Fix null pointer deref in KVCache::ToPtr() - missing .cache = this

[qiuku30]

Problem

Gemma 2B model crashes with a segfault during the prefill phase on a 6GB VM:

[ Reading prompt ] [ BEGIN PHASE: prefill ]
...............Segmentation fault (core dumped)

Root Cause

KVCache::ToPtr() constructs a KVCachePtr but omits the cache back-reference field, leaving it as the default nullptr. When attention.cc:183 calls cache->KOrVDefaultCols(), it dereferences a null pointer.

The bug was introduced during the tiled KV cache refactoring that added the cache field to KVCachePtr for accessing tiled_seq_len.

Debugging Process

1. Confirmed the crash is not purely memory-related (reproduced with seq_len=128, num_threads=1)
2. Confirmed the crash is not SIMD-specific (reproduced on both SSE2 and AVX3_ZEN4 code paths)
3. Compiled with -fsanitize=address and got the exact crash point:

==206353==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000
#0 gcpp::KVCache::KOrVDefaultCols() const /gemma/kv_cache.h:97:12
#1 gcpp::N_AVX3_ZEN4::ComputeQKV /gemma/attention.cc:183:44

Fix

Added the missing .cache = this field initialization in KVCache::ToPtr():

KVCachePtr ToPtr() {
    return KVCachePtr{
        .kv_cache = kv_cache,
        .k_cache = k_cache,
        .v_cache = v_cache,
        .cache = this,   // ← was missing
    };
}
Verification
The fix was tested on a 6GB VM running Gemma 2B Instruct (SFP-compressed):


./gemma --weights 2.0-2b-it-sfp.sbs --tokenizer tokenizer.spm \
  --prompt "Hello" --max_generated_tokens 30

[ Reading prompt ] .........
Hello! My name is Gemma. How can I help you today?

[ Timing info ] Prefill: 583 ms for 15 prompt tokens
[ Timing info ] Generate: 5653 ms for 19 tokens (3.36 tokens/sec)

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[jan-wassenberg]

Hi, thanks for pointing this out. We're unable to merge to the main branch, please retarget to the dev branch and make sure the PR diff only includes the desired change.