[arm] Improve constant handling for usubvsi4.

This patch improves the expansion of usubvsi4 by allowing suitable
constants to be passed directly.  Unlike normal subtraction, either
operand may be a constant (and indeed I have seen cases where both can
be with LTO enabled).  One interesting testcase that improves as a
result of this is:

unsigned f6 (unsigned a)
{
  unsigned x;
  return __builtin_sub_overflow (5U, a, &x) ? 0 : x;
}

Which previously compiled to:

	rsbs	r3, r0, gcc-mirror#5
	cmp	r0, gcc-mirror#5
	movls	r0, r3
	movhi	r0, #0

but now generates the optimal sequence:

	rsbs	r0, r0, gcc-mirror#5
	movcc	r0, #0

	* config/arm/arm.md (usubv<mode>4): Delete expansion.
	(usubvsi4): New pattern.  Allow some immediate values for inputs.
	(usubvdi4): New pattern.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@277187 138bc75d-0d04-0410-961f-82ee72b054a4

Author: rearnsha

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2019-10-18  Richard Earnshaw  <rearnsha@arm.com>
+
+	* config/arm/arm.md (usubv<mode>4): Delete expansion.
+	(usubvsi4): New pattern.  Allow some immediate values for inputs.
+	(usubvdi4): New pattern.
+
 2019-10-18  Richard Earnshaw  <rearnsha@arm.com>
 
 	* config/arm/arm.c (arm_select_cc_mode): Allow either the first

gcc/config/arm/arm.md

@@ -1352,14 +1352,50 @@
   DONE;
 })
 
-(define_expand "usubv<mode>4"
-  [(match_operand:SIDI 0 "register_operand")
-   (match_operand:SIDI 1 "register_operand")
-   (match_operand:SIDI 2 "register_operand")
+(define_expand "usubvsi4"
+  [(match_operand:SI 0 "s_register_operand")
+   (match_operand:SI 1 "arm_rhs_operand")
+   (match_operand:SI 2 "arm_add_operand")
    (match_operand 3 "")]
   "TARGET_32BIT"
 {
-  emit_insn (gen_sub<mode>3_compare1 (operands[0], operands[1], operands[2]));
+  machine_mode mode = CCmode;
+  if (CONST_INT_P (operands[1]) && CONST_INT_P (operands[2]))
+    {
+      /* If both operands are constants we can decide the result statically.  */
+      wi::overflow_type overflow;
+      wide_int val = wi::sub (rtx_mode_t (operands[1], SImode),
+			      rtx_mode_t (operands[2], SImode),
+			      UNSIGNED, &overflow);
+      emit_move_insn (operands[0], GEN_INT (val.to_shwi ()));
+      if (overflow != wi::OVF_NONE)
+	emit_jump_insn (gen_jump (operands[3]));
+      DONE;
+    }
+  else if (CONST_INT_P (operands[2]))
+    emit_insn (gen_cmpsi2_addneg (operands[0], operands[1], operands[2],
+				  GEN_INT (-INTVAL (operands[2]))));
+  else if (CONST_INT_P (operands[1]))
+    {
+      mode = CC_RSBmode;
+      emit_insn (gen_rsb_imm_compare (operands[0], operands[1], operands[2],
+				      GEN_INT (~UINTVAL (operands[1]))));
+    }
+  else
+    emit_insn (gen_subsi3_compare1 (operands[0], operands[1], operands[2]));
+  arm_gen_unlikely_cbranch (LTU, mode, operands[3]);
+
+  DONE;
+})
+
+(define_expand "usubvdi4"
+  [(match_operand:DI 0 "s_register_operand")
+   (match_operand:DI 1 "s_register_operand")
+   (match_operand:DI 2 "s_register_operand")
+   (match_operand 3 "")]
+  "TARGET_32BIT"
+{
+  emit_insn (gen_subdi3_compare1 (operands[0], operands[1], operands[2]));
   arm_gen_unlikely_cbranch (LTU, CCmode, operands[3]);
 
   DONE;