[PATCH][simplify-rtx] (GTU (PLUS a C) (C - 1)) --> (LTU a -C)

[Kyrill Tkachov]

Hi all,

Currently the functions:
int f1(int x, int t)
{
  if (x == -1 || x == -2)
    t = 1;
  return t;
}

int f2(int x, int t)
{
  if (x == -1 || x == -2)
    return 1;
  return t;
}

generate different code on AArch64 even though they have identical 
functionality:
f1:
        add     w0, w0, 2
        cmp     w0, 1
        csinc   w0, w1, wzr, hi
        ret

f2:
        cmn     w0, #2
        csinc   w0, w1, wzr, cc
        ret

The problem is that f2 performs the comparison (LTU w0 -2)
whereas f1 performs (GTU (PLUS w0 2) 1). I think it is possible to simplify the 
f1 form
to the f2 form with the simplify-rtx.c rule added in this patch. With this 
patch the
codegen for both f1 and f2 on aarch64 at -O2 is identical (CMN, CSINC).

Bootstrapped and tested on arm-none-linux-gnueabihf, aarch64-none-linux-gnu, 
x86_64.
What do you think? Is this a correct generalisation of this issue?
If so, ok for trunk?

Thanks,
Kyrill

2016-09-16  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    * simplify-rtx.c (simplify_relational_operation_1): Add transformation
    (GTU (PLUS a C) (C - 1)) --> (LTU a -C).

2016-09-16  Kyrylo Tkachov  <kyrylo.tkac...@arm.com>

    * gcc.target/aarch64/gtu_to_ltu_cmp_1.c: New test.

diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index 14302ea06eccc099ef356ab6c63ac020dd083b0c..4153c7335680068ed3ce08410400ac6abaf30c89 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -4663,6 +4663,19 @@ simplify_relational_operation_1 (enum rtx_code code, machine_mode mode,
 				      cmp_mode, XEXP (op0, 0), new_cmp);
     }
 
+  /* (GTU (PLUS a C) (C - 1)) where C is a non-zero constant can be
+     transformed into (LTU a -C).  */
+  if (code == GTU && GET_CODE (op0) == PLUS && CONST_INT_P (op1)
+      && CONST_INT_P (XEXP (op0, 1))
+      && (UINTVAL (op1) == UINTVAL (XEXP (op0, 1)) - 1)
+      && XEXP (op0, 1) != const0_rtx)
+    {
+      rtx new_cmp
+	= simplify_gen_unary (NEG, cmp_mode, XEXP (op0, 1), cmp_mode);
+      return simplify_gen_relational (LTU, mode, cmp_mode,
+				       XEXP (op0, 0), new_cmp);
+    }
+
   /* Canonicalize (LTU/GEU (PLUS a b) b) as (LTU/GEU (PLUS a b) a).  */
   if ((code == LTU || code == GEU)
       && GET_CODE (op0) == PLUS
diff --git a/gcc/testsuite/gcc.target/aarch64/gtu_to_ltu_cmp_1.c b/gcc/testsuite/gcc.target/aarch64/gtu_to_ltu_cmp_1.c
new file mode 100644
index 0000000000000000000000000000000000000000..81c536c90afe38932c48ed0af24f55e73eeff80e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/gtu_to_ltu_cmp_1.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int
+f1 (int x, int t)
+{
+  if (x == -1 || x == -2)
+    t = 1;
+
+  return t;
+}
+
+/* { dg-final { scan-assembler-times "cmn\\tw\[0-9\]+, #2" 1 } } */