[flang] [llvm] [flang][OpenMP] Prevent `omp.map.info` ops with user-defined mappers from being marked as parial maps (PR #175133)
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Fri Jan 9 00:21:32 PST 2026
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-flang-fir-hlfir
Author: Kareem Ergawy (ergawy)
<details>
<summary>Changes</summary>
The following test was triggering a runtime crash **on the host before launching the kernel**:
```fortran
program test_omp_target_map_bug_v5
implicit none
type nested_type
real, allocatable :: alloc_field(:)
end type nested_type
type nesting_type
integer :: int_field
type(nested_type) :: derived_field
end type nesting_type
type(nesting_type) :: config
allocate(config%derived_field%alloc_field(1))
!$OMP TARGET ENTER DATA MAP(TO:config, config%derived_field%alloc_field)
!$OMP TARGET
config%derived_field%alloc_field(1) = 1.0
!$OMP END TARGET
deallocate(config%derived_field%alloc_field)
end program test_omp_target_map_bug_v5
```
In particular, the runtime was producing a segmentation fault when the test is compiled with any optimization level > 0; if you compile with -O0 the sample ran fine.
After debugging the runtime, it turned out the crash was happening at the point where the runtime calls the default mapper emitted by the compiler for `nesting_type; in particular at this point in the runtime: https://github.com/llvm/llvm-project/blob/c62cd2877cc25a0d708ad22a70c2a57590449c4d/offload/libomptarget/omptarget.cpp#L307.
Bisecting the optimization pipeline using `-mllvm -opt-bisect-limit=N`, the first pass that triggered the issue on `O1` was the `instcombine` pass. Deubbing this further, the issue narrows down to canonalizing `getelementptr` instructions from using struct types (in this case the `nesting_type` in the sample above) to using `i8`. In particular, in `O0`, you would see something like this:
```llvm
define internal void @.omp_mapper._QQFnesting_type_omp_default_mapper(ptr noundef %0, ptr noundef %1, ptr noundef %2, i64 noundef %3, i64 noundef %4, ptr noundef %5) #<!-- -->6 {
entry:
%6 = udiv exact i64 %3, 56
%7 = getelementptr %_QFTnesting_type, ptr %2, i64 %6
....
}
```
```llvm
define internal void @.omp_mapper._QQFnesting_type_omp_default_mapper(ptr noundef %0, ptr noundef %1, ptr noundef %2, i64 noundef %3, i64 noundef %4, ptr noundef %5) #<!-- -->6 {
entry:
%6 = getelementptr i8, ptr %2, i64 %3
....
}
```
The `udiv exact` instruction emitted by the OMP IR Builder (see: https://github.com/llvm/llvm-project/blob/c62cd2877cc25a0d708ad22a70c2a57590449c4d/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp#L9154) allows `instcombine` to assume that `%3` is divisible by the struct size (here `56`) and, therefore, replaces the result of the division with direct GEP on `i8` rather than the struct type.
However, the runtime was calling `@.omp_mapper._QQFnesting_type_omp_default_mapper` not with `56` (the proper struct size) but with `48`!
Debugging this further, I found that the size of `omp.map.info` operation to which the default mapper is attached computes the value of `48` because we set the map to partial (see: https://github.com/llvm/llvm-project/blob/c62cd2877cc25a0d708ad22a70c2a57590449c4d/flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp#L1146 and https://github.com/llvm/llvm-project/blob/c62cd2877cc25a0d708ad22a70c2a57590449c4d/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp#L4501-L4512).
However, I think this is incorrect since the emitted mapper (and user-defined mappers in general) are defined on the whole struct type and should never be marked as partial. Hence, the fix in this PR.
---
Full diff: https://github.com/llvm/llvm-project/pull/175133.diff
3 Files Affected:
- (modified) flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp (+2-1)
- (modified) flang/test/Transforms/omp-map-info-finalization-implicit-field.fir (+25-1)
- (added) offload/test/offloading/fortran/default-mapper-nested-derived-type.f90 (+34)
``````````diff
diff --git a/flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp b/flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp
index 3fe133d63d24d..a60960e739d24 100644
--- a/flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp
+++ b/flang/lib/Optimizer/OpenMP/MapInfoFinalization.cpp
@@ -1143,7 +1143,8 @@ class MapInfoFinalizationPass
newMemberIndices.emplace_back(path);
op.setMembersIndexAttr(builder.create2DI64ArrayAttr(newMemberIndices));
- op.setPartialMap(true);
+ // Set to partial map only if there is no user-defined mapper.
