Make unroll_config optional in WhileLoopUnroller::GetUnrollableLoops.

This change allows more loops to be considered when using GetUnrollableLoops, e.g., in while_initializer_removal pass.

PiperOrigin-RevId: 667782425

xla/service/hlo_unstacker.cc

@@ -110,7 +110,7 @@ struct UnstackerMetadata {
       VLOG(3) << "Prepared module: " << module->name() << " for unstacking.";
     }
     std::vector<std::pair<HloInstruction*, WhileLoopConfig>> loops =
-        WhileLoopUnroller::GetUnrollableLoops(module, {});
+        WhileLoopUnroller::GetUnrollableLoops(module, {}, std::nullopt);
     for (const auto& [instr, while_loop_config] : loops) {
       metadata.unrollable_loop_bodies[instr->while_body()] = while_loop_config;
       metadata.bodies[instr->while_body()] = instr;

xla/service/scan_loop_accumulator_input_unification.cc

@@ -270,7 +270,8 @@ absl::StatusOr<bool> ScanLoopAccumulatorInputUnification::Run(
   // accumulators and inputs that are by definition updated and read fully via
   // dynamic-update-slice and dynamic-sliced within a loop.
   std::vector<std::pair<HloInstruction*, WhileLoopConfig>> unrollable_loops =
-      WhileLoopUnroller::GetUnrollableLoops(module, execution_threads);
+      WhileLoopUnroller::GetUnrollableLoops(module, execution_threads,
+                                            std::nullopt);
 
   // TODO(b/337883537): We might want to simplify compare instructions before
   // this. It helps us identify more inputs and accumulators.

xla/service/while_loop_unroller.cc

@@ -665,7 +665,7 @@ std::optional<int64_t> MatchShapeCoveringDynamicIndexInstruction(
 WhileLoopUnroller::GetUnrollableLoops(
     HloModule* module,
     const absl::flat_hash_set<absl::string_view>& execution_threads,
-    const UnrollConfig& unroll_config) {
+    std::optional<UnrollConfig> unroll_config) {
   // Processing the while loops in the reverse topological order. If the body
   // of while loop A calls while loop B, B comes before A.
   std::vector<HloInstruction*> all_while_ops;
@@ -676,13 +676,16 @@ WhileLoopUnroller::GetUnrollableLoops(
   std::vector<std::pair<HloInstruction*, WhileLoopConfig>> while_loop_configs;
   for (HloInstruction* instr : all_while_ops) {
     std::optional<WhileLoopConfig> config = IsLoopUnrollable(instr);
-    if (config.has_value()) {
-      if (!InitialFeasibilityCheck(instr, config.value(), unroll_config)) {
-        VLOG(3) << "Initial feasibility check failed for " << instr->name();
-        continue;
-      }
-      while_loop_configs.emplace_back(instr, config.value());
+    if (!config.has_value()) {
+      continue;
+    }
+    if (unroll_config.has_value() &&
+        !InitialFeasibilityCheck(instr, config.value(),
+                                 unroll_config.value())) {
+      VLOG(3) << "Initial feasibility check failed for " << instr->name();
+      continue;
     }
+    while_loop_configs.emplace_back(instr, config.value());
   }
   return while_loop_configs;
 }

xla/service/while_loop_unroller.h

@@ -49,7 +49,7 @@ struct WhileLoopConfig {
   int64_t induction_var_idx;
 };
 
-// Config for unrollable while loops.
+// Result for unrolled while loops.
 struct UnrollResult {
   // Whether it's unrolled.
   bool unrolled = false;
@@ -120,12 +120,14 @@ class WhileLoopUnroller : public HloModulePass {
   static std::optional<WhileLoopConfig> IsLoopUnrollable(
       HloInstruction* while_op);
 
-  // Returns the list of unrollable loops in the given module
+  // Returns the list of unrollable loops in the given module. If
+  // `unroll_config` is provided, it will be used to check feasibility according
+  // to InitialFeasibilityCheck method
   static std::vector<std::pair<HloInstruction*, WhileLoopConfig>>
   GetUnrollableLoops(
       HloModule* module,
       const absl::flat_hash_set<absl::string_view>& execution_threads,
-      const UnrollConfig& unroll_config = UnrollConfig());
+      std::optional<UnrollConfig> unroll_config);
 
   // Unrolls the given while loop with the default behaviour set to full unroll.
   // If wrap_in_trivial_loop is set, the unrolled body of the loop will be

xla/service/while_loop_unroller_test.cc

@@ -496,7 +496,7 @@ TEST_F(WhileLoopUnrollerTest, GetUnrollableLoops) {
                           ParseAndReturnVerifiedModule(hlo_string));
 
   auto unrollable_loops =
-      WhileLoopUnroller::GetUnrollableLoops(module.get(), {});
+      WhileLoopUnroller::GetUnrollableLoops(module.get(), {}, std::nullopt);
   // Only while1 and while2 are unrollable
   EXPECT_EQ(unrollable_loops.size(), 2);
 }