A common idiom is to create a DImode value from the "concat" of two SImode
values, using "(long long)hi << 32 | (long long)lo", where the operation
may be ior, xor or plus.  On x86, with -m32, the high and low parts of
a DImode register are actually different SImode registers (typically %edx
and %eax) so ideally this idiom should reduce to two move instructions
(or optimally, just clever register allocation).

Unfortunately, GCC currently performs the IOR operation above on -m32,
and worse allocates DImode registers (split to SImode register pairs)
for both the zero extended HI and LO values.

Hence, for test1 from the new test case below:

typedef int __v4si __attribute__ ((__vector_size__ (16)));
long long test1(__v4si v) {
  unsigned int loVal = (unsigned int)v[0];
  unsigned int hiVal = (unsigned int)v[1];
  return (long long)(loVal) | ((long long)(hiVal) << 32);
}

we currently generate (with -m32 -O2 -msse4.1):

test1:  subl    $28, %esp
        pextrd  $1, %xmm0, %eax
        pmovzxdq        %xmm0, %xmm1
        movq    %xmm1, 8(%esp)
        movl    %eax, %edx
        movl    8(%esp), %eax
        orl     12(%esp), %edx
        addl    $28, %esp
        orb     $0, %ah
        ret

with this patch we now generate:

test1:  pextrd  $1, %xmm0, %edx
        movd    %xmm0, %eax
        ret

The fix is to recognize and split the idiom (hi<<32)|zext(lo) prior
to register allocation on !TARGET_64BIT, simplifying this sequence to
"highpart(dst) = hi; lowpart(dst) = lo".

The one minor complication is that sse.md's define_insn for
*vec_extractv4si_0_zext_sse4 can sometimes interfere with this
optimization.  It turns out that on !TARGET_64BIT, the zero_extend:DI
following vec_select:SI isn't free, and this insn gets split back
into multiple instructions during later passes, but too late to
be optimized away by this patch/reload.  Hence the last hunk of
this patch is to restrict *vec_extractv4si_0_zext_sse4 to TARGET_64BIT.
Checking PR target/80286, where *vec_extractv4si_0_zext_sse4 was
first added, this seems reasonable (but this patch has been tested
both with and without this last change, if it's consider controversial).

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?


2021-12-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/103611
	* config/i386/i386.md (any_or_plus): New code iterator.
	(define_split): Split (HI<<32)|zext(LO) into piece-wise
	move instructions on !TARGET_64BIT.
	* config/i386/sse.md (*vec_extractv4si_0_zext_sse4):
	Restrict to TARGET_64BIT.

gcc/testsuite/ChangeLog
	PR target/103611
	* gcc.target/i386/pr103611-2.c: New test case.


Thanks in advance,
Roger
--