[Flang][OpenMP] Provide an error when the minus operator is used in a reduction

[kiranchandramohan]

Summary

OpenMP 5.2 standard has deprecated the usage of the minus operation in
reductions. The minus operation also is an unpleasant feature with
varied interpretations.

The patch also changes the usage of the minus operator in some existing testcases.

Discussed in https://discourse.llvm.org/t/openmp-runtime-problem-with-subtraction-reduction/64404

[peixin]

Can you add one test case of declare reduction with minus operator?

flang/lib/Semantics/check-omp-structure.cpp

1936

There should be one break here to skip double error msgs.

[kiranchandramohan]

Add break following the minus operator case. Update testcase.

[peixin]

LGTM

[kiranchandramohan]

In D132060#3730783, @peixin wrote:

Can you add one test case of declare reduction with minus operator?

There is some issue with parsing/semantics for declare reduction. I have filed an issue https://github.com/llvm/llvm-project/issues/57219.

Shall we go ahead with this for now?

[peixin]

In D132060#3731891, @kiranchandramohan wrote:

In D132060#3730783, @peixin wrote:

Can you add one test case of declare reduction with minus operator?

There is some issue with parsing/semantics for declare reduction. I have filed an issue https://github.com/llvm/llvm-project/issues/57219.

Shall we go ahead with this for now?

Yes. I saw that issue. Please land this.

flang/lib/Semantics/check-omp-structure.cpp

bool OmpStructureChecker::CheckReductionOperators(
 
   return ok;
 }
 bool OmpStructureChecker::CheckIntrinsicOperator(
     const parser::DefinedOperator::IntrinsicOperator &op) {
 
   switch (op) {
   case parser::DefinedOperator::IntrinsicOperator::Add:
-  case parser::DefinedOperator::IntrinsicOperator::Subtract:
   case parser::DefinedOperator::IntrinsicOperator::Multiply:
   case parser::DefinedOperator::IntrinsicOperator::AND:
   case parser::DefinedOperator::IntrinsicOperator::OR:
   case parser::DefinedOperator::IntrinsicOperator::EQV:
   case parser::DefinedOperator::IntrinsicOperator::NEQV:
     return true;
+  case parser::DefinedOperator::IntrinsicOperator::Subtract:
+    context_.Say(GetContext().clauseSource,
+        "The minus reduction operator is deprecated since OpenMP 5.2 and is not supported in the REDUCTION clause."_err_en_US,
+        ContextDirectiveAsFortran());
+    break;
   default:
     context_.Say(GetContext().clauseSource,
         "Invalid reduction operator in REDUCTION clause."_err_en_US,
         ContextDirectiveAsFortran());
   }
   return false;
 }
 

flang/test/Lower/OpenMP/Todo/reduction-subtract.f90

-! RUN: %not_todo_cmd bbc -emit-fir -fopenmp -o - %s 2>&1 | FileCheck %s
-! RUN: %not_todo_cmd %flang_fc1 -emit-fir -fopenmp -o - %s 2>&1 | FileCheck %s
-
-! CHECK: not yet implemented: Reduction of some intrinsic operators is not supported
-subroutine reduction_subtract
-  integer :: x
-  !$omp parallel
-  !$omp do reduction(-:x)
-  do i=1, 100
-    x = x - i
-  end do
-  !$omp end do
-  !$omp end parallel
-  print *, x
-end subroutine

flang/test/Semantics/OpenMP/omp-firstprivate01.f90

program omp_firstprivate
   !ERROR: FIRSTPRIVATE variable 'b' is PRIVATE in outer context
   !$omp do firstprivate(b)
   do i = 1, 10
     c(i) = a(i) + b(i) + i
   end do
   !$omp end do
   !$omp end parallel
 
-  !$omp parallel reduction(-:a)
+  !$omp parallel reduction(*:a)
   !ERROR: FIRSTPRIVATE variable 'a' is PRIVATE in outer context
   !$omp do firstprivate(a,b)
   do i = 1, 10
-    c(i) =  c(i) - a(i) * b(i) * i
+    c(i) =  c(i) * a(i) * b(i) * i
   end do
   !$omp end do
   !$omp end parallel
 
   !$omp parallel reduction(+:a)
   !ERROR: FIRSTPRIVATE variable 'a' is PRIVATE in outer context
   !$omp sections firstprivate(a, b)
   !$omp section
   c = c * a + b
   !$omp end sections
   !$omp end parallel
 
