Re: [PATCH][AArch64 - v3] Simplify eh_return implementation

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

ping


From: Wilco Dijkstra
Sent: 02 September 2016 12:31
To: Ramana Radhakrishnan; GCC Patches
Cc: nd
Subject: Re: [PATCH][AArch64 - v3] Simplify eh_return implementation
    
Ramana Radhakrishnan wrote:
> Can you please file a PR for this and add some testcases ?  This sounds like a serious enough problem that needs to be looked at probably going back since the dawn of time.

I've created PR77455. Updated patch below:

This patch simplifies the handling of the EH return value.  We force the use of the
frame pointer so the return location is always at FP + 8.  This means we can emit
a simple volatile access in EH_RETURN_HANDLER_RTX without needing md
patterns, splitters and frame offset calculations.  The new implementation also
fixes various bugs in aarch64_final_eh_return_addr, which does not work with
-fomit-frame-pointer, alloca or outgoing arguments.

Bootstrap OK, GCC Regression OK, OK for trunk? Would it be useful to backport
this to GCC6.x?

ChangeLog:

2016-09-02  Wilco Dijkstra  <wdijkstr@arm.com>

        PR77455
gcc/
        * config/aarch64/aarch64.md (eh_return): Remove pattern and splitter.
        * config/aarch64/aarch64.h (AARCH64_EH_STACKADJ_REGNUM): Remove.
        (EH_RETURN_HANDLER_RTX): New define.
        * config/aarch64/aarch64.c (aarch64_frame_pointer_required):
        Force frame pointer in EH return functions.
        (aarch64_expand_epilogue): Add barrier for eh_return.
        (aarch64_final_eh_return_addr): Remove.
        (aarch64_eh_return_handler_rtx): New function.
        * config/aarch64/aarch64-protos.h (aarch64_final_eh_return_addr):
        Remove.
        (aarch64_eh_return_handler_rtx): New prototype.

testsuite/
        * gcc.target/aarch64/eh_return.c: New test.
--
diff --git a/gcc/config/aarch64/aarch64-protos.h b/gcc/config/aarch64/aarch64-protos.h
index 3cdd69b8af1089a839e5d45cda94bc70a15cd777..327c0a97f6f687604afef249b79ac22628418070 100644
--- a/gcc/config/aarch64/aarch64-protos.h
+++ b/gcc/config/aarch64/aarch64-protos.h
@@ -358,7 +358,7 @@ int aarch64_hard_regno_mode_ok (unsigned, machine_mode);
 int aarch64_hard_regno_nregs (unsigned, machine_mode);
 int aarch64_uxt_size (int, HOST_WIDE_INT);
 int aarch64_vec_fpconst_pow_of_2 (rtx);
-rtx aarch64_final_eh_return_addr (void);
+rtx aarch64_eh_return_handler_rtx (void);
 rtx aarch64_mask_from_zextract_ops (rtx, rtx);
 const char *aarch64_output_move_struct (rtx *operands);
 rtx aarch64_return_addr (int, rtx);
diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index 003fec87e41db618570663f28cc2387a87e8252a..fa81e4b853dafcccc08842955288861ec7e7acca 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -400,9 +400,9 @@ extern unsigned aarch64_architecture_version;
 #define ASM_DECLARE_FUNCTION_NAME(STR, NAME, DECL)      \
   aarch64_declare_function_name (STR, NAME, DECL)
 
-/* The register that holds the return address in exception handlers.  */
-#define AARCH64_EH_STACKADJ_REGNUM     (R0_REGNUM + 4)
-#define EH_RETURN_STACKADJ_RTX gen_rtx_REG (Pmode, AARCH64_EH_STACKADJ_REGNUM)
+/* For EH returns X4 contains the stack adjustment.  */
+#define EH_RETURN_STACKADJ_RTX gen_rtx_REG (Pmode, R4_REGNUM)
+#define EH_RETURN_HANDLER_RTX  aarch64_eh_return_handler_rtx ()
 
 /* Don't use __builtin_setjmp until we've defined it.  */
 #undef DONT_USE_BUILTIN_SETJMP
diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index e742c19d76e6c62117aa62a990b9c2945aa06b74..f07d771ea343803e054e03f59c8c1efb698bf474 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -2739,6 +2739,10 @@ aarch64_frame_pointer_required (void)
       && (!crtl->is_leaf || df_regs_ever_live_p (LR_REGNUM)))
     return true;
 
+  /* Force a frame pointer for EH returns so the return address is at FP+8.  */
+  if (crtl->calls_eh_return)
+    return true;
+
   return false;
 }
 
@@ -3298,7 +3302,8 @@ aarch64_expand_epilogue (bool for_sibcall)
                          + cfun->machine->frame.saved_varargs_size) != 0;
 
   /* Emit a barrier to prevent loads from a deallocated stack.  */
-  if (final_adjust > crtl->outgoing_args_size || cfun->calls_alloca)
+  if (final_adjust > crtl->outgoing_args_size || cfun->calls_alloca
+      || crtl->calls_eh_return)
     {
       emit_insn (gen_stack_tie (stack_pointer_rtx, stack_pointer_rtx));
       need_barrier_p = false;
@@ -3366,52 +3371,15 @@ aarch64_expand_epilogue (bool for_sibcall)
     emit_jump_insn (ret_rtx);
 }
 
