[Mlir-commits] [llvm] [mlir] [OMPIRBuilder] Hoist alloca's to entry blocks of compiler-emitted GPU reduction functions (PR #181359)

[llvmlistbot at llvm.org]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-mlir-llvm

Author: Kareem Ergawy (ergawy)

<details>
<summary>Changes</summary>

Fixes a bug in GPU reductions when `-O0` was used to compile GPU reductions. There were invalid memory accesses at runtime for the following example:

```fortran
program test_array_reduction()
  integer :: red_array(1)
  integer :: i

  red_array = 0

  !$omp target teams distribute parallel do reduction(+:red_array)
  do i = 1, 100
    red_array(1) = red_array(1) + 4422
  end do
  !$omp end target teams distribute parallel do

  print *, red_array
end program test_array_reduction
```

The issue was caused by alloca's for some temp values in the combiner region of the reduction op being inlined beyond the entry blocks of the GPU reduction functions emitted by the compiler.

This PR fixes the issue by hoisting all alloca's to the entry block after the reduction functions are completely emitted by the compiler.

---
Full diff: https://github.com/llvm/llvm-project/pull/181359.diff


2 Files Affected:

- (modified) llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp (+6) 
- (modified) mlir/test/Target/LLVMIR/allocatable_gpu_reduction.mlir (+17) 


``````````diff
diff --git a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
index 464ec5b5a2ece..7d446983a3882 100644
--- a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
+++ b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
@@ -4234,6 +4234,12 @@ Expected<Function *> OpenMPIRBuilder::createReductionFunction(
     }
 
   Builder.CreateRetVoid();
+
+  PostDominatorTree PostDomTree(*ReductionFunc);
+  for (llvm::BasicBlock &BB : *ReductionFunc)
+    if (PostDomTree.properlyDominates(&BB, &ReductionFunc->getEntryBlock()))
+      hoistNonEntryAllocasToEntryBlock(BB);
+
   return ReductionFunc;
 }
 
diff --git a/mlir/test/Target/LLVMIR/allocatable_gpu_reduction.mlir b/mlir/test/Target/LLVMIR/allocatable_gpu_reduction.mlir
index 95d12f304aca0..c9ff6de8cc951 100644
--- a/mlir/test/Target/LLVMIR/allocatable_gpu_reduction.mlir
+++ b/mlir/test/Target/LLVMIR/allocatable_gpu_reduction.mlir
@@ -24,6 +24,8 @@ module attributes {dlti.dl_spec = #dlti.dl_spec<"dlti.alloca_memory_space" = 5 :
     "llvm.intr.memcpy"(%5, %arg0, %6) <{isVolatile = false}> : (!llvm.ptr, !llvm.ptr, i32) -> ()
     %7 = llvm.mlir.constant(24 : i32) : i32
     "llvm.intr.memcpy"(%2, %arg1, %7) <{isVolatile = false}> : (!llvm.ptr, !llvm.ptr, i32) -> ()
+    llvm.br ^bb1
+  ^bb1:
     %8 = llvm.getelementptr %5[0, 0] : (!llvm.ptr) -> !llvm.ptr, !llvm.struct<(ptr, i64, i32, i8, i8, i8, i8)>
     %9 = llvm.load %8 : !llvm.ptr -> !llvm.ptr
     %10 = llvm.getelementptr %2[0, 0] : (!llvm.ptr) -> !llvm.ptr, !llvm.struct<(ptr, i64, i32, i8, i8, i8, i8)>
@@ -63,6 +65,21 @@ module attributes {dlti.dl_spec = #dlti.dl_spec<"dlti.alloca_memory_space" = 5 :
   }
 }
 
+// CHECK: define internal void @"{{.*}}$reduction$reduction_func"(ptr noundef %[[ARG_0:.*]], ptr noundef %[[ARG_1:.*]]) {{.*}} {
+// CHECK: entry:
+// CHECK:   %[[TEMP_1:.*]] = alloca { ptr, i64, i32, i8, i8, i8, i8 }
+// CHECK:   %[[TEMP_2:.*]] = alloca { ptr, i64, i32, i8, i8, i8, i8 }
+// CHECK:   br label %[[RED_BODY:omp.reduction.nonatomic.body]]
+// Verify that no allocas are emitted beyond the entry block.
+// CHECK-NOT: alloca
+
+// CHECK: [[RED_BODY]]:
+// CHECK:   %[[TEMP_1_ACAST:.*]] = addrspacecast ptr addrspace(5) %[[TEMP_1]] to ptr
+// CHECK:   %[[TEMP_2_ACAST:.*]] = addrspacecast ptr addrspace(5) %[[TEMP_2]] to ptr
+// CHECK:   call void @llvm.memcpy.p0.p0.i32(ptr %[[TEMP_2_ACAST]], ptr %{{.*}}, i32 24, i1 false)
+// CHECK:   call void @llvm.memcpy.p0.p0.i32(ptr %[[TEMP_1_ACAST]], ptr %{{.*}}, i32 24, i1 false)
+// CHECK: }
+
 // CHECK: define {{.*}} @_omp_reduction_shuffle_and_reduce_func({{.*}}) {{.*}} {
 // CHECK:   %[[REMOTE_RED_LIST:.omp.reduction.remote_reduce_list]] = alloca [1 x ptr], align 8, addrspace(5)
 // CHECK:   %[[RED_ELEM:.omp.reduction.element]] = alloca { ptr, i64, i32, i8, i8, i8, i8 }, align 8, addrspace(5)

``````````

</details>


https://github.com/llvm/llvm-project/pull/181359