rules/BRIEF.md

# Rule-mining brief: new CUDA mutation operators

## Context

We run mutation testing on CUDA kernels to measure how strong a benchmark's
correctness check is. Mutants are produced by DETERMINISTIC rewrite rules
applied to correct kernel source (see ../mutator.py for the 35 current rules
and the Rule dataclass). Each rule is (name, category, regex pattern,
replacement template). Every match site yields one mutant.

## What makes a rule VALUABLE (this is the objective)

A mutant is "high value" when it (a) compiles, (b) SURVIVES the benchmark's
original test inputs — which are aligned shapes, torch.rand[0,1) values,
compared with allclose(atol=rtol=1e-2) — and (c) IS killable by some targeted
input (misaligned shape, signed values, large magnitude, structured spikes).

Measured value rates of existing rules on our pool:
  ceil2floor 100%, const0to1 62%, sync2syncwarp 40%, loop-bound-minus1 39%,
  loop-start-1 35%, sync-remove 36%, lt2le 21%, store-fp16 24%,
  mul2plus 15%, mul2div 12%   <- good
  plus2minus 3%, guard-drop-ternary 0%, expf-fast 0%, smem-index-off1 0%,
  fmin2fmax 0%, negate-store 0%, fdividef-fast 0%   <- bad, being retired

Pattern: rules that break BOUNDARY handling, SYNCHRONIZATION, or PRECISION
produce faults invisible under aligned shapes and small positive values.
Rules that break gross semantics (a+b -> a-b) are caught immediately by any
input and are near-worthless.

## Known expressiveness gaps (highest priority)

Our regex rules CANNOT express these, yet they are the most valuable faults:

1. Identity substitution: replacing a call with its own argument, e.g.
   `fmaxf(val, 0.0f)` -> `val` (removes a ReLU; invisible on positive inputs).
   Generalize: any activation/clamp/saturation wrapper removal.
2. Numerical-stability removal: dropping the max-subtraction in softmax-like
   code, dropping epsilon in a denominator, dropping Kahan compensation.
3. Partial-tail faults: processing only a prefix of a reduction, off-by-one
   in the LAST iteration only, dropping the remainder loop after a vectorized
   main loop.
4. Accumulator staging: using the shared-memory value before the barrier,
   re-initializing an accumulator inside the loop, double-counting an element.
5. Index arithmetic that is wrong only at edges: swapping row/col strides in
   ONE of several index expressions, using the wrong bound variable (M vs N),
   using blockDim where gridDim is meant.

## Your task

Write NEW rules into a fresh file: rules/mined_rules.py

Format (import-safe, no side effects):

```python
from mutator import Rule   # dataclass(name, category, pattern, replacement, note)

MINED_RULES = [
    Rule("name", "category", r"regex", r"replacement", "one-line rationale"),
    ...
]
```

Requirements:
- 25+ new rules, none duplicating an existing rule in ../mutator.py.
- Categories: boundary | sync | precision | indexing | arithmetic | semantic.
- Patterns must be robust regexes that match the CUDA style used in
  ../problems_batch2.py and ../substrates/accepted/*.py (read several of them
  — that is the corpus these rules must fire on).
- Prefer rules targeting the five gap families above; a regex CAN express
  identity substitution when the call and its first argument are both simple
  (e.g. r"fmaxf\((\w+), 0\.0f\)" -> r"\1"). Write several such families.
- Every rule must produce COMPILABLE code (mind types and parens).
- Avoid rules whose mutants would be caught by any input (gross semantics).
- Add a short comment block at the top summarizing your reasoning.

Verify before finishing: run
  python3 -c "import sys; sys.path.insert(0,'.'); from rules.mined_rules import MINED_RULES; print(len(MINED_RULES))"
from the experiment directory, and also verify each pattern compiles as a
regex. Do not modify any existing file.