before the loop
   // with the final one produced by the loop.
   fir.array_merge_store %array_a_dest_init, %array_a_final to %arg0 : !fir.array<?x!fir.type<_QFsTt{m:i32}>>, !fir.array<?x!fir.type<_QFsTt{m:i32}>>, !fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>
   return
 }
 ```
	<!--===- docs/FIRArrayOperations.md

	Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	See https://llvm.org/LICENSE.txt for license information.
	SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

	-->

	# Design: FIR Array operations

	```eval_rst
	.. contents::
	:local:
	```

	## General

	The array operations in FIR model the copy-in/copy-out semantics over Fortran
	statements.

	Fortran language semantics sometimes require the compiler to make a temporary
	copy of an array or array slice. Situations where this can occur include:

	* Passing a non-contiguous array to a procedure that does not declare it as
	assumed-shape.
	* Array expressions, especially those involving `RESHAPE`, `PACK`, and `MERGE`.
	* Assignments of arrays where the array appears on both the left and right-hand
	sides of the assignment.
	* Assignments of `POINTER` arrays.

	There are currently the following operations:
	- `fir.array_load`
	- `fir.array_merge_store`
	- `fir.array_fetch`
	- `fir.array_update`
	- `fir.array_access`
	- `fir.array_amend`

	`array_load`(s) and `array_merge_store` are a pairing that brackets the lifetime
	of the array copies.

	`array_fetch` and `array_update` are defined to work as getter/setter pairs on
	values of elements from loaded array copies. These have "GEP-like" syntax and
	semantics.

	Fortran arrays are implicitly memory bound as are some other Fortran type/kind
	entities. For entities that can be atomically promoted to the value domain,
	we use `array_fetch` and `array_update`.

	`array_access` and `array_amend` are defined to work as getter/setter pairs on
	references to elements in loaded array copies. `array_access` has "GEP-like"
	syntax. `array_amend` annotates which loaded array copy is being written to.
	It is invalid to update an array copy without `array_amend`; doing so will
	result in undefined behavior.
	For those type/kinds that cannot be promoted to values, we must leave them in a
	memory reference domain, and we use `array_access` and `array_amend`.

	## array_load

	This operation taken with `array_merge_store` captures Fortran's
	copy-in/copy-out semantics. One way to think of this is that array_load
	creates a snapshot copy of the entire array. This copy can then be used
	as the "original value" of the array while the array's new value is
	computed. The `array_merge_store` operation is the copy-out semantics, which
	merge the updates with the original array value to produce the final array
	result. This abstracts the copy operations as opposed to always creating
	copies or requiring dependence analysis be performed on the syntax trees
	and before lowering to the IR.

	Load an entire array as a single SSA value.

	```fortran
	real :: a(o:n,p:m)
	...
	... = ... a ...
	```

	One can use `fir.array_load` to produce an ssa-value that captures an
	immutable value of the entire array `a`, as in the Fortran array expression
	shown above. Subsequent changes to the memory containing the array do not
	alter its composite value. This operation lets one load an array as a
	value while applying a runtime shape, shift, or slice to the memory
	reference, and its semantics guarantee immutability.

	```mlir
	%s = fir.shape_shift %lb1, %ex1, %lb2, %ex2 : (index, index, index, index) -> !fir.shapeshift<2>
	// load the entire array 'a'
	%v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shapeshift<2>) -> !fir.array<?x?xf32>
	// a fir.store here into array %a does not change %v
	```

	# array_merge_store

	The `array_merge_store` operation stores a merged array value to memory.


	```fortran
	real :: a(n,m)
	...
	a = ...
	```

	One can use `fir.array_merge_store` to merge/copy the value of `a` in an
	array expression as shown above.

