arm: correctly handle inequality comparisons against max constants [PR100563]

Normally we expect the gimple optimizers to fold away comparisons that
are always true, but at some lower optimization levels this is not
always the case, so the back-end has to be able to generate correct
code in these cases.

In this example, we have a comparison of the form

  (unsigned long long) op <= ~0ULL

which, of course is always true.

Normally, in the arm back-end we handle these expansions where the
immediate cannot be handled directly by adding 1 to the constant and
then adjusting the comparison operator:

  (unsigned long long) op < CONST + 1

but we cannot do that when the constant is already the largest value.

Fortunately, we observe that the comparisons we need to handle this
way are either always true or always false, so instead of forming a
comparison against the maximum value, we can replace it with a
comparison against the minimum value (which just happens to also be a
constant we can handle.  So

  op1 <= ~0ULL -> op1 >= 0U
  op1 > ~0ULL -> op1 < 0U

  op1 <= LONG_LONG_INT_MAX -> op1 >= (-LONG_LONG_INT_MAX - 1)
  op1 > LONG_LONG_INT_MAX -> op1 < (-LONG_LONG_INT_MAX - 1)

gcc:
	PR target/100563
	* config/arm/arm.c (arm_canonicalize_comparison): Correctly
	canonicalize DImode inequality comparisons against the
	maximum integral value.

gcc/testsuite:
	* gcc.dg/pr100563.c: New test.

gcc/config/arm/arm.c

@@ -5563,9 +5563,20 @@ arm_canonicalize_comparison (int *code, rtx *op0, rtx *op1,
 			return;
 		      *op1 = GEN_INT (i + 1);
 		      *code = *code == GT ? GE : LT;
-		      return;
 		    }
-		  break;
+		  else
+		    {
+		      /* GT maxval is always false, LE maxval is always true.
+			 We can't fold that away here as we must make a
+			 comparison, but we can fold them to comparisons
+			 with the same result that can be handled:
+			   op0 GT maxval -> op0 LT minval
+			   op0 LE maxval -> op0 GE minval
+			 where minval = (-maxval - 1).  */
+		      *op1 = GEN_INT (-maxval - 1);
+		      *code = *code == GT ? LT : GE;
+		    }
+		  return;
 
 		case GTU:
 		case LEU:
@@ -5578,9 +5589,19 @@ arm_canonicalize_comparison (int *code, rtx *op0, rtx *op1,
 			return;
 		      *op1 = GEN_INT (i + 1);
 		      *code = *code == GTU ? GEU : LTU;
-		      return;
 		    }
-		  break;
+		  else
+		    {
+		      /* GTU ~0 is always false, LEU ~0 is always true.
+			 We can't fold that away here as we must make a
+			 comparison, but we can fold them to comparisons
+			 with the same result that can be handled:
+			   op0 GTU ~0 -> op0 LTU 0
+			   op0 LEU ~0 -> op0 GEU 0.  */
+		      *op1 = const0_rtx;
+		      *code = *code == GTU ? LTU : GEU;
+		    }
+		  return;
 
 		default:
 		  gcc_unreachable ();

gcc/testsuite/gcc.dg/pr100563.c

@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-Og" } */
+unsigned long long e(void);
+void f(int);
+void a() {
+  short b = -1, c = (int)&b;
+  unsigned long long d = e();
+  f(b >= d);
+}