[PATCH] AArch64: Reassociate CONST in address expressions [PR112573]

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

GCC tends to optimistically create CONST of globals with an immediate offset. 
However it is almost always better to CSE addresses of globals and add immediate
offsets separately (the offset could be merged later in single-use cases).
Splitting CONST expressions with an index in aarch64_legitimize_address fixes part
of PR112573.

Passes regress & bootstrap, OK for commit?

gcc/ChangeLog:
        PR target/112573
        * config/aarch64/aarch64.cc (aarch64_legitimize_address): Reassociate badly
        formed CONST expressions.

gcc/testsuite/ChangeLog:
        PR target/112573
        * gcc.target/aarch64/pr112573.c: Add new test.

---

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 0909b319d16b9a1587314bcfda0a8112b42a663f..9fbc8b62455f48baec533d3dd5e2d9ea995d5a8f 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -12608,6 +12608,20 @@ aarch64_legitimize_address (rtx x, rtx /* orig_x  */, machine_mode mode)
      not to split a CONST for some forms of address expression, otherwise
      it will generate sub-optimal code.  */
 
+  /* First split X + CONST (base, offset) into (base + X) + offset.  */
+  if (GET_CODE (x) == PLUS && GET_CODE (XEXP (x, 1)) == CONST)
+    {
+      poly_int64 offset;
+      rtx base = strip_offset_and_salt (XEXP (x, 1), &offset);
+
+      if (offset.is_constant ())
+      {
+	  base = expand_binop (Pmode, add_optab, base, XEXP (x, 0),
+			       NULL_RTX, true, OPTAB_DIRECT);
+	  x = plus_constant (Pmode, base, offset);
+      }
+    }
+
   if (GET_CODE (x) == PLUS && CONST_INT_P (XEXP (x, 1)))
     {
       rtx base = XEXP (x, 0);
diff --git a/gcc/testsuite/gcc.target/aarch64/pr112573.c b/gcc/testsuite/gcc.target/aarch64/pr112573.c
new file mode 100644
index 0000000000000000000000000000000000000000..be04c0ca86ad9f33975a85f497549955d6d1236d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr112573.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-section-anchors" } */
+
+char a[100];
+
+void f1 (int x, int y)
+{
+  *((a + y) + 3) = x;
+  *((a + y) + 2) = x;
+  *((a + y) + 1) = x;
+  *((a + y) + 0) = x;
+}
+
+/* { dg-final { scan-assembler-times "strb" 4 } } */
+/* { dg-final { scan-assembler-times "adrp" 1 } } */