This patch implements some additional zero-extension and sign-extension
related optimizations in simplify-rtx.cc.  The original motivation comes
from PR rtl-optimization/71775, where in comment #2 Andrew Pinski sees:

Failed to match this instruction:
(set (reg:DI 88 [ _1 ])
    (sign_extend:DI (subreg:SI (ctz:DI (reg/v:DI 86 [ x ])) 0)))

On many platforms the result of DImode CTZ is constrained to be a
small unsigned integer (between 0 and 64), hence the truncation to
32-bits (using a SUBREG) and the following sign extension back to
64-bits are effectively a no-op, so the above should ideally (often)
be simplified to "(set (reg:DI 88) (ctz:DI (reg/v:DI 86 [ x ]))".

To implement this, and some closely related transformations, we build
upon the existing val_signbit_known_clear_p predicate.  In the first
chunk, nonzero_bits knows that FFS and ABS can't leave the sign-bit
bit set, so the simplification of of ABS (ABS (x)) and ABS (FFS (x))
can itself be simplified.  The second transformation is that we can
canonicalized SIGN_EXTEND to ZERO_EXTEND (as in the PR 71775 case above)
when the operand's sign-bit is known to be clear.  The final two chunks
are for SIGN_EXTEND of a truncating SUBREG, and ZERO_EXTEND of a
truncating SUBREG respectively.  The nonzero_bits of a truncating
SUBREG pessimistically thinks that the upper bits may have an
arbitrary value (by taking the SUBREG), so we need look deeper at the
SUBREG's operand to confirm that the high bits are known to be zero.

Unfortunately, for PR rtl-optimization/71775, ctz:DI on x86_64 with
default architecture options is undefined at zero, so we can't be sure
the upper bits of reg:DI 88 will be sign extended (all zeros or all ones).
nonzero_bits knows this, so the above transformations don't trigger,
but the transformations themselves are perfectly valid for other
operations such as FFS, POPCOUNT and PARITY, and on other targets/-march
settings where CTZ is defined at zero.

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.  Testing with CSiBE shows these transformations
trigger on several source files (and with -Os reduces the size of the
code).  Ok for mainline?


2022-07-27  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * simplify_rtx.cc (simplify_unary_operation_1) <ABS>: Simplify
        test as both FFS and ABS result in nonzero_bits returning a
        mask that satisfies val_signbit_known_clear_p.
        <SIGN_EXTEND>: Canonicalize SIGN_EXTEND to ZERO_EXTEND when
        val_signbit_known_clear_p is true of the operand.
        Simplify sign extensions of SUBREG truncations of operands
        that are already suitably (zero) extended.
        <ZERO_EXTEND>: Simplify zero extensions of SUBREG truncations
        of operands that are already suitably zero extended.


Thanks in advance,
Roger
--