llvm(py): introduce BuildCtx + trace hub; refactor if-merge prepass + PHI wiring into module; unify logs; ctx-enable compare/ret/call/boxcall/externcall/typeop/newbox/safepoint; curated smoke option for if-merge; README updates; keep behavior stable

src/llvm_py/prepass/loops.py

@@ -0,0 +1,106 @@
+"""
+Loop prepass utilities
+Detect simple while-shaped loops in MIR(JSON) and return a lowering plan.
+"""
+
+from typing import Dict, List, Any, Optional
+from cfg.utils import build_preds_succs
+
+def detect_simple_while(block_by_id: Dict[int, Dict[str, Any]]) -> Optional[Dict[str, Any]]:
+    """Detect a simple loop pattern: header(branch cond → then/else),
+    a latch that jumps back to header reachable from then, and exit on else.
+    Returns a plan dict or None.
+    """
+    # Build succ and pred maps from JSON quickly
+    preds, succs = build_preds_succs(block_by_id)
+    # Find a header with a branch terminator and else leading to a ret (direct)
+    for b in block_by_id.values():
+        bid = int(b.get('id', 0))
+        term = None
+        if b.get('instructions'):
+            last = b.get('instructions')[-1]
+            if last.get('op') in ('jump','branch','ret'):
+                term = last
+        if term is None and 'terminator' in b:
+            t = b['terminator']
+            if t and t.get('op') in ('jump','branch','ret'):
+                term = t
+        if not term or term.get('op') != 'branch':
+            continue
+        then_bid = int(term.get('then'))
+        else_bid = int(term.get('else'))
+        cond_vid = int(term.get('cond')) if term.get('cond') is not None else None
+        if cond_vid is None:
+            continue
+        # Quick check: else block ends with ret
+        else_blk = block_by_id.get(else_bid)
+        has_ret = False
+        if else_blk is not None:
+            insts = else_blk.get('instructions', [])
+            if insts and insts[-1].get('op') == 'ret':
+                has_ret = True
+            elif else_blk.get('terminator', {}).get('op') == 'ret':
+                has_ret = True
+        if not has_ret:
+            continue
+        # Find a latch that jumps back to header reachable from then
+        latch = None
+        visited = set()
+        stack = [then_bid]
+        while stack:
+            cur = stack.pop()
+            if cur in visited:
+                continue
+            visited.add(cur)
+            cur_blk = block_by_id.get(cur)
+            if cur_blk is None:
+                continue
+            for inst in cur_blk.get('instructions', []) or []:
+                if inst.get('op') == 'jump' and int(inst.get('target')) == bid:
+                    latch = cur
+                    break
+            if latch is not None:
+                break
+            for nx in succs.get(cur, []) or []:
+                if nx not in visited and nx != else_bid:
+                    stack.append(nx)
+        if latch is None:
+            continue
+        # Compose body_insts: collect insts along then-branch region up to latch (inclusive),
+        # excluding any final jump back to header to prevent double edges.
+        collect_order: List[int] = []
+        visited2 = set()
+        stack2 = [then_bid]
+        while stack2:
+            cur = stack2.pop()
+            if cur in visited2 or cur == bid or cur == else_bid:
+                continue
+            visited2.add(cur)
+            collect_order.append(cur)
+            if cur == latch:
+                continue
+            for nx in succs.get(cur, []) or []:
+                if nx not in visited2 and nx != else_bid:
+                    stack2.append(nx)
+        body_insts: List[Dict[str, Any]] = []
+        for bbid in collect_order:
+            blk = block_by_id.get(bbid)
+            if blk is None:
+                continue
+            for inst in blk.get('instructions', []) or []:
+                if inst.get('op') == 'jump' and int(inst.get('target', -1)) == bid:
+                    continue
+                body_insts.append(inst)
+        skip_blocks = set(collect_order)
+        skip_blocks.add(bid)
+        return {
+            'header': bid,
+            'then': then_bid,
+            'else': else_bid,
+            'latch': latch,
+            'exit': else_bid,
+            'cond': cond_vid,
+            'body_insts': body_insts,
+            'skip_blocks': list(skip_blocks),
+        }
+    return None