[LLVM][GPU] Atomic updates support (#853)

1. Helper visitor generates atomic statements with += and -= once again. 2. LLVM visitor now knows how to lower atomic statements for CPUs (trivially) and GPUs. 3. A corresponding IR test was added. `expsyn` test on GPU enabled Co-authored-by: Ioannis Magkanaris <[email protected]>
BlueBrain · May 12, 2022 · 265a02b · 265a02b
1 parent 95782bc
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diff --git a/src/codegen/llvm/codegen_llvm_helper_visitor.cpp b/src/codegen/llvm/codegen_llvm_helper_visitor.cpp
@@ -283,7 +283,7 @@ static void append_statements_from_block(ast::StatementVector& statements,
  * Create atomic statement for given expression of the form a[i] += expression
  * @param var Name of the variable on the LHS (it's an array), e.g. `a`
  * @param var_index Name of the index variable to access variable `var` e.g. `i`
- * @param op_str Operators like += or -=
+ * @param op_str Operators like =, += or -=
  * @param rhs_str expression that will be added or subtracted from `var[var_index]`
  * @return A statement representing atomic operation using `ast::CodegenAtomicStatement`
  */
@@ -299,23 +299,9 @@ static std::shared_ptr<ast::CodegenAtomicStatement> create_atomic_statement(
                                 /*at=*/nullptr,
                                 /*index=*/nullptr);
 
-    // LLVM IR generation is now only supporting assignment (=) and not += or -=
-    // So we need to write increment operation a += b as an assignment operation
-    // a = a + b.
-    // See https://github.com/BlueBrain/nmodl/issues/851
-
-    std::string op(op_str);
-    stringutils::remove_character(op, '=');
-
-    // make sure only + or - operator is used
-    if (op_str != "-" && op_str != "+") {
-        throw std::runtime_error("Unsupported binary operator for atomic statement");
-    }
-
-    auto* rhs = create_expression("{}[{}] {} {} "_format(var, var_index, op, rhs_str));
-    return std::make_shared<ast::CodegenAtomicStatement>(lhs,
-                                                         ast::BinaryOperator{ast::BOP_ASSIGN},
-                                                         rhs);
+    auto op = ast::BinaryOperator(ast::string_to_binaryop(op_str));
+    auto rhs = create_expression(rhs_str);
+    return std::make_shared<ast::CodegenAtomicStatement>(lhs, op, rhs);
 }
 
 /**
@@ -420,22 +406,7 @@ void CodegenLLVMHelperVisitor::ion_write_statements(BlockType type,
         index_statements.push_back(visitor::create_statement(index_statement));
 
         // pass ion variable to write and its index
-
-        // lhs variable
-        std::string lhs = "{}[{}] "_format(ion_varname, index_varname);
-
-        // lets turn a += b into a = a + b if applicable
-        // note that this is done in order to facilitate existing implementation in the llvm
-        // backend which doesn't support += or -= operators.
-        std::string statement;
-        if (!op.compare("+=")) {
-            statement = "{} = {} + {}"_format(lhs, lhs, rhs);
-        } else if (!op.compare("-=")) {
-            statement = "{} = {} - {}"_format(lhs, lhs, rhs);
-        } else {
-            statement = "{} {} {}"_format(lhs, op, rhs);
-        }
-        body_statements.push_back(visitor::create_statement(statement));
+        body_statements.push_back(create_atomic_statement(ion_varname, index_varname, op, rhs));
     };
 
     /// iterate over all ions and create write ion statements for given block type

diff --git a/src/codegen/llvm/codegen_llvm_visitor.cpp b/src/codegen/llvm/codegen_llvm_visitor.cpp
@@ -512,31 +512,73 @@ void CodegenLLVMVisitor::visit_boolean(const ast::Boolean& node) {
     ir_builder.create_boolean_constant(node.get_value());
 }
 
-/**
- * Currently, this functions is very similar to visiting the binary operator. However, the
- * difference here is that the writes to the LHS variable must be atomic. These has a particular
- * use case in synapse kernels. For simplicity, we choose not to support atomic writes at this
- * stage and emit a warning.
- *
- * \todo support this properly.
- */
 void CodegenLLVMVisitor::visit_codegen_atomic_statement(const ast::CodegenAtomicStatement& node) {
-    if (platform.is_cpu_with_simd())
-        logger->warn("Atomic operations are not supported");
+    // Get the variable node that need an atomic update.
+    const auto& var = std::dynamic_pointer_cast<ast::VarName>(node.get_lhs());
+    if (!var)
+        throw std::runtime_error("Error: only 'VarName' update is supported\n");
 
