[Bug tree-optimization/116142] vec_widen_smult_{odd,even}_M ineffective for a simple widening dot product (vect_used_by_reduction is not set?)

[xry111 at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116142

--- Comment #9 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
In the following call stack:

#0  single_imm_use (var=0x7ffff564a3a8, use_p=0x7fffffffc9d8, 
    stmt=0x7fffffffc9e0) at ../../gcc/gcc/ssa-iterators.h:427
#1  0x0000000001aabcde in vec_info::lookup_single_use (this=0x33a1b80, 
    lhs=0x7ffff564a3a8) at ../../gcc/gcc/tree-vectorizer.cc:581
#2  0x0000000001a1a40a in supportable_widening_operation (vinfo=0x33a1b80, 
    code=..., stmt_info=0x3482b50, vectype_out=0x7ffff595a1f8, 
    vectype_in=0x7ffff595a000, code1=0x7fffffffcb90, code2=0x7fffffffcb94, 
    multi_step_cvt=0x7fffffffcba0, interm_types=0x7fffffffcc48)
    at ../../gcc/gcc/tree-vect-stmts.cc:14196
#3  0x00000000019f7c2d in vectorizable_conversion (vinfo=0x33a1b80, 
    stmt_info=0x3482b50, gsi=0x0, vec_stmt=0x0, slp_node=0x33b4cc8, 
    cost_vec=0x7fffffffd190) at ../../gcc/gcc/tree-vect-stmts.cc:5438
#4  0x0000000001a16ecc in vect_analyze_stmt (vinfo=0x33a1b80, 
    stmt_info=0x3482b50, need_to_vectorize=0x7fffffffcf73, node=0x33b4cc8, 
    node_instance=0x32fb6e0, cost_vec=0x7fffffffd190)
    at ../../gcc/gcc/tree-vect-stmts.cc:13291

"ptr == ptr->next" is satisfied (the comment says it means "there aren't any
uses whatsoever" but I don't really understand it: I knows almost nothing about
tree vectorization) and single_imm_use directly returns false.