b11365

ggml-cpu : fix soft_max_back wrong output when dst aliases src1 (#27096)

• ggml-cpu : fix soft_max_back wrong output when dst aliases src1

GGML_OP_SOFT_MAX_BACK is listed in ggml_op_can_inplace, so the graph
allocator may assign dst to alias either src0 (dy) or src1 (y).

The result was built in several steps:

ggml_vec_cpy_f32  (nc, dx, dy);
ggml_vec_acc1_f32 (nc, dx, -dot_y_dy);
ggml_vec_mul_f32  (nc, dx, dx, y);
ggml_vec_scale_f32(nc, dx, scale);

When dst aliases src1, the first step overwrites y and the third step
then reads the overwritten values, so the output is silently wrong.
Aliasing dst with src0 is unaffected. The CUDA kernel completes its
reduction before writing and is already safe.

Replace the sequence with a single fused loop that reads both sources
before writing, which is correct under either aliasing.

Add a regression test that marks dy as a graph output so the allocator
is forced to alias dst with y, asserts that the alias actually
happened, and compares against values computed on the host.

• cont : remove comment

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Co-authored-by: Georgi Gerganov ggerganov@gmail.com

Website:

• https://llama.app

Attestations:

• https://github.com/ggml-org/llama.cpp/attestations/52336742

macOS/iOS:

• macOS Apple Silicon (arm64)
• macOS Apple Silicon (arm64, KleidiAI enabled) DISABLED
• macOS Intel (x64)
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• Ubuntu x64 (CPU)
• Ubuntu arm64 (CPU)
• Ubuntu s390x (CPU)
• Ubuntu x64 (Vulkan)
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• Ubuntu x64 (CUDA 12) - CUDA 12.8 libraries
• Ubuntu x64 (CUDA 13) - CUDA 13.4 libraries
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• Windows x64 (CUDA 12) - CUDA 12.4 DLLs
• Windows x64 (CUDA 13) - CUDA 13.4 DLLs
• Windows arm64 (CUDA 13) - CUDA 13.4 DLLs
• Windows x64 (Vulkan)
• Windows x64 (OpenVINO)
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