-    // Support only assignment for now.
+    // Evaluate RHS of the update.
     llvm::Value* rhs = accept_and_get(node.get_rhs());
-    if (node.get_atomic_op().get_value() != ast::BinaryOp::BOP_ASSIGN)
-        throw std::runtime_error(
-            "Error: only assignment is supported for CodegenAtomicStatement\n");
-    const auto& var = dynamic_cast<ast::VarName*>(node.get_lhs().get());
-    if (!var)
-        throw std::runtime_error("Error: only 'VarName' assignment is supported\n");
 
-    // Process the assignment as if it was non-atomic.
-    if (platform.is_cpu_with_simd())
-        logger->warn("Treating write as non-atomic");
-    write_to_variable(*var, rhs);
+    // First, check if it is an atomic write only and we can return early.
+    // Otherwise, extract what kind of atomic update we want to make.
+    ast::BinaryOp atomic_op = node.get_atomic_op().get_value();
+    if (atomic_op == ast::BinaryOp::BOP_ASSIGN) {
+        write_to_variable(*var, rhs);
+        return;
+    }
+    ast::BinaryOp op = ir_builder.extract_atomic_op(atomic_op);
+
+    // For different platforms, we handle atomic updates differently!
+    if (platform.is_cpu_with_simd()) {
+        throw std::runtime_error("Error: no atomic update support for SIMD CPUs\n");
+    } else if (platform.is_gpu()) {
+        const auto& identifier = var->get_name();
+
+        // We only need to support atomic updates to instance struct members.
+        if (!identifier->is_codegen_instance_var())
+            throw std::runtime_error("Error: atomic updates for non-instance variable\n");
+
+        const auto& node = std::dynamic_pointer_cast<ast::CodegenInstanceVar>(identifier);
+        const auto& instance_name = node->get_instance_var()->get_node_name();
+        const auto& member_node = node->get_member_var();
+        const auto& member_name = member_node->get_node_name();
+
+        if (!instance_var_helper.is_an_instance_variable(member_name))
+            throw std::runtime_error("Error: " + member_name +
+                                     " is not a member of the instance variable\n");
+
+        llvm::Value* instance_ptr = ir_builder.create_load(instance_name);
+        int member_index = instance_var_helper.get_variable_index(member_name);
+        llvm::Value* member_ptr = ir_builder.get_struct_member_ptr(instance_ptr, member_index);
+
+        // Some sanity checks.
+        auto codegen_var_with_type = instance_var_helper.get_variable(member_name);
+        if (!codegen_var_with_type->get_is_pointer())
+            throw std::runtime_error(
+                "Error: atomic updates are allowed on pointer variables only\n");
+        const auto& member_var_name = std::dynamic_pointer_cast<ast::VarName>(member_node);
+        if (!member_var_name->get_name()->is_indexed_name())
+            throw std::runtime_error("Error: " + member_name + " is not an IndexedName\n");
+        const auto& member_indexed_name = std::dynamic_pointer_cast<ast::IndexedName>(
+            member_var_name->get_name());
+        if (!member_indexed_name->get_length()->is_name())
+            throw std::runtime_error("Error: " + member_name + " must be indexed with a variable!");
+
+        llvm::Value* i64_index = get_index(*member_indexed_name);
+        llvm::Value* instance_member = ir_builder.create_load(member_ptr);
+        llvm::Value* ptr = ir_builder.create_inbounds_gep(instance_member, i64_index);
+
+        ir_builder.create_atomic_op(ptr, rhs, op);
+    } else {
+        // For non-SIMD CPUs, updates don't have to be atomic at all!
+        llvm::Value* lhs = accept_and_get(node.get_lhs());
+        ir_builder.create_binary_op(lhs, rhs, op);
+        llvm::Value* result = ir_builder.pop_last_value();
+
+        write_to_variable(*var, result);
+    }
 }
 
 // Generating FOR loop in LLVM IR creates the following structure:

diff --git a/src/codegen/llvm/llvm_ir_builder.cpp b/src/codegen/llvm/llvm_ir_builder.cpp
@@ -293,6 +293,28 @@ void IRBuilder::create_array_alloca(const std::string& name,
     create_alloca(name, array_type);
 }
 
