simplify-rtx: Optimize (x - 1) * y + y [PR98334]

We don't try to optimize for signed x, y (int) (x - 1U) * y + y
into x * y, we can't do that with signed x * y, because the former
is well defined for INT_MIN and -1, while the latter is not.
We could perhaps optimize it during isel or some very late optimization
where we'd turn magically flag_wrapv, but we don't do that yet.

This patch optimizes it in simplify-rtx.c, such that we can optimize
it during combine.

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/98334
	* simplify-rtx.c (simplify_context::simplify_binary_operation_1):
	Optimize (X - 1) * Y + Y to X * Y or (X + 1) * Y - Y to X * Y.

	* gcc.target/i386/pr98334.c: New test.

2 files changed, 92 insertions, 0 deletions

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index e041b8e..571e233 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -2602,6 +2602,42 @@ simplify_context::simplify_binary_operation_1 (rtx_code code,
 	      return (set_src_cost (tem, int_mode, speed)
 		      <= set_src_cost (orig, int_mode, speed) ? tem : 0);
 	    }
+
+	  /* Optimize (X - 1) * Y + Y to X * Y.  */
+	  lhs = op0;
+	  rhs = op1;
+	  if (GET_CODE (op0) == MULT)
+	    {
+	      if (((GET_CODE (XEXP (op0, 0)) == PLUS
+		    && XEXP (XEXP (op0, 0), 1) == constm1_rtx)
+		   || (GET_CODE (XEXP (op0, 0)) == MINUS
+		       && XEXP (XEXP (op0, 0), 1) == const1_rtx))
+		  && rtx_equal_p (XEXP (op0, 1), op1))
+		lhs = XEXP (XEXP (op0, 0), 0);
+	      else if (((GET_CODE (XEXP (op0, 1)) == PLUS
+			 && XEXP (XEXP (op0, 1), 1) == constm1_rtx)
+			|| (GET_CODE (XEXP (op0, 1)) == MINUS
+			    && XEXP (XEXP (op0, 1), 1) == const1_rtx))
+		       && rtx_equal_p (XEXP (op0, 0), op1))
+		lhs = XEXP (XEXP (op0, 1), 0);
+	    }
+	  else if (GET_CODE (op1) == MULT)
+	    {
+	      if (((GET_CODE (XEXP (op1, 0)) == PLUS
+		    && XEXP (XEXP (op1, 0), 1) == constm1_rtx)
+		   || (GET_CODE (XEXP (op1, 0)) == MINUS
+		       && XEXP (XEXP (op1, 0), 1) == const1_rtx))
+		  && rtx_equal_p (XEXP (op1, 1), op0))
+		rhs = XEXP (XEXP (op1, 0), 0);
+	      else if (((GET_CODE (XEXP (op1, 1)) == PLUS
+			 && XEXP (XEXP (op1, 1), 1) == constm1_rtx)
+			|| (GET_CODE (XEXP (op1, 1)) == MINUS
+			    && XEXP (XEXP (op1, 1), 1) == const1_rtx))
+		       && rtx_equal_p (XEXP (op1, 0), op0))
+		rhs = XEXP (XEXP (op1, 1), 0);
+	    }
+	  if (lhs != op0 || rhs != op1)
+	    return simplify_gen_binary (MULT, int_mode, lhs, rhs);
 	}
 
       /* (plus (xor X C1) C2) is (xor X (C1^C2)) if C2 is signbit.  */
@@ -2789,6 +2825,26 @@ simplify_context::simplify_binary_operation_1 (rtx_code code,
 	      return (set_src_cost (tem, int_mode, speed)
 		      <= set_src_cost (orig, int_mode, speed) ? tem : 0);
 	    }
+
+	  /* Optimize (X + 1) * Y - Y to X * Y.  */
+	  lhs = op0;
+	  if (GET_CODE (op0) == MULT)
+	    {
+	      if (((GET_CODE (XEXP (op0, 0)) == PLUS
+		    && XEXP (XEXP (op0, 0), 1) == const1_rtx)
+		   || (GET_CODE (XEXP (op0, 0)) == MINUS
+		       && XEXP (XEXP (op0, 0), 1) == constm1_rtx))
+		  && rtx_equal_p (XEXP (op0, 1), op1))
+		lhs = XEXP (XEXP (op0, 0), 0);
+	      else if (((GET_CODE (XEXP (op0, 1)) == PLUS
+			 && XEXP (XEXP (op0, 1), 1) == const1_rtx)
+			|| (GET_CODE (XEXP (op0, 1)) == MINUS
+			    && XEXP (XEXP (op0, 1), 1) == constm1_rtx))
+		       && rtx_equal_p (XEXP (op0, 0), op1))
+		lhs = XEXP (XEXP (op0, 1), 0);
+	    }
+	  if (lhs != op0)
+	    return simplify_gen_binary (MULT, int_mode, lhs, op1);
 	}
 
       /* (a - (-b)) -> (a + b).  True even for IEEE.  */
diff --git a/gcc/testsuite/gcc.target/i386/pr98334.c b/gcc/testsuite/gcc.target/i386/pr98334.c
new file mode 100644
index 0000000..283abc5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr98334.c
@@ -0,0 +1,36 @@
+/* PR rtl-optimization/98334 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fomit-frame-pointer -fno-stack-protector" } */
+/* { dg-final { scan-assembler-not "\taddl\t" } } */
+/* { dg-final { scan-assembler-not "\tsubl\t" } } */
+/* { dg-final { scan-assembler-not "\tleal\t" } } */
+
+int
+foo (int i, unsigned int n)
+{
+  int result = 0;
+  while (n > 0)
+    {
+      result += i;
+      n -= 1;
+    }
+  return result;
+}
+
+int
+bar (int x, int y)
+{
+  return (int) (y - 1U) * x + x;
+}
+
+int
+baz (int x, int y)
+{
+  return (y - 1) * x + x;
+}
+
+int
+qux (int x, int y)
+{
+  return x * (int) (y + 1U) - x;
+}