[PATCH v3 0/1] LoongArch: Fix eh_return epilogue for normal returns

[PATCH v3 0/1] LoongArch: Fix eh_return epilogue for normal returns

[Yang Yujie]

Updates:
v1 -> v2: Add a test case.
v2 -> v3: Fix code format.

Yang Yujie (1):
  LoongArch: Fix eh_return epilogue for normal returns

 gcc/config/loongarch/loongarch-protos.h       |  2 +-
 gcc/config/loongarch/loongarch.cc             | 41 ++++++++++++-------
 gcc/config/loongarch/loongarch.md             | 18 +++++++-
 .../loongarch/eh_return-normal-return.c       | 32 +++++++++++++++
 4 files changed, 76 insertions(+), 17 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/loongarch/eh_return-normal-return.c

-- 
2.43.0

[PATCH v3] LoongArch: Fix eh_return epilogue for normal returns

[Yang Yujie]

On LoongArch, the regitsters $r4 - $r7 (EH_RETURN_DATA_REGNO) will be saved
and restored in the function prologue and epilogue if the given function calls
__builtin_eh_return.  This causes the return value to be overwritten on normal
return paths and breaks a rare case of libgcc's _Unwind_RaiseException.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc: Do not restore the saved eh_return
	data registers ($r4-$r7) for a normal return of a function that calls
	__builtin_eh_return elsewhere.
	* config/loongarch/loongarch-protos.h: Same.
	* config/loongarch/loongarch.md: Same.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/eh_return-normal-return.c: New test.
---
 gcc/config/loongarch/loongarch-protos.h       |  2 +-
 gcc/config/loongarch/loongarch.cc             | 41 ++++++++++++-------
 gcc/config/loongarch/loongarch.md             | 18 +++++++-
 .../loongarch/eh_return-normal-return.c       | 32 +++++++++++++++
 4 files changed, 76 insertions(+), 17 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/loongarch/eh_return-normal-return.c

diff --git a/gcc/config/loongarch/loongarch-protos.h b/gcc/config/loongarch/loongarch-protos.h
index cb8fc36b086..af20b5d7132 100644
--- a/gcc/config/loongarch/loongarch-protos.h
+++ b/gcc/config/loongarch/loongarch-protos.h
@@ -60,7 +60,7 @@ enum loongarch_symbol_type {
 extern rtx loongarch_emit_move (rtx, rtx);
 extern HOST_WIDE_INT loongarch_initial_elimination_offset (int, int);
 extern void loongarch_expand_prologue (void);
-extern void loongarch_expand_epilogue (bool);
+extern void loongarch_expand_epilogue (int);
 extern bool loongarch_can_use_return_insn (void);
 \f
 extern bool loongarch_symbolic_constant_p (rtx, enum loongarch_symbol_type *);
diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
index 3545e66a10e..9c0e0dd1b73 100644
--- a/gcc/config/loongarch/loongarch.cc
+++ b/gcc/config/loongarch/loongarch.cc
@@ -1015,20 +1015,30 @@ loongarch_save_restore_reg (machine_mode mode, int regno, HOST_WIDE_INT offset,
 
 static void
 loongarch_for_each_saved_reg (HOST_WIDE_INT sp_offset,
-			      loongarch_save_restore_fn fn)
+			      loongarch_save_restore_fn fn,
+			      bool skip_eh_data_regs_p)
 {
   HOST_WIDE_INT offset;
 
   /* Save the link register and s-registers.  */
   offset = cfun->machine->frame.gp_sp_offset - sp_offset;
   for (int regno = GP_REG_FIRST; regno <= GP_REG_LAST; regno++)
-    if (BITSET_P (cfun->machine->frame.mask, regno - GP_REG_FIRST))
-      {
-	if (!cfun->machine->reg_is_wrapped_separately[regno])
-	  loongarch_save_restore_reg (word_mode, regno, offset, fn);
+    {
+      /* Special care needs to be taken for $r4-$r7 (EH_RETURN_DATA_REGNO)
+	 when returning normally from a function that calls __builtin_eh_return.
+	 In this case, these registers are saved but should not be restored,
+	 or the return value may be clobbered.  */
 
