[Patch, Vectorizer, SVE] fmin/fmax builtin reduction support

[Alejandro Martinez Vicente]

Hi all,
 
Loops that use the fmin/fmax builtins can be vectorized even without
-ffast-math using SVE's FMINNM/FMAXNM instructions. This is an example:
 
double
f (double *x, int n)
{
  double res = 100.0;
  for (int i = 0; i < n; ++i)
    res = __builtin_fmin (res, x[i]);
  return res;
}

Before this patch, the compiler would generate this code (-march=armv8.2-a+sve
-O2 -ftree-vectorize):

0000000000000000 <f>:
   0:   7100003f        cmp     w1, #0x0
   4:   5400018d        b.le    34 <f+0x34>
   8:   51000422        sub     w2, w1, #0x1
   c:   91002003        add     x3, x0, #0x8
  10:   d2e80b21        mov     x1, #0x4059000000000000
  14:   9e670020        fmov    d0, x1
  18:   8b224c62        add     x2, x3, w2, uxtw #3
  1c:   d503201f        nop
  20:   fc408401        ldr     d1, [x0],#8
  24:   1e617800        fminnm  d0, d0, d1
  28:   eb02001f        cmp     x0, x2
  2c:   54ffffa1        b.ne    20 <f+0x20>
  30:   d65f03c0        ret
  34:   d2e80b20        mov     x0, #0x4059000000000000
  38:   9e670000        fmov    d0, x0
  3c:   d65f03c0        ret

After this patch, this is the code that gets generated:

0000000000000000 <f>:
   0:   7100003f        cmp     w1, #0x0
   4:   5400020d        b.le    44 <f+0x44>
   8:   d2800002        mov     x2, #0x0
   c:   25d8e3e0        ptrue   p0.d
  10:   93407c21        sxtw    x1, w1
  14:   90000003        adrp    x3, 0 <f>
  18:   25804001        mov     p1.b, p0.b
  1c:   91000063        add     x3, x3, #0x0
  20:   85c0e060        ld1rd   {z0.d}, p0/z, [x3]
  24:   25e11fe0        whilelo p0.d, xzr, x1
  28:   a5e24001        ld1d    {z1.d}, p0/z, [x0, x2, lsl #3]
  2c:   04f0e3e2        incd    x2
  30:   65c58020        fminnm  z0.d, p0/m, z0.d, z1.d
  34:   25e11c40        whilelo p0.d, x2, x1
  38:   54ffff81        b.ne    28 <f+0x28>  // b.any
  3c:   65c52400        fminnmv d0, p1, z0.d
  40:   d65f03c0        ret
  44:   d2e80b20        mov     x0, #0x4059000000000000
  48:   9e670000        fmov    d0, x0
  4c:   d65f03c0        ret

This patch extends the support for reductions to include calls to internal
functions, in addition to assign statements. For this purpose, in most places
where a tree_code would be used, a code_helper is used instead. The code_helper
allows to hold either a tree_code or combined_fn.

This patch implements these tasks:

- Detect a reduction candidate based on a call to an internal function
  (currently only fmin or fmax).
- Process the reduction using code_helper. This means that at several places
  we have to check whether this is as assign-based reduction or a call-based
  reduction.
- Add new internal functions for the fmin/fmax reductions and for conditional
  fmin/fmax. In architectures where ieee fmin/fmax reductions are available, it
  is still possible to vectorize the loop using unconditional instructions.
- Update SVE's md to support these new reductions.
- Add new SVE tests to check that the optimal code is being generated.

I tested this patch in an aarch64 machine bootstrapping the compiler and
running the checks.
 
Alejandro
 
gcc/Changelog:
 
2018-12-18  Alejandro Martinez  <alejandro.martinezvicente@arm.com>

	* gimple-match.h (code_helper_for_stmnt): New function to get a
	code_helper from an statement.
	* internal-fn.def: New reduc_fmax_scal and reduc_fmin_scal optabs for
	ieee fp max/min reductions
	* optabs.def: Likewise.
	* tree-vect-loop.c (reduction_fn_for_scalar_code): Changed function
	signature to accept code_helper instead of tree_code. Handle the
	fmax/fmin builtins.
	(needs_fold_left_reduction_p): Likewise.
	(check_reduction_path): Likewise.
	(vect_is_simple_reduction): Use code_helper instead of tree_code. Check
	for supported call-based reductions. Extend support for both
	assignment-based and call-based reductions.
	(vect_model_reduction_cost): Extend cost-model support to call-based
	reductions (just use MAX expression).
	(get_initial_def_for_reduction): Use code_helper instead of tree_code.
	Extend support for both assignment-based and call-based reductions.
	(vect_create_epilog_for_reduction): Likewise.
	(vectorizable_reduction): Likewise.
	* tree-vectorizer.h: include gimple-match.h for code_helper. Use
	code_helper in check_reduction_path signature.
	* config/aarch64/aarch64-sve.md: Added define_expand to capture new
	reduc_fmax_scal and reduc_fmin_scal optabs.
	* config/aarch64/iterators.md: New FMAXMINNMV and fmaxmin_uns iterators
	to support the new define_expand.
 
gcc/testsuite/Changelog:
 
2018-12-18  Alejandro Martinez  <alejandro.martinezvicente@arm.com>

	* gcc.target/aarch64/sve/reduc_9.c: New test to check
	SVE-vectorized reductions without -ffast-math.
	* gcc.target/aarch64/sve/reduc_10.c: New test to check
	SVE-vectorized builtin reductions without -ffast-math.
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