	```mlir
	%v = fir.array_load %a(%shape) : ...
	%r = fir.array_update %v, %f, %i, %j : (!fir.array<?x?xf32>, f32, index, index) -> !fir.array<?x?xf32>
	fir.array_merge_store %v, %r to %a : !fir.ref<!fir.array<?x?xf32>>
	```

	This operation merges the original loaded array value, `%v`, with the
	chained updates, `%r`, and stores the result to the array at address, `%a`.

	This operation taken with `array_load`'s captures Fortran's
	copy-in/copy-out semantics. The first operands of `array_merge_store` is the
	result of the initial `array_load` operation. While this value could be
	retrieved by reference chasing through the different array operations it is
	useful to have it on hand directly for analysis passes since this directly
	defines the "bounds" of the Fortran statement represented by these operations.
	The intention is to allow copy-in/copy-out regions to be easily delineated,
	analyzed, and optimized.

	## array_fetch

	The `array_fetch` operation fetches the value of an element in an array value.

	```fortran
	real :: a(n,m)
	...
	... a ...
	... a(r,s+1) ...
	```

	One can use `fir.array_fetch` to fetch the (implied) value of `a(i,j)` in
	an array expression as shown above. It can also be used to extract the
	element `a(r,s+1)` in the second expression.

	```mlir
	%s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
	// load the entire array 'a'
	%v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
	// fetch the value of one of the array value's elements
	%1 = fir.array_fetch %v, %i, %j : (!fir.array<?x?xf32>, index, index) -> f32
	```

	It is only possible to use `array_fetch` on an `array_load` result value or a
	value that can be trace back transitively to an `array_load` as the dominating
	source. Other array operation such as `array_update` can be in between.

	## array_update

	The `array_update` operation is used to update the value of an element in an
	array value. A new array value is returned where all element values of the input
	array are identical except for the selected element which is the value passed in
	the update.

	```fortran
	real :: a(n,m)
	...
	a = ...
	```

	One can use `fir.array_update` to update the (implied) value of `a(i,j)`
	in an array expression as shown above.

	```mlir
	%s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
	// load the entire array 'a'
	%v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
	// update the value of one of the array value's elements
	// %r_{ij} = %f if (i,j) = (%i,%j), %v_{ij} otherwise
	%r = fir.array_update %v, %f, %i, %j : (!fir.array<?x?xf32>, f32, index, index) -> !fir.array<?x?xf32>
	fir.array_merge_store %v, %r to %a : !fir.ref<!fir.array<?x?xf32>>
	```

	An array value update behaves as if a mapping function from the indices
	to the new value has been added, replacing the previous mapping. These
	mappings can be added to the ssa-value, but will not be materialized in
	memory until the `fir.array_merge_store` is performed.
	`fir.array_update` can be seen as an array access with a notion that the array
	will be changed at the accessed position when `fir.array_merge_store` is
	performed.

	## array_access

	The `array_access` provides a reference to a single element from an array value.
	This is not a view in the immutable array, otherwise it couldn't be stored to.
	It can be see as a logical copy of the element and its position in the array.
	Tis reference can be written to and modified withoiut changing the original
	array.

	The `array_access` operation is used to fetch the memory reference of an element
	in an array value.

	```fortran
	real :: a(n,m)
	...
	... a ...
	... a(r,s+1) ...
	```

	One can use `fir.array_access` to recover the implied memory reference to
	the element `a(i,j)` in an array expression `a` as shown above. It can also
	be used to recover the reference element `a(r,s+1)` in the second
	expression.

