[mlir][OpenMP] Convert omp.cancel sections to LLVMIR (#137193)

tblah · web-flow · commit 7b70fc74d039 · 2025-04-29T17:19:40.000+01:00
This is quite ugly but it is the best I could think of. The old
FiniCBWrapper was way too brittle depending upon the exact block
structure inside of the section, and could be confused by any control
flow in the section (e.g. an if clause on cancel). The wording in the
comment and variable names didn't seem to match where it was actually
branching too as well.

Clang's (non-OpenMPIRBuilder) lowering for cancel inside of sections
branches to a block containing __kmpc_for_static_fini.

This was hard to achieve here because sometimes the FiniCBWrapper has to
run before the worksharing loop finalization has been crated.

To get around this ordering issue I created a dummy branch to a dummy
block, which is then fixed later once all of the information is
available.
diff --git a/clang/lib/CodeGen/CGStmtOpenMP.cpp b/clang/lib/CodeGen/CGStmtOpenMP.cpp
@@ -4345,8 +4345,9 @@ void CodeGenFunction::EmitOMPSectionsDirective(const OMPSectionsDirective &S) {
     using InsertPointTy = llvm::OpenMPIRBuilder::InsertPointTy;
     using BodyGenCallbackTy = llvm::OpenMPIRBuilder::StorableBodyGenCallbackTy;
 
-    auto FiniCB = [this](InsertPointTy IP) {
-      OMPBuilderCBHelpers::FinalizeOMPRegion(*this, IP);
+    auto FiniCB = [](InsertPointTy IP) {
+      // Don't FinalizeOMPRegion because this is done inside of OMPIRBuilder for
+      // sections.
       return llvm::Error::success();
     };
 
diff --git a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
@@ -2172,23 +2172,19 @@ OpenMPIRBuilder::InsertPointOrErrorTy OpenMPIRBuilder::createSections(
   if (!updateToLocation(Loc))
     return Loc.IP;
 
+  // FiniCBWrapper needs to create a branch to the loop finalization block, but
+  // this has not been created yet at some times when this callback runs.
+  SmallVector<BranchInst *> CancellationBranches;
   auto FiniCBWrapper = [&](InsertPointTy IP) {
     if (IP.getBlock()->end() != IP.getPoint())
       return FiniCB(IP);
     // This must be done otherwise any nested constructs using FinalizeOMPRegion
     // will fail because that function requires the Finalization Basic Block to
     // have a terminator, which is already removed by EmitOMPRegionBody.
     // IP is currently at cancelation block.
-    // We need to backtrack to the condition block to fetch
-    // the exit block and create a branch from cancelation
-    // to exit block.
-    IRBuilder<>::InsertPointGuard IPG(Builder);
-    Builder.restoreIP(IP);
-    auto *CaseBB = IP.getBlock()->getSinglePredecessor();
-    auto *CondBB = CaseBB->getSinglePredecessor()->getSinglePredecessor();
-    auto *ExitBB = CondBB->getTerminator()->getSuccessor(1);
-    Instruction *I = Builder.CreateBr(ExitBB);
-    IP = InsertPointTy(I->getParent(), I->getIterator());
+    BranchInst *DummyBranch = Builder.CreateBr(IP.getBlock());
+    IP = InsertPointTy(DummyBranch->getParent(), DummyBranch->getIterator());
+    CancellationBranches.push_back(DummyBranch);
     return FiniCB(IP);
   };
 
@@ -2251,6 +2247,9 @@ OpenMPIRBuilder::InsertPointOrErrorTy OpenMPIRBuilder::createSections(
     return WsloopIP.takeError();
   InsertPointTy AfterIP = *WsloopIP;
 
+  BasicBlock *LoopFini = AfterIP.getBlock()->getSinglePredecessor();
+  assert(LoopFini && "Bad structure of static workshare loop finalization");
+
   // Apply the finalization callback in LoopAfterBB
   auto FiniInfo = FinalizationStack.pop_back_val();
   assert(FiniInfo.DK == OMPD_sections &&
@@ -2264,6 +2263,12 @@ OpenMPIRBuilder::InsertPointOrErrorTy OpenMPIRBuilder::createSections(
     AfterIP = {FiniBB, FiniBB->begin()};
   }
 
