i386: Introduce V2QImode vectorized arithmetic [PR103861]

i386: Introduce V2QImode vectorized arithmetic [PR103861]

This patch adds basic V2QImode infrastructure and V2QImode arithmetic
operations (plus, minus and neg).  The patched compiler can emit SSE
vectorized QImode operations (e.g. PADDB) with partial QImode vector,
and also synthesized double HI/LO QImode operations with integer registers.

The testcase:

typedef char __v2qi __attribute__ ((__vector_size__ (2)));
__v2qi plus  (__v2qi a, __v2qi b) { return a + b; };

compiles with -O2 to:

        movl    %edi, %edx
        movl    %esi, %eax
        addb    %sil, %dl
        addb    %ah, %dh
        movl    %edx, %eax
        ret

which is much better than what the unpatched compiler produces:

        movl    %edi, %eax
        movl    %esi, %edx
        xorl    %ecx, %ecx
        movb    %dil, %cl
        movsbl  %dh, %edx
        movsbl  %ah, %eax
        addl    %edx, %eax
        addb    %sil, %cl
        movb    %al, %ch
        movl    %ecx, %eax
        ret

The V2QImode vectorization does not require vector registers, so it can
be enabled by default also for 32-bit targets without SSE.

The patch also enables vectorized V2QImode sign/zero extends.

2021-12-30  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:

	PR target/103861
	* config/i386/i386.h (VALID_SSE2_REG_MODE): Add V2QImode.
	(VALID_INT_MODE_P): Ditto.
	* config/i386/i386.c (ix86_secondary_reload): Handle
	V2QImode reloads from SSE register to memory.
	(vector_mode_supported_p): Always return true for V2QImode.
	* config/i386/i386.md (*subqi_ext<mode>_2): New insn pattern.
	(*negqi_ext<mode>_2): Ditto.
	* config/i386/mmx.md (movv2qi): New expander.
	(movmisalignv2qi): Ditto.
	(*movv2qi_internal): New insn pattern.
	(*pushv2qi2): Ditto.
	(negv2qi2 and splitters): Ditto.
	(<plusminus:insn>v2qi3 and splitters): Ditto.

gcc/testsuite/ChangeLog:

	PR target/103861
	* gcc.dg/store_merging_18.c (dg-options): Add -fno-tree-vectorize.
	* gcc.dg/store_merging_29.c (dg-options): Ditto.
	* gcc.target/i386/pr103861.c: New test.
	* gcc.target/i386/pr92658-avx512vl.c (dg-final):
	Remove vpmovqb scan-assembler xfail.
	* gcc.target/i386/pr92658-sse4.c (dg-final):
	Remove pmovzxbq scan-assembler xfail.
	* gcc.target/i386/pr92658-sse4-2.c (dg-final):
	Remove pmovsxbq scan-assembler xfail.
	* gcc.target/i386/warn-vect-op-2.c (dg-warning): Adjust warnings.