[PATCH 1/2] Match: Simplify unsigned scalar sat_sub(x -1) to (x - x != 0)

[Li Xu]

From: xuli <xu...@eswincomputing.com>

When the imm operand op1=1 in the unsigned scalar sat_sub form2 below,
we can simplify (x != 0 ? x + max : 0) to (x - x != 0), thereby eliminating
a branch instruction.

Form2:
T __attribute__((noinline))             \
sat_u_sub_imm##IMM##_##T##_fmt_2 (T x)  \
{                                       \
  return x >= (T)IMM ? x - (T)IMM : 0;  \
}

Take below form 2 as example:
DEF_SAT_U_SUB_IMM_FMT_2(uint8_t, 1)

Before this patch:
__attribute__((noinline))
uint8_t sat_u_sub_imm1_uint8_t_fmt_2 (uint8_t x)
{
  uint8_t _1;
  uint8_t _3;

  <bb 2> [local count: 1073741824]:
  if (x_2(D) != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 4>; [50.00%]

  <bb 3> [local count: 536870912]:
  _3 = x_2(D) + 255;

  <bb 4> [local count: 1073741824]:
  # _1 = PHI <x_2(D)(2), _3(3)>
  return _1;

}

Assembly code:
sat_u_sub_imm1_uint8_t_fmt_2:
        beq     a0,zero,.L2
        addiw   a0,a0,-1
        andi    a0,a0,0xff
.L2:
        ret

After this patch:
__attribute__((noinline))
uint8_t sat_u_sub_imm1_uint8_t_fmt_2 (uint8_t x)
{
  _Bool _1;
  unsigned char _2;
  uint8_t _4;

  <bb 2> [local count: 1073741824]:
  _1 = x_3(D) != 0;
  _2 = (unsigned char) _1;
  _4 = x_3(D) - _2;
  return _4;

}

Assembly code:
sat_u_sub_imm1_uint8_t_fmt_2:
        snez    a5,a0
        subw    a0,a0,a5
        andi    a0,a0,0xff
        ret

The below test suites are passed for this patch:
1. The rv64gcv fully regression tests.
2. The x86 bootstrap tests.
3. The x86 fully regression tests.

Signed-off-by: Li Xu <xu...@eswincomputing.com>
gcc/ChangeLog:

        * match.pd: Simplify (x != 0 ? x + max : 0) to (x - x != 0).
---
 gcc/match.pd | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/gcc/match.pd b/gcc/match.pd
index 0455dfa6993..9dcb9555cc0 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3383,6 +3383,22 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   }
   (if (wi::eq_p (sum, wi::uhwi (0, precision)))))))
 
+/* The boundary condition for case 10: IMM = 1:
+   SAT_U_SUB = X >= IMM ? (X - IMM) : 0.
+   simplify (X != 0 ? X + max : 0) to (X - X != 0).  */
+(simplify
+ (cond (ne @0 integer_zerop) (plus @0 INTEGER_CST@1) integer_zerop)
+ (if (INTEGRAL_TYPE_P (type) && TYPE_UNSIGNED (type)
+     && types_match (type, @0))
+  (with
+   {
+      unsigned precision = TYPE_PRECISION (type);
+      wide_int max = wi::mask (precision, false, precision);
+      wide_int c1 = wi::to_wide (@1);
+   }
+   (if (wi::eq_p (c1, max))
+   (minus @0 (convert (ne @0 { build_zero_cst (type); })))))))
+
 /* Signed saturation sub, case 1:
    T minus = (T)((UT)X - (UT)Y);
    SAT_S_SUB = (X ^ Y) & (X ^ minus) < 0 ? (-(T)(X < 0) ^ MAX) : minus;
-- 
2.17.1