-	offset -= UNITS_PER_WORD;
-      }
+      if (BITSET_P (cfun->machine->frame.mask, regno - GP_REG_FIRST))
+	{
+	  if (!(cfun->machine->reg_is_wrapped_separately[regno]
+		|| (skip_eh_data_regs_p
+		&& GP_ARG_FIRST <= regno && regno < GP_ARG_FIRST + 4)))
+	    loongarch_save_restore_reg (word_mode, regno, offset, fn);
+
+	  offset -= UNITS_PER_WORD;
+	}
+    }
 
   /* This loop must iterate over the same space as its companion in
      loongarch_compute_frame_info.  */
@@ -1297,7 +1307,7 @@ loongarch_expand_prologue (void)
 			    GEN_INT (-step1));
       RTX_FRAME_RELATED_P (emit_insn (insn)) = 1;
       size -= step1;
-      loongarch_for_each_saved_reg (size, loongarch_save_reg);
+      loongarch_for_each_saved_reg (size, loongarch_save_reg, false);
     }
 
   /* Set up the frame pointer, if we're using one.  */
@@ -1382,11 +1392,11 @@ loongarch_can_use_return_insn (void)
   return reload_completed && cfun->machine->frame.total_size == 0;
 }
 
-/* Expand an "epilogue" or "sibcall_epilogue" pattern; SIBCALL_P
-   says which.  */
+/* Expand function epilogue for the following insn patterns:
+   "epilogue" (style == 0) / "sibcall_epilogue" (1) / "eh_return" (2).  */
 
 void
-loongarch_expand_epilogue (bool sibcall_p)
+loongarch_expand_epilogue (int style)
 {
   /* Split the frame into two.  STEP1 is the amount of stack we should
      deallocate before restoring the registers.  STEP2 is the amount we
@@ -1403,7 +1413,8 @@ loongarch_expand_epilogue (bool sibcall_p)
   bool need_barrier_p
     = (get_frame_size () + cfun->machine->frame.arg_pointer_offset) != 0;
 
-  if (!sibcall_p && loongarch_can_use_return_insn ())
+  /* Handle simple returns.  */
+  if (style == 0 && loongarch_can_use_return_insn ())
     {
       emit_jump_insn (gen_return ());
       return;
@@ -1479,7 +1490,8 @@ loongarch_expand_epilogue (bool sibcall_p)
 
   /* Restore the registers.  */
   loongarch_for_each_saved_reg (frame->total_size - step2,
-				loongarch_restore_reg);
+				loongarch_restore_reg,
+				crtl->calls_eh_return && style != 2);
 
   if (need_barrier_p)
     loongarch_emit_stack_tie ();
@@ -1504,7 +1516,8 @@ loongarch_expand_epilogue (bool sibcall_p)
     emit_insn (gen_add3_insn (stack_pointer_rtx, stack_pointer_rtx,
 			      EH_RETURN_STACKADJ_RTX));
 
-  if (!sibcall_p)
+  /* Emit return unless doing sibcall.  */
+  if (style != 1)
     emit_jump_insn (gen_simple_return_internal (ra));
 }
 
diff --git a/gcc/config/loongarch/loongarch.md b/gcc/config/loongarch/loongarch.md
index 7a101dd64b7..61b27293489 100644
--- a/gcc/config/loongarch/loongarch.md
+++ b/gcc/config/loongarch/loongarch.md
@@ -3197,7 +3197,7 @@ (define_expand "epilogue"
   [(const_int 2)]
   ""
 {
-  loongarch_expand_epilogue (false);
+  loongarch_expand_epilogue (0);
   DONE;
 })
 
@@ -3205,7 +3205,7 @@ (define_expand "sibcall_epilogue"
   [(const_int 2)]
   ""
 {
-  loongarch_expand_epilogue (true);
+  loongarch_expand_epilogue (1);
   DONE;
 })
 
@@ -3262,6 +3262,20 @@ (define_expand "eh_return"
     emit_insn (gen_eh_set_ra_di (operands[0]));
   else
     emit_insn (gen_eh_set_ra_si (operands[0]));
+
+  emit_jump_insn (gen_eh_return_internal ());
+  emit_barrier ();
+  DONE;
+})
+
+(define_insn_and_split "eh_return_internal"
+  [(eh_return)]
+  ""
+  "#"
+  "epilogue_completed"
+  [(const_int 0)]
+{
+  loongarch_expand_epilogue (2);
   DONE;
 })
 
