Hi All,

This adds an implementation for the new optab for unsigned pow2 bitmask for
AArch64.

The implementation rewrites:

   x = y / (2 ^ (sizeof (y)/2)-1

into e.g. (for bytes)

   (x + ((x + 257) >> 8)) >> 8

where it's required that the additions be done in double the precision of x
such that we don't lose any bits during an overflow.

Essentially the sequence decomposes the division into doing two smaller
divisions, one for the top and bottom parts of the number and adding the results
back together.

To account for the fact that shift by 8 would be division by 256 we add 1 to
both parts of x such that when 255 we still get 1 as the answer.

Because the amount we shift are half the original datatype we can use the
halfing instructions the ISA provides to do the operation instead of using
actual shifts.

For AArch64 this means we generate for:

void draw_bitmap1(uint8_t* restrict pixel, uint8_t level, int n)
{
  for (int i = 0; i < (n & -16); i+=1)
    pixel[i] = (pixel[i] * level) / 0xff;
}

the following:

	movi    v3.16b, 0x1
	umull2  v1.8h, v0.16b, v2.16b
	umull   v0.8h, v0.8b, v2.8b
	addhn   v5.8b, v1.8h, v3.8h
	addhn   v4.8b, v0.8h, v3.8h
	uaddw   v1.8h, v1.8h, v5.8b
	uaddw   v0.8h, v0.8h, v4.8b
	uzp2    v0.16b, v0.16b, v1.16b

instead of:

	umull   v2.8h, v1.8b, v5.8b
	umull2  v1.8h, v1.16b, v5.16b
	umull   v0.4s, v2.4h, v3.4h
	umull2  v2.4s, v2.8h, v3.8h
	umull   v4.4s, v1.4h, v3.4h
	umull2  v1.4s, v1.8h, v3.8h
	uzp2    v0.8h, v0.8h, v2.8h
	uzp2    v1.8h, v4.8h, v1.8h
	shrn    v0.8b, v0.8h, 7
	shrn2   v0.16b, v1.8h, 7

Which results in significantly faster code.

Thanks for Wilco for the concept.

Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.

Ok for master?

Thanks,
Tamar

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (udiv_pow2_bitmask<mode>2): New.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/div-by-bitmask.c: New test.

--- inline copy of patch -- 
diff --git a/gcc/config/aarch64/aarch64-simd.md b/gcc/config/aarch64/aarch64-simd.md
index 18733428f3fb91d937346aa360f6d1fe13ca1eae..6b0405924a03a243949a6741f4c0e989d9ca2869 100644
--- a/gcc/config/aarch64/aarch64-simd.md
+++ b/gcc/config/aarch64/aarch64-simd.md
@@ -4845,6 +4845,57 @@ (define_expand "aarch64_<sur><addsub>hn2<mode>"
   }
 )
 
+;; div optimizations using narrowings
+;; we can do the division e.g. shorts by 255 faster by calculating it as
+;; (x + ((x + 257) >> 8)) >> 8 assuming the operation is done in
+;; double the precision of x.
+;;
+;; If we imagine a short as being composed of two blocks of bytes then
+;; adding 257 or 0b0000_0001_0000_0001 to the number is equivalen to
+;; adding 1 to each sub component:
+;;
+;;      short value of 16-bits
+;; ┌──────────────┬────────────────┐
+;; │              │                │
+;; └──────────────┴────────────────┘
+;;   8-bit part1 ▲  8-bit part2   ▲
+;;               │                │
+;;               │                │
+;;              +1               +1
+;;
+;; after the first addition, we have to shift right by 8, and narrow the
+;; results back to a byte.  Remember that the addition must be done in
+;; double the precision of the input.  Since 8 is half the size of a short
+;; we can use a narrowing halfing instruction in AArch64, addhn which also
+;; does the addition in a wider precision and narrows back to a byte.  The
+;; shift itself is implicit in the operation as it writes back only the top
+;; half of the result. i.e. bits 2*esize-1:esize.
+;;
+;; Since we have narrowed the result of the first part back to a byte, for
+;; the second addition we can use a widening addition, uaddw.
+;;
+;; For the finaly shift, since it's unsigned arithmatic we emit an ushr by 8
+;; to shift and the vectorizer.
+;;
+;; The shift is later optimized by combine to a uzp2 with movi #0.
+(define_expand "udiv_pow2_bitmask<mode>2"
+  [(match_operand:VQN 0 "register_operand")
+   (match_operand:VQN 1 "register_operand")]
+  "TARGET_SIMD"
+{
+  rtx addend = gen_reg_rtx (<MODE>mode);
+  rtx val = aarch64_simd_gen_const_vector_dup (<VNARROWQ2>mode, 1);
+  emit_move_insn (addend, lowpart_subreg (<MODE>mode, val, <VNARROWQ2>mode));
+  rtx tmp1 = gen_reg_rtx (<VNARROWQ>mode);
+  rtx tmp2 = gen_reg_rtx (<MODE>mode);
+  emit_insn (gen_aarch64_addhn<mode> (tmp1, operands[1], addend));
+  unsigned bitsize = GET_MODE_UNIT_BITSIZE (<VNARROWQ>mode);
+  rtx shift_vector = aarch64_simd_gen_const_vector_dup (<MODE>mode, bitsize);
+  emit_insn (gen_aarch64_uaddw<Vnarrowq> (tmp2, operands[1], tmp1));
+  emit_insn (gen_aarch64_simd_lshr<mode> (operands[0], tmp2, shift_vector));
+  DONE;
+})
+
 ;; pmul.
 
