PR #25275: Propagate loop metadata in double_buffer_loop_unrolling.

Imported from GitHub PR #25275

Currently, this pass drops the `known_induction_variable` and `known_init_step` fields in `WhileLoopBackendConfig`. This change keeps them if they were present before and updates them with the updated value, just like `known_trip_count`. This is only done for double buffered loops, not for fully unrolled ones.
Copybara import of the project:

--
6bd4459 by Johannes Reifferscheid <jreiffers@nvidia.com>:

Keep loop metadata after loop unrolling.

--
369ad9b by Johannes Reifferscheid <jreiffers@nvidia.com>:

Fix init update and add tests.

Merging this change closes #25275

FUTURE_COPYBARA_INTEGRATE_REVIEW=#25275 from jreiffers:keep-metadata 369ad9b
PiperOrigin-RevId: 750112934

Author: jreiffers

xla/service/gpu/transforms/double_buffer_loop_unrolling.cc

@@ -519,6 +519,21 @@ absl::StatusOr<bool> DoubleBufferingUnroll(HloInstruction* while_instr,
 
   WhileLoopBackendConfig new_config;
   new_config.mutable_known_trip_count()->set_n(exact_trip_count / 2);
+
+  // Keep known induction variable metadata if it was present before.
+  if (config.has_known_induction_variable()) {
+    *new_config.mutable_known_induction_variable() =
+        config.known_induction_variable();
+  }
+
+  // Update the init/step metadata if it was present before.
+  if (config.has_known_init_step()) {
+    int64_t step = config.known_init_step().step();
+    new_config.mutable_known_init_step()->set_step(step * 2);
+    new_config.mutable_known_init_step()->set_init(
+        config.known_init_step().init() + (peel_one_iteration ? step : 0));
+  }
+
   TF_RETURN_IF_ERROR(while_instr->set_backend_config(new_config));
   return true;  // changed
 }

xla/service/gpu/transforms/double_buffer_loop_unrolling_test.cc

@@ -92,7 +92,9 @@ ENTRY main {
   param_0 = f32[] parameter(0)
   param_2 = s32[] constant(0)
   tuple = (f32[], s32[]) tuple(param_0, param_2)
-  ROOT while = (f32[], s32[]) while(tuple), condition=condition, body=body, backend_config={"known_trip_count":{"n":"10"}}
+  ROOT while = (f32[], s32[]) while(tuple), condition=condition, body=body,
+      backend_config={"known_trip_count":{"n":"10"},
+                      "known_induction_variable":{"tuple_index":"1"}}
 })";
 
   TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<xla::HloModule> module,
@@ -110,6 +112,7 @@ ENTRY main {
       WhileLoopBackendConfig config,
       while_instruction->backend_config<WhileLoopBackendConfig>());
   EXPECT_EQ(config.known_trip_count().n(), 5);
+  EXPECT_EQ(config.known_induction_variable().tuple_index(), 1);
   EXPECT_EQ(CountInstructions((*while_instruction->while_body()),
                               HloOpcode::kAllReduceStart),
             2);
@@ -1397,6 +1400,94 @@ ENTRY main {
   EXPECT_THAT(double_buffer.Run(module.get()), IsOkAndHolds(false));
 }
 
+TEST_F(GpuLoopDoubleBufferTransformerTest, UpdateInitStepOddTripCount) {
+  absl::string_view kModuleString = R"(
+    HloModule m
+      condition {
+      input_tuple = (s32[]) parameter(0)
+      iter = s32[] get-tuple-element(input_tuple), index=0
+      c12 = s32[] constant(12)
+      ROOT continue = pred[] compare(iter, c12), direction=LT
+    }
+
+    body {
+      input_tuple = (s32[]) parameter(0)
+      iter = s32[] get-tuple-element(input_tuple), index=0
+      c2 = s32[] constant(2)
+      next_iter = s32[] add(iter, c2)
+      ROOT output_tuple = (s32[]) tuple(next_iter)
+    }
+
+    ENTRY main {
+      c3 = s32[] constant(3)
+      tuple = (s32[]) tuple(c3)
+      // Values: 3, 5, 7, 9, 11
+      ROOT while = (s32[]) while(tuple), condition=condition, body=body,
+        backend_config={"known_trip_count":{"n":"5"},
+                        "known_init_step":{"init":"3","step":"2"}}
+    })";
+
+  TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<xla::HloModule> module,
+                          ParseAndReturnVerifiedModule(kModuleString));
+  DoubleBufferLoopUnrolling unroller(
+      DoubleBufferLoopUnrolling::UnrollStrategy::kDoubleBuffer);
+  TF_ASSERT_OK_AND_ASSIGN(bool changed, unroller.Run(module.get()));
+  EXPECT_TRUE(changed);
+
+  HloInstruction* while_instruction = hlo_query::GetFirstInstructionWithOpcode(
+      *module->entry_computation(), HloOpcode::kWhile);
+  TF_ASSERT_OK_AND_ASSIGN(
+      WhileLoopBackendConfig config,
+      while_instruction->backend_config<WhileLoopBackendConfig>());
+  EXPECT_EQ(config.known_trip_count().n(), 2);
+  EXPECT_EQ(config.known_init_step().init(), 5);
+  EXPECT_EQ(config.known_init_step().step(), 4);
+}
+
+TEST_F(GpuLoopDoubleBufferTransformerTest, UpdateInitStepEvenTripCount) {
+  absl::string_view kModuleString = R"(
+    HloModule m
+      condition {
+      input_tuple = (s32[]) parameter(0)
+      iter = s32[] get-tuple-element(input_tuple), index=0
+      c14 = s32[] constant(14)
+      ROOT continue = pred[] compare(iter, c14), direction=LT
+    }
+
+    body {
+      input_tuple = (s32[]) parameter(0)
+      iter = s32[] get-tuple-element(input_tuple), index=0
+      c2 = s32[] constant(2)
+      next_iter = s32[] add(iter, c2)
+      ROOT output_tuple = (s32[]) tuple(next_iter)
+    }
+
+    ENTRY main {
+      c3 = s32[] constant(3)
+      tuple = (s32[]) tuple(c3)
+      // Values: 3, 5, 7, 9, 11, 13
+      ROOT while = (s32[]) while(tuple), condition=condition, body=body,
+        backend_config={"known_trip_count":{"n":"6"},
+                        "known_init_step":{"init":"3","step":"2"}}
+    })";
+
+  TF_ASSERT_OK_AND_ASSIGN(std::unique_ptr<xla::HloModule> module,
+                          ParseAndReturnVerifiedModule(kModuleString));
+  DoubleBufferLoopUnrolling unroller(
+      DoubleBufferLoopUnrolling::UnrollStrategy::kDoubleBuffer);
+  TF_ASSERT_OK_AND_ASSIGN(bool changed, unroller.Run(module.get()));
+  EXPECT_TRUE(changed);
+
+  HloInstruction* while_instruction = hlo_query::GetFirstInstructionWithOpcode(
+      *module->entry_computation(), HloOpcode::kWhile);
+  TF_ASSERT_OK_AND_ASSIGN(
+      WhileLoopBackendConfig config,
+      while_instruction->backend_config<WhileLoopBackendConfig>());
+  EXPECT_EQ(config.known_trip_count().n(), 3);
+  EXPECT_EQ(config.known_init_step().init(), 3);
+  EXPECT_EQ(config.known_init_step().step(), 4);
+}
+
 }  // namespace
 }  // namespace gpu
 }  // namespace xla