cprop1 pass does not consider paradoxical subreg and for (insn 22) claims
that it equals 8 elements of HImodeby setting REG_EQUAL note:

(insn 21 19 22 4 (set (reg:V4QI 98)
        (mem/u/c:V4QI (symbol_ref/u:DI ("*.LC1") [flags 0x2]) [0  S4
A32])) "pr110206.c":12:42 1530 {*movv4qi_internal}
     (expr_list:REG_EQUAL (const_vector:V4QI [
                (const_int -52 [0xffffffffffffffcc]) repeated x4
            ])
        (nil)))
(insn 22 21 23 4 (set (reg:V8HI 100)
        (zero_extend:V8HI (vec_select:V8QI (subreg:V16QI (reg:V4QI 98) 0)
                (parallel [
                        (const_int 0 [0])
                        (const_int 1 [0x1])
                        (const_int 2 [0x2])
                        (const_int 3 [0x3])
                        (const_int 4 [0x4])
                        (const_int 5 [0x5])
                        (const_int 6 [0x6])
                        (const_int 7 [0x7])
                    ])))) "pr110206.c":12:42 7471 {sse4_1_zero_extendv8qiv8hi2}
     (expr_list:REG_EQUAL (const_vector:V8HI [
                (const_int 204 [0xcc]) repeated x8
            ])
        (expr_list:REG_DEAD (reg:V4QI 98)
            (nil))))

We rely on the "undefined" vals to have a specific value (from the earlier
REG_EQUAL note) but actual code generation doesn't ensure this (it doesn't
need to).  That said, the issue isn't the constant folding per-se but that
we do not actually constant fold but register an equality that doesn't hold.

    PR target/110206

gcc/ChangeLog:

    * fwprop.cc (contains_paradoxical_subreg_p): Move to ...
    * rtlanal.cc (contains_paradoxical_subreg_p): ... here.
    * rtlanal.h (contains_paradoxical_subreg_p): Add prototype.
    * cprop.cc (try_replace_reg): Do not set REG_EQUAL note
    when the original source contains a paradoxical subreg.

gcc/testsuite/ChangeLog:

    * gcc.dg/torture/pr110206.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

OK for mainline and backports?

Uros.