[PATCH] Fix PR46932: Block auto increment on frame pointer

[Wilco Dijkstra]

Block auto increment on frame pointer references.  This is never
beneficial since the SFP expands into SP+C or FP+C during register
allocation.  The generated code for the testcase is now as expected:

        str     x30, [sp, -32]!
        strb    w0, [sp, 31]
        add     x0, sp, 31
        bl      foo3
        ldr     x30, [sp], 32
        ret

On SPEC2017 codesize improves uniformly across the board.

ChangeLog:
2017-07-20  Wilco Dijkstra  <wdijk...@arm.com>

        PR middle-end/46932
        * auto-inc-dec.c (parse_add_or_inc): Block autoinc on sfp.

gcc/testsuite/

        * gcc.dg/pr46932.c: New testcase.

--

diff --git a/gcc/auto-inc-dec.c b/gcc/auto-inc-dec.c
index 
91fa5cf0bbe04b8a2c2c2b9209d245a327de0ffd..db1bd5bba2cee9fbf24d6d522505ac292688aab9
 100644
--- a/gcc/auto-inc-dec.c
+++ b/gcc/auto-inc-dec.c
@@ -769,6 +769,12 @@ parse_add_or_inc (rtx_insn *insn, bool before_mem)
   inc_insn.pat = pat;
   inc_insn.reg_res = SET_DEST (pat);
   inc_insn.reg0 = XEXP (SET_SRC (pat), 0);
+
+  /* Block any auto increment of the frame pointer since it expands into
+     an addition and cannot be removed by copy propagation.  */
+  if (inc_insn.reg0 == frame_pointer_rtx)
+    return false;
+
   if (rtx_equal_p (inc_insn.reg_res, inc_insn.reg0))
     inc_insn.form = before_mem ? FORM_PRE_INC : FORM_POST_INC;
   else
diff --git a/gcc/testsuite/gcc.dg/pr46932.c b/gcc/testsuite/gcc.dg/pr46932.c
new file mode 100644
index 
0000000000000000000000000000000000000000..b96febcc095fbec2cc4e02d1155871220d17cc99
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr46932.c
@@ -0,0 +1,13 @@
+/* { dg-options "-O2 -fdump-rtl-auto_inc_dec" } */
+
+/* Check that accesses based on the frame pointer do not
+   use auto increment.  */
+
+extern void foo (char*);
+void t01 (char t)
+{
+  char c = t;
+  foo (&c);
+}
+
+/* { dg-final { scan-rtl-dump-not "success" "auto_inc_dec" } } */