	```mlir
	%s = fir.shape %n, %m : (index, index) -> !fir.shape<2>
	// load the entire array 'a'
	%v = fir.array_load %a(%s) : (!fir.ref<!fir.array<?x?xf32>>, !fir.shape<2>) -> !fir.array<?x?xf32>
	// fetch the value of one of the array value's elements
	%1 = fir.array_access %v, %i, %j : (!fir.array<?x?xf32>, index, index) -> !fir.ref<f32>
	```

	It is only possible to use `array_access` on an `array_load` result value or a
	value that can be trace back transitively to an `array_load` as the dominating
	source. Other array operation such as `array_amend` can be in between.

	`array_access` if mainly used with `character`'s arrays and arrays of derived
	types where because they might have a non-compile time sizes that would be
	useless too load entirely or too big to load.

	Here is a simple example with a `character` array assignment.

	Fortran
	```
	subroutine foo(c1, c2, n)
	integer(8) :: n
	character(n) :: c1(:), c2(:)
	c1 = c2
	end subroutine
	```

	It results in this cleaned-up FIR:
	```
	func @_QPfoo(%arg0: !fir.box<!fir.array<?x!fir.char<1,?>>>, %arg1: !fir.box<!fir.array<?x!fir.char<1,?>>>, %arg2: !fir.ref<i64>) {
	%0 = fir.load %arg2 : !fir.ref<i64>
	%c0 = arith.constant 0 : index
	%1:3 = fir.box_dims %arg0, %c0 : (!fir.box<!fir.array<?x!fir.char<1,?>>>, index) -> (index, index, index)
	%2 = fir.array_load %arg0 : (!fir.box<!fir.array<?x!fir.char<1,?>>>) -> !fir.array<?x!fir.char<1,?>>
	%3 = fir.array_load %arg1 : (!fir.box<!fir.array<?x!fir.char<1,?>>>) -> !fir.array<?x!fir.char<1,?>>
	%c1 = arith.constant 1 : index
	%4 = arith.subi %1#1, %c1 : index
	%5 = fir.do_loop %arg3 = %c0 to %4 step %c1 unordered iter_args(%arg4 = %2) -> (!fir.array<?x!fir.char<1,?>>) {
	%6 = fir.array_access %3, %arg3 : (!fir.array<?x!fir.char<1,?>>, index) -> !fir.ref<!fir.char<1,?>>
	%7 = fir.array_access %arg4, %arg3 : (!fir.array<?x!fir.char<1,?>>, index) -> !fir.ref<!fir.char<1,?>>
	%false = arith.constant false
	%8 = fir.convert %7 : (!fir.ref<!fir.char<1,?>>) -> !fir.ref<i8>
	%9 = fir.convert %6 : (!fir.ref<!fir.char<1,?>>) -> !fir.ref<i8>
	fir.call @llvm.memmove.p0i8.p0i8.i64(%8, %9, %0, %false) : (!fir.ref<i8>, !fir.ref<i8>, i64, i1) -> ()
	%10 = fir.array_amend %arg4, %7 : (!fir.array<?x!fir.char<1,?>>, !fir.ref<!fir.char<1,?>>) -> !fir.array<?x!fir.char<1,?>>
	fir.result %10 : !fir.array<?x!fir.char<1,?>>
	}
	fir.array_merge_store %2, %5 to %arg0 : !fir.array<?x!fir.char<1,?>>, !fir.array<?x!fir.char<1,?>>, !fir.box<!fir.array<?x!fir.char<1,?>>>
	return
	}
	func private @llvm.memmove.p0i8.p0i8.i64(!fir.ref<i8>, !fir.ref<i8>, i64, i1)
	}
	```

	`fir.array_access` and `fir.array_amend` split the two purposes of
	`fir.array_update` into two distinct operations to work on type/kind that must
	reside in the memory reference domain. `fir.array_access` captures the array
	access semantics and `fir.array_amend` denotes which `fir.array_access` is the
	lhs.

	We do not want to start loading the entire `!fir.ref<!fir.char<1,?>>` here since
	it has dynamic length, and even if constant, could be too long to do so.

	## array_amend

	The `array_amend` operation marks an array value as having been changed via a
	reference obtain by an `array_access`. It acts as a logical transaction log
	that is used to merge the final result back with an `array_merge_store`
	operation.

