An analysis of backend UNSPECs reveals that two of the most common UNSPECs
across target backends are for copysign and bit reversal.  This patch
adds RTX codes for these expressions to allow their representation to
be standardized, and them to optimized by the middle-end RTL optimizers.

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{-32} with
no new failures.  Ok for mainline?


2023-05-06  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * doc/rtl.texi (bitreverse, copysign): Document new RTX codes.
        * rtl.def (BITREVERSE, COPYSIGN): Define new RTX codes.
        * simplify-rtx.cc (simplify_unary_operation_1): Optimize
        NOT (BITREVERSE x) as BITREVERSE (NOT x).
        Optimize POPCOUNT (BITREVERSE x) as POPCOUNT x.
        Optimize PARITY (BITREVERSE x) as PARITY x.
        Optimize BITREVERSE (BITREVERSE x) as x.
        (simplify_const_unary_operation) <case BITREVERSE>: Evaluate
        BITREVERSE of a constant integer at compile-time.
        (simplify_binary_operation_1) <case COPYSIGN>:  Optimize
        COPY_SIGN (x, x) as x.  Optimize COPYSIGN (x, C) as ABS x
        or NEG (ABS x) for constant C.  Optimize COPYSIGN (ABS x, y)
        and COPYSIGN (NEG x, y) as COPYSIGN (x, y).  Optimize
        COPYSIGN (x, ABS y) as ABS x.
        Optimize COPYSIGN (COPYSIGN (x, y), z) as COPYSIGN (x, z).
        Optimize COPYSIGN (x, COPYSIGN (y, z)) as COPYSIGN (x, z).
        (simplify_const_binary_operation): Evaluate COPYSIGN of constant
        arguments at compile-time.
        * wide-int.cc (wide_int_storage::bitreverse): Provide a
        wide_int implementation, based upon bswap implementation.
        * wide-int.g (wide_int_storage::bitreverse): Prototype here.


Thanks in advance,
Roger
--