+ast::BinaryOp IRBuilder::extract_atomic_op(ast::BinaryOp op) {
+    switch (op) {
+    case ast::BinaryOp::BOP_SUB_ASSIGN:
+        return ast::BinaryOp::BOP_SUBTRACTION;
+    case ast::BinaryOp::BOP_ADD_ASSIGN:
+        return ast::BinaryOp::BOP_ADDITION;
+    default:
+        throw std::runtime_error("Error: only atomic addition and subtraction is supported\n");
+    }
+}
+
+void IRBuilder::create_atomic_op(llvm::Value* ptr, llvm::Value* update, ast::BinaryOp op) {
+    if (op == ast::BinaryOp::BOP_SUBTRACTION) {
+        update = builder.CreateFNeg(update);
+    }
+    builder.CreateAtomicRMW(llvm::AtomicRMWInst::FAdd,
+                            ptr,
+                            update,
+                            llvm::MaybeAlign(),
+                            llvm::AtomicOrdering::SequentiallyConsistent);
+}
+
 void IRBuilder::create_binary_op(llvm::Value* lhs, llvm::Value* rhs, ast::BinaryOp op) {
     // Check that both lhs and rhs have the same types.
     if (lhs->getType() != rhs->getType())

diff --git a/src/codegen/llvm/llvm_ir_builder.hpp b/src/codegen/llvm/llvm_ir_builder.hpp
@@ -146,6 +146,9 @@ class IRBuilder {
         return vectorize && mask;
     }
 
+    /// Extracts binary operator (+ or -) from atomic update (+= or =-).
+    ast::BinaryOp extract_atomic_op(ast::BinaryOp op);
+
     /// Generates LLVM IR to allocate the arguments of the function on the stack.
     void allocate_function_arguments(llvm::Function* function,
                                      const ast::CodegenVarWithTypeVector& nmodl_arguments);
@@ -158,6 +161,9 @@ class IRBuilder {
     /// Generates LLVM IR for the given binary operator.
     void create_binary_op(llvm::Value* lhs, llvm::Value* rhs, ast::BinaryOp op);
 
+    /// Generates LLVM IR for the given atomic operator.
+    void create_atomic_op(llvm::Value* ptr, llvm::Value* update, ast::BinaryOp op);
+
     /// Generates LLVM IR for the bitcast instruction.
     llvm::Value* create_bitcast(llvm::Value* value, llvm::Type* dst_type);
 
@@ -304,13 +310,13 @@ class IRBuilder {
     /// Pops the last visited value from the value stack.
     llvm::Value* pop_last_value();
 
+    /// Generates an inbounds GEP instruction for the given value and returns calculated address.
+    llvm::Value* create_inbounds_gep(llvm::Value* variable, llvm::Value* index);
+
   private:
     /// Generates an inbounds GEP instruction for the given name and returns calculated address.
     llvm::Value* create_inbounds_gep(const std::string& variable_name, llvm::Value* index);
 
-    /// Generates an inbounds GEP instruction for the given value and returns calculated address.
-    llvm::Value* create_inbounds_gep(llvm::Value* variable, llvm::Value* index);
-
     /// Returns a scalar constant of the provided type.
     template <typename C, typename V>
     llvm::Value* get_scalar_constant(llvm::Type* type, V value);

diff --git a/test/benchmark/CMakeLists.txt b/test/benchmark/CMakeLists.txt
@@ -44,9 +44,8 @@ if(NMODL_ENABLE_PYTHON_BINDINGS)
     set_tests_properties(
       "PyJIT/${modfile_name}" PROPERTIES ENVIRONMENT
                                          PYTHONPATH=${PROJECT_BINARY_DIR}/lib:$ENV{PYTHONPATH})
-    # Disable running the expsyn.mod on GPU because atomic instructions are not supported yet on GPU
-    # See https://github.com/BlueBrain/nmodl/issues/834
-    if(NMODL_ENABLE_LLVM_CUDA AND NOT ${modfile} STREQUAL "${NMODL_PROJECT_SOURCE_DIR}/test/benchmark/kernels/expsyn.mod")
+
+    if(NMODL_ENABLE_LLVM_CUDA)
       add_test(NAME "PyJIT/${modfile_name}_gpu"
                COMMAND ${PYTHON_EXECUTABLE} ${NMODL_PROJECT_SOURCE_DIR}/test/benchmark/benchmark.py
                       --file ${modfile} --gpu ${extra_args})