+  // Now we can fix the dummy branch to point to the right place
+  for (BranchInst *DummyBranch : CancellationBranches) {
+    assert(DummyBranch->getNumSuccessors() == 1);
+    DummyBranch->setSuccessor(0, LoopFini);
+  }
+
   return AfterIP;
 }
 
diff --git a/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp b/mlir/lib/Target/LLVMIR/Dialect/OpenMP/OpenMPToLLVMIRTranslation.cpp
@@ -161,7 +161,8 @@ static LogicalResult checkImplementationStatus(Operation &op) {
   auto checkCancelDirective = [&todo](auto op, LogicalResult &result) {
     omp::ClauseCancellationConstructType cancelledDirective =
         op.getCancelDirective();
-    if (cancelledDirective != omp::ClauseCancellationConstructType::Parallel)
+    if (cancelledDirective != omp::ClauseCancellationConstructType::Parallel &&
+        cancelledDirective != omp::ClauseCancellationConstructType::Sections)
       result = todo("cancel directive construct type not yet supported");
   };
   auto checkDepend = [&todo](auto op, LogicalResult &result) {
@@ -1688,10 +1689,11 @@ convertOmpSections(Operation &opInst, llvm::IRBuilderBase &builder,
   auto finiCB = [&](InsertPointTy codeGenIP) { return llvm::Error::success(); };
 
   allocaIP = findAllocaInsertPoint(builder, moduleTranslation);
+  bool isCancellable = constructIsCancellable(sectionsOp);
   llvm::OpenMPIRBuilder::LocationDescription ompLoc(builder);
   llvm::OpenMPIRBuilder::InsertPointOrErrorTy afterIP =
       moduleTranslation.getOpenMPBuilder()->createSections(
-          ompLoc, allocaIP, sectionCBs, privCB, finiCB, false,
+          ompLoc, allocaIP, sectionCBs, privCB, finiCB, isCancellable,
           sectionsOp.getNowait());
 
