This patch adds a convenient post-reload splitter for setting/updating
the highpart of a TImode variable, using the recently added
split_double_concat infrastructure.

For the new test case below:

__int128 foo(__int128 x, unsigned long long y)
{
  __int128 t = (__int128)y << 64;
  __int128 r = (x & ~0ull) | t;
  return r;
}

mainline GCC with -O2 currently generates:

foo:    movq    %rdi, %rcx
        xorl    %eax, %eax
        xorl    %edi, %edi
        orq     %rcx, %rax
        orq     %rdi, %rdx
        ret

with this patch, GCC instead now generates the much better:

foo:    movq    %rdi, %rcx
        movq    %rcx, %rax
        ret

[which interestingly shows the deeper (ABI) issue that I'm trying
to fix, this new define_insn_and_split being the next piece].

It turns out that the -m32 equivalent of this testcase, already
avoids using explict orl/xor instructions, as it gets optimized
(in combine) by a completely different path.  Given that this idiom
isn't seen in 32-bit code (and therefore this pattern wouldn't match),
and also that the shorter 32-bit AND bitmask is represented as a
CONST_INT rather than a CONST_WIDE_INT, this new define_insn_and_split
is implemented for just TARGET_64BIT rather than contort a "generic"
implementation using DWI mode iterators.

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?  Please let me know if you'd
prefer a different pattern name [insv seemed better than mov].


2023-01-05  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (any_or_plus): Move definition earlier.
	(*insvti_highpart_1): New define_insn_and_split to overwrite
	(insv) the highpart of a TImode register/memory.

gcc/testsuite/ChangeLog
	* gcc.target/i386/insvti_highpart-1.c: New test case.


Thanks in advance,
Roger
--