	```mlir
	// fetch the value of one of the array value's elements
	%1 = fir.array_access %v, %i, %j : (!fir.array<?x?xT>, index, index) -> !fir.ref<T>
	// modify the element by storing data using %1 as a reference
	%2 = ... %1 ...
	// mark the array value
	%new_v = fir.array_amend %v, %2 : (!fir.array<?x?xT>, !fir.ref<T>) -> !fir.array<?x?xT>
	```

	## Example

	Here is an example of a FIR code using several array operations together. The
	example below is a simplified version of the FIR code comiing from the
	following Fortran code snippet.

	```fortran
	subroutine s(a,l,u)
	type t
	integer m
	end type t
	type(t) :: a(:)
	integer :: l, u
	forall (i=l:u)
	a(i) = a(u-i+1)
	end forall
	end
	```

	```
	func @_QPs(%arg0: !fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>, %arg1: !fir.ref<i32>, %arg2: !fir.ref<i32>) {
	%l = fir.load %arg1 : !fir.ref<i32>
	%l_index = fir.convert %l : (i32) -> index
	%u = fir.load %arg2 : !fir.ref<i32>
	%u_index = fir.convert %u : (i32) -> index
	%c1 = arith.constant 1 : index
	// This is the "copy-in" array used on the RHS of the expression. It will be indexed into and loaded at each iteration.
	%array_a_src = fir.array_load %arg0 : (!fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>) -> !fir.array<?x!fir.type<_QFsTt{m:i32}>>

	// This is the "seed" for the "copy-out" array on the LHS. It'll flow from iteration to iteration and gets
	// updated at each iteration.
	%array_a_dest_init = fir.array_load %arg0 : (!fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>) -> !fir.array<?x!fir.type<_QFsTt{m:i32}>>

	%array_a_final = fir.do_loop %i = %l_index to %u_index step %c1 unordered iter_args(%array_a_dest = %array_a_dest_init) -> (!fir.array<?x!fir.type<_QFsTt{m:i32}>>) {
	// Compute indexing for the RHS and array the element.
	%u_minus_i = arith.subi %u_index, %i : index // u-i
	%u_minus_i_plus_one = arith.addi %u_minus_i, %c1: index // u-i+1
	%a_src_ref = fir.array_access %array_a_src, %u_minus_i_plus_one {Fortran.offsets} : (!fir.array<?x!fir.type<_QFsTt{m:i32}>>, index) -> !fir.ref<!fir.type<_QFsTt{m:i32}>>
	%a_src_elt = fir.load %a_src_ref : !fir.ref<!fir.type<_QFsTt{m:i32}>>

	// Get the reference to the element in the array on the LHS
	%a_dst_ref = fir.array_access %array_a_dest, %i {Fortran.offsets} : (!fir.array<?x!fir.type<_QFsTt{m:i32}>>, index) -> !fir.ref<!fir.type<_QFsTt{m:i32}>>

	// Store the value, and update the array
	fir.store %a_src_elt to %a_dst_ref : !fir.ref<!fir.type<_QFsTt{m:i32}>>
	%updated_array_a = fir.array_amend %array_a_dest, %a_dst_ref : (!fir.array<?x!fir.type<_QFsTt{m:i32}>>, !fir.ref<!fir.type<_QFsTt{m:i32}>>) -> !fir.array<?x!fir.type<_QFsTt{m:i32}>>

	// Forward the current updated array to the next iteration.
	fir.result %updated_array_a : !fir.array<?x!fir.type<_QFsTt{m:i32}>>
	}
	// Store back the result by merging the initial value loaded before the loop
	// with the final one produced by the loop.
	fir.array_merge_store %array_a_dest_init, %array_a_final to %arg0 : !fir.array<?x!fir.type<_QFsTt{m:i32}>>, !fir.array<?x!fir.type<_QFsTt{m:i32}>>, !fir.box<!fir.array<?x!fir.type<_QFsTt{m:i32}>>>
	return
	}
	```