Swap cmp operands for effective match (#783)

tools/isledecomp/isledecomp/compare/asm/fixes.py

@@ -0,0 +1,106 @@
+from difflib import SequenceMatcher
+from typing import List
+
+
+ALLOWED_JUMP_SWAPS = (
+    ("ja", "jb"),
+    ("jae", "jbe"),
+    ("jb", "ja"),
+    ("jbe", "jae"),
+    ("jg", "jl"),
+    ("jge", "jle"),
+    ("jl", "jg"),
+    ("jle", "jge"),
+    ("je", "je"),
+    ("jne", "jne"),
+)
+
+
+def jump_swap_ok(a: str, b: str) -> bool:
+    """For the instructions a,b, are they both jump instructions
+    that are compatible with a swapped cmp operand order?"""
+    # Grab the mnemonic
+    (jmp_a, _, __) = a.partition(" ")
+    (jmp_b, _, __) = b.partition(" ")
+
+    return (jmp_a, jmp_b) in ALLOWED_JUMP_SWAPS
+
+
+def is_operand_swap(a: str, b: str) -> bool:
+    """This is a hack to avoid parsing the operands. It's not as simple as
+    breaking on the comma because templates or string literals interfere
+    with this. Instead we check:
+        1. Do both strings use the exact same set of characters?
+        2. If we do break on ', ', is the first token of each different?
+    2 is needed to catch an edge case like:
+        cmp eax, dword ptr [ecx + 0x1234]
+        cmp ecx, dword ptr [eax + 0x1234]
+    """
+    return a.partition(", ")[0] != b.partition(", ")[0] and sorted(a) == sorted(b)
+
+
+def can_cmp_swap(orig: List[str], recomp: List[str]) -> bool:
+    # Make sure we have 1 cmp and 1 jmp for both
+    if len(orig) != 2 or len(recomp) != 2:
+        return False
+
+    if not orig[0].startswith("cmp") or not recomp[0].startswith("cmp"):
+        return False
+
+    if not orig[1].startswith("j") or not recomp[1].startswith("j"):
+        return False
+
+    # Checking two things:
+    # Are the cmp operands flipped?
+    # Is the jump instruction compatible with a flip?
+    return is_operand_swap(orig[0], recomp[0]) and jump_swap_ok(orig[1], recomp[1])
+
+
+def patch_jump(a: str, b: str) -> str:
+    """For jump instructions a, b, return `(mnemonic_a) (operand_b)`.
+    The reason to do it this way (instead of just returning `a`) is that
+    the jump instructions might use different displacement offsets
+    or labels. If we just replace `b` with `a`, this diff would be
+    incorrectly eliminated."""
+    (mnemonic_a, _, __) = a.partition(" ")
+    (_, __, operand_b) = b.partition(" ")
+
+    return mnemonic_a + " " + operand_b
+
+
+def patch_cmp_swaps(
+    sm: SequenceMatcher, orig_asm: List[str], recomp_asm: List[str]
+) -> bool:
+    """Can we resolve the diffs between orig and recomp by patching
+    swapped cmp instructions?
+    For example:
+        cmp eax, ebx            cmp ebx, eax
+        je .label               je .label
+
+        cmp eax, ebx            cmp ebx, eax
+        ja .label               jb .label
+    """
+
+    # Copy the instructions so we can patch
+    # TODO: If we change our strategy to allow multiple rounds of patching,
+    # we should modify the recomp array directly.
+    new_asm = recomp_asm[::]
+
+    codes = sm.get_opcodes()
+
+    for code, i1, i2, j1, j2 in codes:
+        # To save us the trouble of finding "compatible" cmp instructions
+        # use the diff information we already have.
+        if code != "replace":
+            continue
+
+        # If the ranges in orig and recomp are not equal, use the shorter one
+        for i, j in zip(range(i1, i2), range(j1, j2)):
+            if can_cmp_swap(orig_asm[i : i + 2], recomp_asm[j : j + 2]):
+                # Patch cmp
+                new_asm[j] = orig_asm[i]
+
+                # Patch the jump if necessary
+                new_asm[j + 1] = patch_jump(orig_asm[i + 1], recomp_asm[j + 1])
+
+    return orig_asm == new_asm

tools/isledecomp/isledecomp/compare/core.py

@@ -11,6 +11,7 @@ from isledecomp.parser import DecompCodebase
 from isledecomp.dir import walk_source_dir
 from isledecomp.types import SymbolType
 from isledecomp.compare.asm import ParseAsm, can_resolve_register_differences
+from isledecomp.compare.asm.fixes import patch_cmp_swaps
 from .db import CompareDb, MatchInfo
 from .diff import combined_diff
 from .lines import LinesDb
@@ -470,7 +471,9 @@ class Compare:
         if ratio != 1.0:
             # Check whether we can resolve register swaps which are actually
             # perfect matches modulo compiler entropy.
-            is_effective_match = can_resolve_register_differences(orig_asm, recomp_asm)
+            is_effective_match = patch_cmp_swaps(
+                diff, orig_asm, recomp_asm
+            ) or can_resolve_register_differences(orig_asm, recomp_asm)
             unified_diff = combined_diff(
                 diff, orig_combined, recomp_combined, context_size=10
             )