-/* Return the place to copy the exception unwinding return address to.
-   This will probably be a stack slot, but could (in theory be the
-   return register).  */
+/* Implement EH_RETURN_HANDLER_RTX.  The return address is stored at FP + 8.
+   The access needs to be volatile to prevent it from being removed.  */
 rtx
-aarch64_final_eh_return_addr (void)
+aarch64_eh_return_handler_rtx (void)
 {
-  HOST_WIDE_INT fp_offset;
-
-  aarch64_layout_frame ();
-
-  fp_offset = cfun->machine->frame.frame_size
-             - cfun->machine->frame.hard_fp_offset;
-
-  if (cfun->machine->frame.reg_offset[LR_REGNUM] < 0)
-    return gen_rtx_REG (DImode, LR_REGNUM);
-
-  /* DSE and CSELIB do not detect an alias between sp+k1 and fp+k2.  This can
-     result in a store to save LR introduced by builtin_eh_return () being
-     incorrectly deleted because the alias is not detected.
-     So in the calculation of the address to copy the exception unwinding
-     return address to, we note 2 cases.
-     If FP is needed and the fp_offset is 0, it means that SP = FP and hence
-     we return a SP-relative location since all the addresses are SP-relative
-     in this case.  This prevents the store from being optimized away.
-     If the fp_offset is not 0, then the addresses will be FP-relative and
-     therefore we return a FP-relative location.  */
-
-  if (frame_pointer_needed)
-    {
-      if (fp_offset)
-        return gen_frame_mem (DImode,
-                             plus_constant (Pmode, hard_frame_pointer_rtx, UNITS_PER_WORD));
-      else
-        return gen_frame_mem (DImode,
-                             plus_constant (Pmode, stack_pointer_rtx, UNITS_PER_WORD));
-    }
-
-  /* If FP is not needed, we calculate the location of LR, which would be
-     at the top of the saved registers block.  */
-
-  return gen_frame_mem (DImode,
-                       plus_constant (Pmode,
-                                      stack_pointer_rtx,
-                                      fp_offset
-                                      + cfun->machine->frame.saved_regs_size
-                                      - 2 * UNITS_PER_WORD));
+  rtx tmp = gen_frame_mem (Pmode,
+    plus_constant (Pmode, hard_frame_pointer_rtx, UNITS_PER_WORD));
+  MEM_VOLATILE_P (tmp) = true;
+  return tmp;
 }
 
 /* Output code to add DELTA to the first argument, and then jump
diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index 21f5a6aba74d28f04b9391ba917453a4cd7de1af..7d86aa7c0cb2fdb30889badc172d2270eeadb1e5 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -591,25 +591,6 @@
   [(set_attr "type" "branch")]
 )
 
-(define_insn "eh_return"
-  [(unspec_volatile [(match_operand:DI 0 "register_operand" "r")]
-    UNSPECV_EH_RETURN)]
-  ""
-  "#"
-  [(set_attr "type" "branch")]
-
-)
-
-(define_split
-  [(unspec_volatile [(match_operand:DI 0 "register_operand" "")]
-    UNSPECV_EH_RETURN)]
-  "reload_completed"
-  [(set (match_dup 1) (match_dup 0))]
-  {
-    operands[1] = aarch64_final_eh_return_addr ();
-  }
-)
-
 (define_insn "*cb<optab><mode>1"
   [(set (pc) (if_then_else (EQL (match_operand:GPI 0 "register_operand" "r")
                                 (const_int 0))
diff --git a/gcc/testsuite/gcc.target/aarch64/eh_return.c b/gcc/testsuite/gcc.target/aarch64/eh_return.c
new file mode 100644
index 0000000000000000000000000000000000000000..32179488085ed4c84aa77007565c875d09c4197c
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/eh_return.c
@@ -0,0 +1,82 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fno-inline" } */
+
+#include <stdlib.h>
+#include <stdio.h>
+
+int val, test, failed;
+
+int main (void);
+
+void
+eh0 (void *p)
+{
+  val = (int)(long)p & 7;
+  if (val)
+    abort ();
+}
+
+void
+eh1 (void *p, int x)
+{
+  void *q = __builtin_alloca (x);
+  eh0 (q);
+  __builtin_eh_return (0, p);
+}
+
+void
+eh2a (int a,int b,int c,int d,int e,int f,int g,int h, void *p)
+{
+  val = a + b + c + d + e + f + g + h +  (int)(long)p & 7;
+}
+
+void
+eh2 (void *p)
+{
+  eh2a (val, val, val, val, val, val, val, val, p);
+  __builtin_eh_return (0, p);
+}
+
+
+void
+continuation (void)
+{
+  test++;
+  main ();
+}
+
+void
+fail (void)
+{
+  failed = 1;
+  printf ("failed\n");
+  continuation ();
+}
+
+void
+do_test1 (void)
+{
+  if (!val)
+    eh1 (continuation, 100);
+  fail ();
+}
+
+void
+do_test2 (void)
+{
+  if (!val)
+    eh2 (continuation);
+  fail ();
+}
+
+int
+main (void)
+{
+  if (test == 0)
+    do_test1 ();
+  if (test == 1)
+    do_test2 ();
+  if (failed || test != 2)
+    exit (1);
+  exit (0);
+}