diff --git a/test/unit/codegen/codegen_llvm_ir.cpp b/test/unit/codegen/codegen_llvm_ir.cpp
@@ -1360,7 +1360,7 @@ SCENARIO("Vectorised derivative block", "[visitor][llvm][derivative]") {
                 }
                 g = (g-rhs)/0.001
                 mech->ion_dinadv[ion_dinadv_id] = mech->ion_dinadv[ion_dinadv_id]+(dina-mech->ina[id])/0.001
-                mech->ion_ina[ion_ina_id] = mech->ion_ina[ion_ina_id]+mech->ina[id]
+                mech->ion_ina[ion_ina_id] += mech->ina[id]
                 mech->vec_rhs[node_id] = mech->vec_rhs[node_id]-rhs
                 mech->vec_d[node_id] = mech->vec_d[node_id]+g
             })";
@@ -1807,4 +1807,38 @@ SCENARIO("GPU kernel body IR generation", "[visitor][llvm][gpu]") {
             REQUIRE(!std::regex_search(module_string, m, log_old_call));
         }
     }
+
+    GIVEN("For current update with atomic addition ") {
+        std::string nmodl_text = R"(
+            NEURON {
+                SUFFIX test
+                USEION na READ ena WRITE ina
+            }
+
+            STATE { }
+
+            ASSIGNED {
+                v (mV)
+                ena (mV)
+                ina (mA/cm2)
+            }
+
+            BREAKPOINT {
+                SOLVE states METHOD cnexp
+            }
+
+            DERIVATIVE states { }
+        )";
+
+        THEN("corresponding LLVM atomic instruction is generated") {
+            std::string module_string = run_gpu_llvm_visitor(nmodl_text,
+                                                             /*opt_level=*/0,
+                                                             /*use_single_precision=*/false);
+            std::smatch m;
+
+            // Check for atomic addition.
+            std::regex add(R"(atomicrmw fadd double\* %.*, double %.* seq_cst)");
+            REQUIRE(std::regex_search(module_string, m, add));
+        }
+    }
 }
diff --git a/test/unit/codegen/codegen_llvm_visitor.cpp b/test/unit/codegen/codegen_llvm_visitor.cpp
@@ -462,8 +462,8 @@ SCENARIO("Channel: Derivative and breakpoint block llvm transformations",
                     g = (g-rhs)/0.001
                     mech->ion_dinadv[ion_dinadv_id] = mech->ion_dinadv[ion_dinadv_id]+(dina-mech->ina[id])/0.001
                     mech->ion_dikdv[ion_dikdv_id] = mech->ion_dikdv[ion_dikdv_id]+(dik-mech->ik[id])/0.001
-                    mech->ion_ina[ion_ina_id] = mech->ion_ina[ion_ina_id]+mech->ina[id]
-                    mech->ion_ik[ion_ik_id] = mech->ion_ik[ion_ik_id]+mech->ik[id]
+                    mech->ion_ina[ion_ina_id] += mech->ina[id]
+                    mech->ion_ik[ion_ik_id] += mech->ik[id]
                     mech->vec_rhs[node_id] = mech->vec_rhs[node_id]-rhs
                     mech->vec_d[node_id] = mech->vec_d[node_id]+g
                 }
@@ -593,7 +593,7 @@ SCENARIO("Synapse: Derivative and breakpoint block llvm transformations",
                     }
                     mech->g[id] = (mech->g[id]-rhs)/0.001
                     mech->ion_dinadv[ion_dinadv_id] = mech->ion_dinadv[ion_dinadv_id]+(dina-mech->ina[id])/0.001*1.e2/mech->node_area[node_area_id]
-                    mech->ion_ina[ion_ina_id] = mech->ion_ina[ion_ina_id]+mech->ina[id]*(1.e2/mech->node_area[node_area_id])
+                    mech->ion_ina[ion_ina_id] += mech->ina[id]*(1.e2/mech->node_area[node_area_id])
                     mfactor = 1.e2/mech->node_area[node_area_id]
                     mech->g[id] = mech->g[id]*mfactor
                     rhs = rhs*mfactor