   if (failed(handleError(afterIP, opInst)))
diff --git a/mlir/test/Target/LLVMIR/openmp-cancel.mlir b/mlir/test/Target/LLVMIR/openmp-cancel.mlir
@@ -80,3 +80,79 @@ llvm.func @cancel_parallel_if(%arg0 : i1) {
 // CHECK:         br label %[[VAL_23]]
 // CHECK:       omp.par.exit.exitStub:                            ; preds = %[[VAL_31]], %[[VAL_26]]
 // CHECK:         ret void
+
+llvm.func @cancel_sections_if(%cond : i1) {
+  omp.sections {
+    omp.section {
+      omp.cancel cancellation_construct_type(sections) if(%cond)
+      omp.terminator
+    }
+    omp.terminator
+  }
+  llvm.return
+}
+// CHECK-LABEL: define void @cancel_sections_if
+// CHECK:         %[[VAL_0:.*]] = alloca i32, align 4
+// CHECK:         %[[VAL_1:.*]] = alloca i32, align 4
+// CHECK:         %[[VAL_2:.*]] = alloca i32, align 4
+// CHECK:         %[[VAL_3:.*]] = alloca i32, align 4
+// CHECK:         br label %[[VAL_4:.*]]
+// CHECK:       entry:                                            ; preds = %[[VAL_5:.*]]
+// CHECK:         br label %[[VAL_6:.*]]
+// CHECK:       omp_section_loop.preheader:                       ; preds = %[[VAL_4]]
+// CHECK:         store i32 0, ptr %[[VAL_1]], align 4
+// CHECK:         store i32 0, ptr %[[VAL_2]], align 4
+// CHECK:         store i32 1, ptr %[[VAL_3]], align 4
+// CHECK:         %[[VAL_7:.*]] = call i32 @__kmpc_global_thread_num(ptr @1)
+// CHECK:         call void @__kmpc_for_static_init_4u(ptr @1, i32 %[[VAL_7]], i32 34, ptr %[[VAL_0]], ptr %[[VAL_1]], ptr %[[VAL_2]], ptr %[[VAL_3]], i32 1, i32 0)
+// CHECK:         %[[VAL_8:.*]] = load i32, ptr %[[VAL_1]], align 4
+// CHECK:         %[[VAL_9:.*]] = load i32, ptr %[[VAL_2]], align 4
+// CHECK:         %[[VAL_10:.*]] = sub i32 %[[VAL_9]], %[[VAL_8]]
+// CHECK:         %[[VAL_11:.*]] = add i32 %[[VAL_10]], 1
+// CHECK:         br label %[[VAL_12:.*]]
+// CHECK:       omp_section_loop.header:                          ; preds = %[[VAL_13:.*]], %[[VAL_6]]
+// CHECK:         %[[VAL_14:.*]] = phi i32 [ 0, %[[VAL_6]] ], [ %[[VAL_15:.*]], %[[VAL_13]] ]
+// CHECK:         br label %[[VAL_16:.*]]
+// CHECK:       omp_section_loop.cond:                            ; preds = %[[VAL_12]]
+// CHECK:         %[[VAL_17:.*]] = icmp ult i32 %[[VAL_14]], %[[VAL_11]]
+// CHECK:         br i1 %[[VAL_17]], label %[[VAL_18:.*]], label %[[VAL_19:.*]]
+// CHECK:       omp_section_loop.body:                            ; preds = %[[VAL_16]]
+// CHECK:         %[[VAL_20:.*]] = add i32 %[[VAL_14]], %[[VAL_8]]
+// CHECK:         %[[VAL_21:.*]] = mul i32 %[[VAL_20]], 1
+// CHECK:         %[[VAL_22:.*]] = add i32 %[[VAL_21]], 0
+// CHECK:         switch i32 %[[VAL_22]], label %[[VAL_23:.*]] [
+// CHECK:           i32 0, label %[[VAL_24:.*]]
+// CHECK:         ]
+// CHECK:       omp_section_loop.body.case:                       ; preds = %[[VAL_18]]
+// CHECK:         br label %[[VAL_25:.*]]
+// CHECK:       omp.section.region:                               ; preds = %[[VAL_24]]
+// CHECK:         br i1 %[[VAL_26:.*]], label %[[VAL_27:.*]], label %[[VAL_28:.*]]
+// CHECK:       9:                                                ; preds = %[[VAL_25]]
+// CHECK:         %[[VAL_29:.*]] = call i32 @__kmpc_global_thread_num(ptr @1)
+// CHECK:         %[[VAL_30:.*]] = call i32 @__kmpc_cancel(ptr @1, i32 %[[VAL_29]], i32 3)
+// CHECK:         %[[VAL_31:.*]] = icmp eq i32 %[[VAL_30]], 0
+// CHECK:         br i1 %[[VAL_31]], label %[[VAL_32:.*]], label %[[VAL_33:.*]]
+// CHECK:       .split:                                           ; preds = %[[VAL_27]]
+// CHECK:         br label %[[VAL_34:.*]]
+// CHECK:       12:                                               ; preds = %[[VAL_25]]
+// CHECK:         br label %[[VAL_34]]
+// CHECK:       13:                                               ; preds = %[[VAL_28]], %[[VAL_32]]
+// CHECK:         br label %[[VAL_35:.*]]
+// CHECK:       omp.region.cont:                                  ; preds = %[[VAL_34]]
+// CHECK:         br label %[[VAL_23]]
+// CHECK:       omp_section_loop.body.sections.after:             ; preds = %[[VAL_35]], %[[VAL_18]]
+// CHECK:         br label %[[VAL_13]]
+// CHECK:       omp_section_loop.inc:                             ; preds = %[[VAL_23]]
+// CHECK:         %[[VAL_15]] = add nuw i32 %[[VAL_14]], 1
+// CHECK:         br label %[[VAL_12]]
+// CHECK:       omp_section_loop.exit:                            ; preds = %[[VAL_33]], %[[VAL_16]]
+// CHECK:         call void @__kmpc_for_static_fini(ptr @1, i32 %[[VAL_7]])
+// CHECK:         %[[VAL_36:.*]] = call i32 @__kmpc_global_thread_num(ptr @1)
+// CHECK:         call void @__kmpc_barrier(ptr @2, i32 %[[VAL_36]])
+// CHECK:         br label %[[VAL_37:.*]]
+// CHECK:       omp_section_loop.after:                           ; preds = %[[VAL_19]]
+// CHECK:         br label %[[VAL_38:.*]]
+// CHECK:       omp_section_loop.aftersections.fini:              ; preds = %[[VAL_37]]
+// CHECK:         ret void
+// CHECK:       .cncl:                                            ; preds = %[[VAL_27]]
+// CHECK:         br label %[[VAL_19]]
diff --git a/mlir/test/Target/LLVMIR/openmp-todo.mlir b/mlir/test/Target/LLVMIR/openmp-todo.mlir
@@ -42,22 +42,6 @@ llvm.func @cancel_wsloop(%lb : i32, %ub : i32, %step: i32) {
 
 // -----
 
-llvm.func @cancel_sections() {
-  // expected-error@below {{LLVM Translation failed for operation: omp.sections}}
-  omp.sections {
-    omp.section {
-      // expected-error@below {{not yet implemented: Unhandled clause cancel directive construct type not yet supported in omp.cancel operation}}
-      // expected-error@below {{LLVM Translation failed for operation: omp.cancel}}
-      omp.cancel cancellation_construct_type(sections)
-      omp.terminator
-    }
-    omp.terminator
-  }
-  llvm.return
-}
-
-// -----
-
 llvm.func @cancel_taskgroup() {
   // expected-error@below {{LLVM Translation failed for operation: omp.taskgroup}}
   omp.taskgroup {