+ op.setPartialMap(op.getMapperIdAttr() == nullptr);
return mlir::WalkResult::advance();
});
diff --git a/flang/test/Transforms/omp-map-info-finalization-implicit-field.fir b/flang/test/Transforms/omp-map-info-finalization-implicit-field.fir
index 632525b4b43c9..d3e8125d2ee3d 100644
--- a/flang/test/Transforms/omp-map-info-finalization-implicit-field.fir
+++ b/flang/test/Transforms/omp-map-info-finalization-implicit-field.fir
@@ -15,13 +15,24 @@ fir.global internal @_QFEdst_record : !record_t {
fir.has_value %0 : !record_t
}
+omp.declare_mapper @record_mapper : !record_t {
+^bb0(%arg0: !fir.ref<!record_t>):
+ %0 = omp.map.info var_ptr(%arg0: !fir.ref<!record_t>, !record_t) map_clauses(implicit, tofrom) capture(ByRef) -> !fir.ref<!record_t>
+ omp.declare_mapper.info map_entries(%0: !fir.ref<!record_t>)
+}
+
func.func @_QQmain() {
%6 = fir.address_of(@_QFEdst_record) : !fir.ref<!record_t>
%7:2 = hlfir.declare %6 {uniq_name = "_QFEdst_record"} : (!fir.ref<!record_t>) -> (!fir.ref<!record_t>, !fir.ref<!record_t>)
%16 = omp.map.info var_ptr(%7#1 : !fir.ref<!record_t>, !record_t) map_clauses(implicit, tofrom) capture(ByRef) -> !fir.ref<!record_t> {name = "dst_record"}
- omp.target map_entries(%16 -> %arg0 : !fir.ref<!record_t>) {
+ %17 = omp.map.info var_ptr(%7#1 : !fir.ref<!record_t>, !record_t) map_clauses(implicit, tofrom) capture(ByRef) mapper(@record_mapper) -> !fir.ref<!record_t> {name = "dst_record_with_mapper"}
+ omp.target map_entries(%16 -> %arg0, %17 -> %arg1 : !fir.ref<!record_t>, !fir.ref<!record_t>) {
%20:2 = hlfir.declare %arg0 {uniq_name = "_QFEdst_record"} : (!fir.ref<!record_t>) -> (!fir.ref<!record_t>, !fir.ref<!record_t>)
+ %21:2 = hlfir.declare %arg1 {uniq_name = "_QFEdst_record"} : (!fir.ref<!record_t>) -> (!fir.ref<!record_t>, !fir.ref<!record_t>)
+
%23 = hlfir.designate %20#0{"to_implicitly_map"} {fortran_attrs = #fir.var_attrs<allocatable>} : (!fir.ref<!record_t>) -> !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
+
+ %24 = hlfir.designate %21#0{"to_implicitly_map"} {fortran_attrs = #fir.var_attrs<allocatable>} : (!fir.ref<!record_t>) -> !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
omp.terminator
}
return
@@ -56,6 +67,19 @@ func.func @_QQmain() {
// CHECK-SAME: [1], [1, 0] : {{.*}}) -> {{.*}}> {name =
// CHECK-SAME: "dst_record", partial_map = true}
+// Verify map ops when using a mapper:
+// Implicit field mapping is the same as for the non-mapper case.
+// CHECK: omp.map.info
+// CHECK: omp.map.info
+
+// Verify that partial-map is not set if the map info op uses a user-defined (or
+// compiler-emitted) mapper.
+// CHECK: %[[RECORD_MAP_MAPPER:.*]] = omp.map.info var_ptr(
+// CHECK-SAME: %[[RECORD_DECL]]#1 : {{.*}}) map_clauses(
+// CHECK-SAME: implicit, tofrom) capture(ByRef) mapper(@record_mapper)
+// CHECK-SAME: members(%{{.*}}, %{{.*}} : [1], [1, 0] : {{.*}}) -> {{.*}}> {name =
+// CHECK-SAME: "dst_record_with_mapper"}
+
// CHECK: omp.target map_entries(
// CHECK-SAME: %[[RECORD_MAP]] -> %{{[^[:space:]]+}},
// CHECK-SAME: %[[FIELD_MAP]] -> %{{[^[:space:]]+}},
diff --git a/offload/test/offloading/fortran/default-mapper-nested-derived-type.f90 b/offload/test/offloading/fortran/default-mapper-nested-derived-type.f90
new file mode 100644
index 0000000000000..5d69fa072fd63
--- /dev/null
+++ b/offload/test/offloading/fortran/default-mapper-nested-derived-type.f90
@@ -0,0 +1,34 @@
+! Regression test for default mappers emitted for nested derived types. Some
+! optimization passes and instrumentation callbacks cause crashes in emitted
+! mappers and this test guards against such crashes.
+
+! REQUIRES: flang, amdgpu
+
+! RUN: %libomptarget-compile-fortran-generic
+! RUN: env LIBOMPTARGET_INFO=16 %libomptarget-run-generic 2>&1 | %fcheck-generic
+
+program test_omp_target_map_bug_v5
+ implicit none
+ type nested_type
+ real, allocatable :: alloc_field(:)
+ end type nested_type
+
+ type nesting_type
+ integer :: int_field
+ type(nested_type) :: derived_field
+ end type nesting_type
+
+ type(nesting_type) :: config
+
+ allocate(config%derived_field%alloc_field(1))
+
+ !$OMP TARGET ENTER DATA MAP(TO:config, config%derived_field%alloc_field)
+
+ !$OMP TARGET
+ config%derived_field%alloc_field(1) = 1.0
+ !$OMP END TARGET
+
+ deallocate(config%derived_field%alloc_field)
+end program test_omp_target_map_bug_v5
+
+! CHECK: "PluginInterface" device {{[0-9]+}} info: Launching kernel {{.*}}
``````````
</details>
https://github.com/llvm/llvm-project/pull/175133
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