This patch improves the cost/gain calculation used during the i386 backend's
SImode/DImode scalar-to-vector (STV) conversion pass.  The current code
handles loads and stores, but doesn't consider that converting other
scalar operations with a memory destination, requires an explicit load
before and an explicit store after the vector equivalent.

To ease the review, the significant change looks like:

         /* For operations on memory operands, include the overhead
            of explicit load and store instructions.  */
         if (MEM_P (dst))
           igain += !optimize_insn_for_size_p ()
                    ? (m * (ix86_cost->int_load[2]
                            + ix86_cost->int_store[2])
                       - (ix86_cost->sse_load[sse_cost_idx] +
                          ix86_cost->sse_store[sse_cost_idx]))
                    : -COSTS_N_BYTES (8);

however the patch itself is complicated by a change in indentation
which leads to a number of lines with only whitespace changes.
For architectures where integer load/store costs are the same as
vector load/store costs, there should be no change without -Os/-Oz.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?


2024-01-06  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR target/113231
        * config/i386/i386-features.cc (compute_convert_gain): Include
        the overhead of explicit load and store (movd) instructions when
        converting non-store scalar operations with memory destinations.

gcc/testsuite/ChangeLog
        PR target/113231
        * gcc.target/i386/pr113231.c: New test case.


Thanks again,
Roger
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