-  !$omp parallel reduction(-:a)
+  !$omp parallel reduction(*:a)
   !ERROR: FIRSTPRIVATE variable 'a' is PRIVATE in outer context
   !$omp task firstprivate(a,b)
-  c =  c - a * b
+  c =  c * a * b
   !$omp end task
   !$omp end parallel
 
   !$omp parallel reduction(+:b)
   !ERROR: FIRSTPRIVATE variable 'b' is PRIVATE in outer context
   !$omp taskloop firstprivate(b)
   do i = 1, 10
     c(i) = a(i) + b(i) + i

flang/test/Semantics/OpenMP/omp-reduction-subtract.f90

+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
+! OpenMP Version 5.2
+! Minus operation is deprecated in reduction
+
+subroutine reduction_subtract
+  integer :: x
+  !ERROR: The minus reduction operator is deprecated since OpenMP 5.2 and is not supported in the REDUCTION clause.
+  !$omp do reduction(-:x)
+  do i=1, 100
+    x = x - i
+  end do
+  !$omp end do
+end subroutine

flang/test/Semantics/OpenMP/omp-reduction02.f90

 ! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
 ! OpenMP Version 4.5
 ! 2.15.3.6 Reduction Clause
 program omp_reduction
 
   integer :: i
   integer :: k = 10
   integer :: j = 10
 
   !ERROR: 'k' appears in more than one data-sharing clause on the same OpenMP directive
-  !$omp parallel do reduction(+:k), reduction(-:k)
+  !$omp parallel do reduction(+:k), reduction(*:k)
   do i = 1, 10
     k = k + 1
+    k = k * 3
   end do
   !$omp end parallel do
 
   !ERROR: 'k' appears in more than one data-sharing clause on the same OpenMP directive
-  !$omp parallel do reduction(+:k), reduction(-:j), reduction(+:k)
+  !$omp parallel do reduction(+:k), reduction(*:j), reduction(+:k)
   do i = 1, 10
     k = k + 1
+    j = j * 3
   end do
   !$omp end parallel do
 
   !ERROR: 'k' appears in more than one data-sharing clause on the same OpenMP directive
-  !$omp parallel do reduction(+:j), reduction(-:k), reduction(+:k)
+  !$omp parallel do reduction(+:j), reduction(*:k), reduction(+:k)
   do i = 1, 10
+    j = j + 1
     k = k + 1
+    k = k * 3
   end do
   !$omp end parallel do
 
   !ERROR: 'k' appears in more than one data-sharing clause on the same OpenMP directive
-  !$omp parallel do reduction(+:j), reduction(-:k), private(k)
+  !$omp parallel do reduction(+:j), reduction(*:k), private(k)
   do i = 1, 10
+    j = j + 1
     k = k + 1
+    k = k * 3
   end do
   !$omp end parallel do
 end program omp_reduction

flang/test/Semantics/OpenMP/omp-reduction04.f90

 ! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
 ! OpenMP Version 4.5
 ! 2.15.3.6 Reduction Clause
 program omp_Reduction
   integer :: i
   integer, parameter :: k = 10
   common /c/ a, b
 
   !ERROR: Variable 'k' on the REDUCTION clause is not definable
   !$omp parallel do reduction(+:k)
   do i = 1, 10
     l = k + 1
   end do
   !$omp end parallel do
 
   !ERROR: Variable 'c' on the REDUCTION clause is not definable
-  !$omp parallel do reduction(-:/c/)
+  !$omp parallel do reduction(*:/c/)
   do i = 1, 10
     l = k + 1
   end do
   !$omp end parallel do
 end program omp_Reduction

flang/test/Semantics/OpenMP/omp-reduction07.f90

 ! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp
 ! OpenMP Version 4.5
 ! 2.15.3.6 Reduction Clause
 program omp_reduction
 
-  integer :: i,j,l
+  integer :: a,i,j,l
   integer :: k = 10
   !$omp parallel private(k)
   !ERROR: REDUCTION variable 'k' is PRIVATE in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
   !$omp do reduction(+:k)
   do i = 1, 10
     k = k + 1
   end do
   !$omp end do
   !$omp end parallel
 
 
-  !$omp parallel private(j),reduction(-:k)
+  !$omp parallel private(j),reduction(+:k)
   !ERROR: REDUCTION variable 'k' is REDUCTION in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
   !$omp do reduction(+:k)
   do i = 1, 10
     k = k + 1
   end do
   !$omp end do
   !$omp end parallel
 
