analyze-jitc-log

name	analyze-jitc-log
description	Analyze jitc.log to find JIT compilation issues — fragment gaps, branch overflow, bad codegen
allowed-tools	Bash, Read, Grep
argument-hint	["log-file"]

Analyze jitc.log

The jitc.log file contains all translated PPC→AArch64 code. Each entry shows:

• PPC instruction offset + disassembly
• Native AArch64 addresses + instruction encoding + disassembly

Steps

1. Read the END of the log first — the most recently compiled code is at the bottom:

   tail -100 ${ARGUMENTS:-jitc.log}
   

2. Check for fragment gaps — look for large jumps in native addresses between consecutive instructions. Two consecutive AArch64 instructions should be 4 bytes apart. If the gap is >512 bytes, a new fragment was allocated:

   python3 -c "
   import re, sys
   lines = open('${ARGUMENTS:-jitc.log}').readlines()
   prev_addr = None
   for line in lines:
       m = re.match(r'\s+([0-9a-f]+)\s+[0-9a-f]+\s+', line)
       if m and len(m.group(1)) > 6:  # native address (long hex)
           addr = int(m.group(1), 16)
           if prev_addr and addr - prev_addr > 512:
               print(f'FRAGMENT GAP: {prev_addr:x} -> {addr:x} (distance: {addr - prev_addr} bytes)')
           prev_addr = addr
   "
   

3. Check for conditional branch overflow risk — any B.cc (0x54xxxxxx) instruction followed by a fragment gap means the conditional branch might not reach its target. B.cc range is ±1MB (±0x100000):

   • Look for 54 prefix instructions near fragment boundaries
   • Check if the gap exceeds 1MB

4. Check for unresolved fixups — 14000000 is B #0 (branch to self), which means a fixup was never resolved:

   grep '14000000' ${ARGUMENTS:-jitc.log}
   

5. Check for GEN_INTERPRET overhead — look for the pattern: store current_opc, compute pc from base+offset, store pc, store npc, mov x0 cpu, blr interpreter. Count how many bytes each PPC instruction takes in native code.

6. Identify the PPC page being compiled — the first column shows PPC offsets within the page. The current_code_base (stored at CPU offset 904) gives the page base EA.

Key addresses

• CPU state offsets: pc=800, npc=804, current_opc=808, current_code_base=904
• X19 = JITC pointer, X20 = CPU state pointer
• Fragment size = 512 bytes
• Translation cache starts around 0x1050xxxxx or 0x1014xxxxx

Common issues

• Fragment gap + conditional branch = potential range overflow (root cause of prior kernel stall)
• Unresolved fixup (14000000) = compiler bug, will branch to self infinitely
• emitBLR crossing fragment = the MOVZ+MOVK+BLR sequence split across fragments (should be prevented by emitAssure)