Simple O(1) inlining rewrite (#552)

The inline pass (as part of a `Walk` rewrite) has O(n^2) time complexity
which is a performance issue.

An extra factor of `n` is coming from `inline.py` where each block is
split in two, and the inline region is inserted in the middle. Splitting
blocks in two comes with the extra O(n) factor:
```python
after_block = ir.Block()
stmt = call_like.next_stmt
while stmt is not None:
    stmt.detach()
    after_block.stmts.append(stmt)
    stmt = call_like.next_stmt
```

This PR introduces a partial workaround. Simple regions with just a
single block are inlined by inserting all of their statements directly.
Since statements form a linked list, this is O(1).

For my test case, I observe that this fix reduces runtime and brings the
time complexity of the inline pass back to O(n).

<img width="489" height="358" alt="image"
src="https://github.com/user-attachments/assets/3b11560f-670f-45d3-84b0-0959693f47b4"
/>

However, we should refactor the inline pass to scale linearly even in
the general case.

---------

Co-authored-by: Casey Duckering <[email protected]>

Author: rafaelha

src/kirin/rewrite/inline.py

@@ -1,4 +1,4 @@
-from typing import Callable
+from typing import Callable, cast
 from dataclasses import dataclass
 
 from kirin import ir
@@ -68,6 +68,121 @@ def inline_call_like(
             args (tuple[ir.SSAValue, ...]): the arguments of the call (first one is the callee)
             region (ir.Region): the region of the callee
         """
+        if not call_like.parent_block:
+            return
+
+        if not call_like.parent_region:
+            return
+
+        # NOTE: we cannot change region because it may be used elsewhere
+        inline_region: ir.Region = region.clone()
+
+        # Preserve source information by attributing inlined code to the call site
+        if call_like.source is not None:
+            for block in inline_region.blocks:
+                if block.source is None:
+                    block.source = call_like.source
+                for stmt in block.stmts:
+                    if stmt.source is None:
+                        stmt.source = call_like.source
+
+        if self._can_use_simple_inline(inline_region):
+            return self._inline_simple(call_like, args, inline_region.blocks[0])
+
+        return self._inline_complex(call_like, args, inline_region)
+
+    def _can_use_simple_inline(self, inline_region: ir.Region) -> bool:
+        """Check if we can use the fast path for simple single-block inlining.
+
+        Args:
+            inline_region: The cloned region to be inlined
+
+        Returns:
+            True if simple inline is possible (single block with simple return)
+        """
+        if len(inline_region.blocks) != 1:
+            return False
+
+        block = inline_region.blocks[0]
+
+        # Last statement must be a simple return
+        if not isinstance(block.last_stmt, func.Return):
+            return False
+
+        return True
+
+    def _inline_simple(
+        self,
+        call_like: ir.Statement,
+        args: tuple[ir.SSAValue, ...],
+        func_block: ir.Block,
+    ):
+        """Fast path: inline single-block function by splicing statements.
+
+        For simple functions with no control flow, we just clone the function's
+        statements and insert them before the call site.
+        No new blocks are created, no statement parent updates are needed.
+
+        Complexity: O(k) where k = number of statements in function (typically small)
+
+        Args:
+            call_like: The call statement to replace
+            args: Arguments to the call (first is callee, rest are parameters)
+            func_block: The single block from the cloned function region
+        """
+        ssa_map: dict[ir.SSAValue, ir.SSAValue] = {}
+        for func_arg, call_arg in zip(func_block.args, args):
+            ssa_map[func_arg] = call_arg
+            if func_arg.name and call_arg.name is None:
+                call_arg.name = func_arg.name
+
+        for stmt in func_block.stmts:
+            if isinstance(stmt, func.Return):
+                return_value = ssa_map.get(stmt.value, stmt.value)
+
+                if call_like.results:
+                    for call_result in call_like.results:
+                        call_result.replace_by(return_value)
+
+                # Don't insert the return statement itself
+                break
+
+            new_stmt = stmt.from_stmt(
+                stmt,
+                args=[ssa_map.get(arg, arg) for arg in stmt.args],
+                regions=[r.clone(ssa_map) for r in stmt.regions],
+                successors=stmt.successors,  # successors are empty for simple stmts
+            )
+
+            new_stmt.insert_before(call_like)
+
+            # Update SSA mapping for newly created results
+            for old_result, new_result in zip(stmt.results, new_stmt.results):
+                ssa_map[old_result] = new_result
+                if old_result.name:
+                    new_result.name = old_result.name
+
+        call_like.delete()
+
+    def _inline_complex(
+        self,
+        call_like: ir.Statement,
+        args: tuple[ir.SSAValue, ...],
+        inline_region: ir.Region,
+    ):
+        """Inline multi-block function with control flow.
+
+        This handles the general case where the function has multiple blocks
+
+        Complexity: O(n+k) where n = statements after call site (due to moving them)
+            and k = number of statements in function.
+
+        Args:
+            call_like: The call statement to replace
+            args: Arguments to the call
+            inline_region: The cloned function region to inline
+        """
+
         # <stmt>
         # <stmt>
         # <br (a, b, c)>
@@ -89,26 +204,8 @@ def inline_call_like(
         #     split the current block into two, and replace the return with
         #     the branch instruction
         # 4. remove the call
-        if not call_like.parent_block:
-            return
-
-        if not call_like.parent_region:
-            return
-
-        # NOTE: we cannot change region because it may be used elsewhere
-        inline_region: ir.Region = region.clone()
-
-        # Preserve source information by attributing inlined code to the call site
-        if call_like.source is not None:
-            for block in inline_region.blocks:
-                if block.source is None:
-                    block.source = call_like.source
-                for stmt in block.stmts:
-                    if stmt.source is None:
-                        stmt.source = call_like.source
-
-        parent_block: ir.Block = call_like.parent_block
-        parent_region: ir.Region = call_like.parent_region
+        parent_block: ir.Block = cast(ir.Block, call_like.parent_block)
+        parent_region: ir.Region = cast(ir.Region, call_like.parent_region)
 
         # wrap what's after invoke into a block
         after_block = ir.Block()
@@ -150,4 +247,3 @@ def inline_call_like(
                 successor=entry_block,
             ).insert_before(call_like)
         call_like.delete()
-        return