Diagnosing a near-match
You're at 95%. The function is almost right, and the diff shows one or two stubborn instructions. This is the most valuable moment in decompiling, because a near-match is a diagnostic: the shape of the gap names the bug. Here's how to read the common ones.
Symptom → likely cause
| What you see in the diff | Likely cause |
|---|---|
A stray extsb (or extsh) you don't want | Used char/short where the value is unsigned — try u8/u16 |
cmpw where target has cmplw (or cmpwi/cmplwi) | Wrong signedness on the compared value — flip signed/unsigned |
| Swapped registers (right ops, wrong homes) | Local declaration order differs — reorder your decls |
| One extra or missing instruction | Wrong idiom — e.g. & 0xff7f vs & ~0x80, or a manual mask vs a bitfield |
| Same instructions, reordered | Instruction scheduling — a decl/expression order or pragma issue |
Wrong offset in a load/store (8(r3) vs c(r3)) | Wrong struct layout — a field is the wrong size or missing |
Signedness shows up as a single opcode
This is the cleanest near-match there is. Compare two unsigned values that feed a branch and you get cmplw; compare them as signed int and you get cmpw:
; target (unsigned) ; your build (int)
cmplw r3,r4 cmpw r3,r4Identical everywhere else — one letter in one mnemonic. The fix is one letter in your C too: type the operand unsigned/u32 instead of int. The diff is telling you the type.
A stray extsb is a type leak
If your build inserts extsb r3,r3 (sign-extend byte) that the target doesn't have, you declared a byte-sized value as signed char when the original was u8. The compiler is dutifully sign-extending a value the original code treated as unsigned. Change the type, the extsb evaporates.
The method
1. Find the first diverging instruction (everything below it may be fallout). 2. Name the category from the table — is this a type, an order, or an idiom? 3. Form one hypothesis and make one change. 4. Re-diff. Did the first divergence move down? Then you fixed something real.
A near-match is not "stuck." It's a function handing you a labeled clue.