> -----Original Message-----
> From: Liu, Hongtao <[email protected]>
> Sent: Monday, September 14, 2026 5:45 PM
> To: H.J. Lu <[email protected]>; GCC Patches <[email protected]>;
> Uros Bizjak <[email protected]>
> Subject: RE: [PATCH v3] x86: Adjust cost for vector integer conversion without
> SSE4.1
>
> > + int scale = 0;
> > + if (scalar_p)
> > + {
> > + if (stmt_info
> > + && stmt_info->stmt
> > + && gimple_code (stmt_info->stmt) == GIMPLE_ASSIGN)
> > + {
> > + tree lhs = gimple_assign_lhs (stmt_info->stmt);
> > + if (TREE_CODE (lhs) == ARRAY_REF)
> > + {
> > + tree index = TREE_OPERAND (lhs, 1);
> > + /* Two more instructions are needed to access non-zero
> > + array element. */
> > + if (TREE_CODE (index) != INTEGER_CST
> > + || compare_tree_int (index, 0) != 0)
> > + scale = 2;
> > + }
> > + }
> > + }
>
> Not fully understand this part, why access non-zero array elements should be
> penalized with additional cost, is it testcase specific or it's general?
>
> > + else if (vector_int_conversion_p && !TARGET_SSE4_1)
> > + {
> > + /* Two instructions are needed to emulate vector integer
> > + conversions. One more instruction is needed to emulate
> > + 2-element vector conversion of QImode and HImode. */
> > + machine_mode rhs_mode = TYPE_MODE (rhs_type);
> > + rhs_mode = GET_MODE_INNER (rhs_mode);
> > + scale = (rhs_mode == SImode || GET_MODE_NUNITS (mode) != 2
> > + ? 2 : 3);
> > + }
>
> It's reasonable to increase cost for signed conversion w/o sse4.1, but for
> unsigned conversion it's not needed, also I don't think GET_MODE_NUNITS
> (mode) == 2 makes difference from 4/8, it looks tricky.
>
> For the original PR, you're trying to prevent vectorization for 2-elemenet
> reduction.
> Revising per-stmt cost to prevent 2-element integer vectorization may hurt
> others.
> Maybe we can add a specific heuristic to add additional cost for 2-element
> integer vectorization when vector width is less equal WORD, and this should
> solve the issue in the PR and don't regress others.
When VF is 2, vector width <= WORD, and there're cross-unit movement in the
vectorization(i.e vec_contruct/vec_deconstruct/vec_to_scalar/scalar_to_vec.)
Pentise the vectorization with extra cost
>
>
> Thanks,
> Hongtao
>
>
> --
> H.J.
