[PR87815]Don't generate shift sequence for load replacement in DSE when the mode size is not compile-time constant

[Renlin Li]

Hi all,

The patch adds a check if the gap is compile-time constant.

This happens when dse decides to replace the load with previous store value.
The problem is that, shift sequence could not accept compile-time non-constant
mode operand.

Another issue raised from this issue is the inefficient code-generation for
general data manipulation over mask/predicate register.
In sve, some general data processing instructions don't apply on predicate
registers directly. In the worst(this) case, memory load/store is generated to 
reload
the value into a general purpose register for further data processing.
We need to improve that.

aarch64 sve test Okay, Okay to commit?

Regards,
Renlin

gcc/ChangeLog:

2018-10-31  Renlin Li  <renlin...@arm.com>

        PR target/87815
        * dse.c (get_stored_val): Add check for compile-time
          constantness of gap.

gcc/testsuite/ChangeLog:

2018-10-31  Renlin Li  <renlin...@arm.com>

        PR target/87815
        * gcc.target/aarch64/sve/pr87815.c: New.

diff --git a/gcc/dse.c b/gcc/dse.c
index cfebfa0e110be56f17337dcb152984d782528889..21d166d92fcc2c2a4dd6d04bb7a7247b79b81a62 100644
--- a/gcc/dse.c
+++ b/gcc/dse.c
@@ -1841,7 +1841,7 @@ get_stored_val (store_info *store_info, machine_mode read_mode,
   else
     gap = read_offset - store_info->offset;
 
-  if (maybe_ne (gap, 0))
+  if (gap.is_constant () && maybe_ne (gap, 0))
     {
       poly_int64 shift = gap * BITS_PER_UNIT;
       poly_int64 access_size = GET_MODE_SIZE (read_mode) + gap;
diff --git a/gcc/testsuite/gcc.target/aarch64/sve/pr87815.c b/gcc/testsuite/gcc.target/aarch64/sve/pr87815.c
new file mode 100644
index 0000000000000000000000000000000000000000..628cedb2acce82a86b61944eb6184d7fdbb2d656
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/sve/pr87815.c
@@ -0,0 +1,13 @@
+/* { dg-do compile { target aarch64_asm_sve_ok } } */
+/* { dg-options "-O3" } */
+int a, b, d;
+short e;
+
+void f ()
+{
+  for (int i = 0; i < 8; i++)
+    {
+      e = b >= 2 ?: a >> b;
+      d = e && b;
+    }
+}