 (define_insn "aarch64_pmul<mode>"
diff --git a/gcc/testsuite/gcc.target/aarch64/div-by-bitmask.c b/gcc/testsuite/gcc.target/aarch64/div-by-bitmask.c
new file mode 100644
index 0000000000000000000000000000000000000000..c03aee695ef834fbe3533a21d54a218160b0007d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/div-by-bitmask.c
@@ -0,0 +1,70 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O2 -std=c99 -fdump-tree-vect -save-temps" } */
+/* { dg-final { check-function-bodies "**" "" "" { target { le } } } } */
+
+#include <stdint.h>
+
+/*
+** draw_bitmap1:
+** ...
+** 	umull2	v[0-9]+.8h, v[0-9]+.16b, v[0-9]+.16b
+** 	umull	v[0-9]+.8h, v[0-9]+.8b, v[0-9]+.8b
+** 	addhn	v[0-9]+.8b, v[0-9]+.8h, v[0-9]+.8h
+** 	addhn	v[0-9]+.8b, v[0-9]+.8h, v[0-9]+.8h
+** 	uaddw	v[0-9]+.8h, v[0-9]+.8h, v[0-9]+.8b
+** 	uaddw	v[0-9]+.8h, v[0-9]+.8h, v[0-9]+.8b
+** 	uzp2	v[0-9]+.16b, v[0-9]+.16b, v[0-9]+.16b
+** ...
+*/
+void draw_bitmap1(uint8_t* restrict pixel, uint8_t level, int n)
+{
+  for (int i = 0; i < (n & -16); i+=1)
+    pixel[i] = (pixel[i] * level) / 0xff;
+}
+
+void draw_bitmap2(uint8_t* restrict pixel, uint8_t level, int n)
+{
+  for (int i = 0; i < (n & -16); i+=1)
+    pixel[i] = (pixel[i] * level) / 0xfe;
+}
+
+/*
+** draw_bitmap3:
+** ...
+** 	umull2	v[0-9]+.4s, v[0-9]+.8h, v[0-9]+.8h
+** 	umull	v[0-9]+.4s, v[0-9]+.4h, v[0-9]+.4h
+** 	addhn	v[0-9]+.4h, v[0-9]+.4s, v[0-9]+.4s
+** 	addhn	v[0-9]+.4h, v[0-9]+.4s, v[0-9]+.4s
+** 	uaddw	v[0-9]+.4s, v[0-9]+.4s, v[0-9]+.4h
+** 	uaddw	v[0-9]+.4s, v[0-9]+.4s, v[0-9]+.4h
+** 	uzp2	v[0-9]+.8h, v[0-9]+.8h, v[0-9]+.8h
+** ...
+*/
+void draw_bitmap3(uint16_t* restrict pixel, uint16_t level, int n)
+{
+  for (int i = 0; i < (n & -16); i+=1)
+    pixel[i] = (pixel[i] * level) / 0xffffU;
+}
+
+/*
+** draw_bitmap4:
+** ...
+** 	umull2	v[0-9]+.2d, v[0-9]+.4s, v[0-9]+.4s
+** 	umull	v[0-9]+.2d, v[0-9]+.2s, v[0-9]+.2s
+** 	addhn	v[0-9]+.2s, v[0-9]+.2d, v[0-9]+.2d
+** 	addhn	v[0-9]+.2s, v[0-9]+.2d, v[0-9]+.2d
+** 	uaddw	v[0-9]+.2d, v[0-9]+.2d, v[0-9]+.2s
+** 	uaddw	v[0-9]+.2d, v[0-9]+.2d, v[0-9]+.2s
+** 	uzp2	v[0-9]+.4s, v[0-9]+.4s, v[0-9]+.4s
+** ...
+*/
+/* Costing for long vectorization seems off, so disable
+   the cost model to test the codegen.  */
+__attribute__ ((optimize("-fno-vect-cost-model")))
+void draw_bitmap4(uint32_t* restrict pixel, uint32_t level, int n)
+{
+  for (int i = 0; i < (n & -16); i+=1)
+    pixel[i] = (pixel[i] * (uint64_t)level) / 0xffffffffUL;
+}
+
+/* { dg-final { scan-tree-dump-times "\.DIV_POW2_BITMASK" 6 "vect" } } */




--