ggml-org/llama.cpp b11365

on GitHub

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

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