   !$omp parallel private(j),firstprivate(k)
   !ERROR: REDUCTION variable 'k' is FIRSTPRIVATE in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
   !$omp do reduction(min:k)
   do i = 1, 10
     k = k + 1
   end do
   !$omp end do
   !$omp end parallel
 
 
   !$omp parallel private(l,j),firstprivate(k)
   !ERROR: REDUCTION variable 'k' is FIRSTPRIVATE in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
   !ERROR: REDUCTION variable 'j' is PRIVATE in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
-  !$omp sections reduction(ior:k) reduction(-:j)
+  !$omp sections reduction(ior:k) reduction(*:j)
   do i = 1, 10
-    k = k + 1
+    k = ior(k, 1)
+    j = j * 3
   end do
   !$omp end sections
   !$omp end parallel
 
 !$omp sections private(k)
   !ERROR: A worksharing region may not be closely nested inside a worksharing, explicit task, taskloop, critical, ordered, atomic, or master region
   !ERROR: REDUCTION variable 'k' is PRIVATE in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
   !$omp do reduction(+:k) reduction(max:j)
do j = 1,10
     end do
     !$omp end do
   end do
   !$omp end target
 !$omp end sections
 
 !$omp parallel reduction(+:a)
 !ERROR: REDUCTION variable 'a' is REDUCTION in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
-!$omp sections reduction(-:a)
-a = 10
+!$omp sections reduction(*:a)
+a = a + 10
 !$omp end sections
 !$omp end parallel
 
-!$omp parallel reduction(-:a)
+!$omp parallel reduction(*:a)
 !$omp end parallel
 
 
 !$omp parallel reduction(+:a)
 !ERROR: REDUCTION clause is not allowed on the WORKSHARE directive
 !ERROR: REDUCTION variable 'a' is REDUCTION in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
-!$omp workshare reduction(-:a)
-a = 10
+!$omp workshare reduction(*:a)
+a = a + 10
 !$omp end workshare
 !$omp end parallel
 
-!$omp parallel reduction(-:a)
+!$omp parallel reduction(*:a)
 !$omp end parallel
 
 
 !$omp parallel reduction(+:a)
 !ERROR: REDUCTION clause is not allowed on the SINGLE directive
 !ERROR: REDUCTION variable 'a' is REDUCTION in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
-!$omp single reduction(-:a)
-a = 10
+!$omp single reduction(*:a)
+a = a + 10
 !$omp end single
 !$omp end parallel
 
-!$omp parallel reduction(-:a)
+!$omp parallel reduction(+:a)
 !$omp end parallel
 
 
 !$omp parallel reduction(+:a)
 !ERROR: REDUCTION clause is not allowed on the SINGLE directive
 !ERROR: REDUCTION variable 'a' is REDUCTION in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
 !$omp single reduction(iand:a)
-a = 10
+a = a + 10
 !$omp end single
 !$omp end parallel
 
 !$omp parallel reduction(iand:a)
 !$omp end parallel
 
 !$omp parallel reduction(ieor:a)
 !ERROR: REDUCTION variable 'a' is REDUCTION in outer context must be shared in the parallel regions to which any of the worksharing regions arising from the worksharing construct bind.
-!$omp sections reduction(-:a)
-a = 10
+!$omp sections reduction(+:a)
+a = ieor(a, 10)
 !$omp end sections
 !$omp end parallel
 
 !$omp parallel reduction(ieor:a)
 !$omp end parallel
 
 end program omp_reduction

flang/test/Semantics/OpenMP/omp-reduction09.f90

program omp_reduction
   !DEF: /omp_reduction/OtherConstruct6/OtherConstruct1/i (OmpPrivate, OmpPreDetermined) HostAssoc INTEGER(4)
   do i=1,10
     !DEF: /omp_reduction/OtherConstruct6/OtherConstruct1/k (OmpReduction) HostAssoc INTEGER(4)
     k = k+1
   end do
   !$omp end do
   !$omp end parallel
 
-  !$omp do reduction(-:k) reduction(*:j) reduction(-:l)
+  !$omp do reduction(+:k) reduction(*:j) reduction(+:l)
   !DEF: /omp_reduction/OtherConstruct7/i (OmpPrivate, OmpPreDetermined) HostAssoc INTEGER(4)
   do i=1,10
     !DEF: /omp_reduction/OtherConstruct7/k (OmpReduction) HostAssoc INTEGER(4)
     k = k+1
   end do
   !$omp end do