diff --git a/gcc/testsuite/gcc.target/loongarch/eh_return-normal-return.c b/gcc/testsuite/gcc.target/loongarch/eh_return-normal-return.c
new file mode 100644
index 00000000000..da7e98a35bd
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/eh_return-normal-return.c
@@ -0,0 +1,32 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+#include <stdlib.h>
+
+int foo ()  __attribute__((noinline));
+int main ();
+
+int
+foo () {
+
+  int t;
+
+  /* prevent optimization using asm */
+  asm ("" : "=r" (t) : "0" (-1));
+  asm ("" : "=r" (t) : "0" (t ? 1 : 0));
+
+  if (t == 0)
+    /* never reached */
+    __builtin_eh_return (0, __builtin_return_address (0));
+  else
+    return 202312;
+}
+
+int
+main ()
+{
+  if (foo() == 202312)
+    return 0; 
+  else
+    abort ();
+}
-- 
2.43.0

Re: [PATCH v3] LoongArch: Fix eh_return epilogue for normal returns

[Xi Ruoyao]

On Thu, 2023-12-07 at 09:40 +0800, Yang Yujie wrote:
>  static void
>  loongarch_for_each_saved_reg (HOST_WIDE_INT sp_offset,
> -			      loongarch_save_restore_fn fn)
> +			      loongarch_save_restore_fn fn,
> +			      bool skip_eh_data_regs_p)
>  {
>    HOST_WIDE_INT offset;
>  
>    /* Save the link register and s-registers.  */
>    offset = cfun->machine->frame.gp_sp_offset - sp_offset;
>    for (int regno = GP_REG_FIRST; regno <= GP_REG_LAST; regno++)
> -    if (BITSET_P (cfun->machine->frame.mask, regno - GP_REG_FIRST))
> -      {
> -	if (!cfun->machine->reg_is_wrapped_separately[regno])
> -	  loongarch_save_restore_reg (word_mode, regno, offset, fn);
> +    {
> +      /* Special care needs to be taken for $r4-$r7 (EH_RETURN_DATA_REGNO)
> +	 when returning normally from a function that calls __builtin_eh_return.
> +	 In this case, these registers are saved but should not be restored,
> +	 or the return value may be clobbered.  */
>  
> -	offset -= UNITS_PER_WORD;
> -      }
> +      if (BITSET_P (cfun->machine->frame.mask, regno - GP_REG_FIRST))
> +	{
> +	  if (!(cfun->machine->reg_is_wrapped_separately[regno]
> +		|| (skip_eh_data_regs_p
> +		&& GP_ARG_FIRST <= regno && regno < GP_ARG_FIRST + 4)))
> +	    loongarch_save_restore_reg (word_mode, regno, offset, fn);
> +
> +	  offset -= UNITS_PER_WORD;
> +	}
> +    }

I don't like this pair of {} for the for statement.  It's not necessary
and it changes the indent level, causing the diff hard to review.

Otherwise LGTM.  I'm not sure why I didn't notice the eh_return issue
when I learnt shrink wrapping from RISC-V...

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University

Re: [PATCH v3] LoongArch: Fix eh_return epilogue for normal returns

[Yang Yujie]

On Thu, Dec 07, 2023 at 11:02:58AM +0800, Xi Ruoyao wrote:
> 
> I don't like this pair of {} for the for statement.  It's not necessary
> and it changes the indent level, causing the diff hard to review.
> 
> Otherwise LGTM.  I'm not sure why I didn't notice the eh_return issue
> when I learnt shrink wrapping from RISC-V...
>

Thanks for the review!  This problem on LoongArch was first noticed in a
failed libphobos test case, and the fix is partially borrowed from i386,
which seemed to be the only architecture without this issue.

So despite the extra braces (which I'd say I prefer to have because of the
new block of comment inserted on top of the if statement :P), I am going to
ask Lulu for pushing this.

Yujie

Re: [PATCH v3] LoongArch: Fix eh_return epilogue for normal returns

[Xi Ruoyao]

On Thu, 2023-12-07 at 14:18 +0800, Yang Yujie wrote:
> On Thu, Dec 07, 2023 at 11:02:58AM +0800, Xi Ruoyao wrote:
> > 
> > I don't like this pair of {} for the for statement.  It's not necessary
> > and it changes the indent level, causing the diff hard to review.
> > 
> > Otherwise LGTM.  I'm not sure why I didn't notice the eh_return issue
> > when I learnt shrink wrapping from RISC-V...
> > 
> 
> Thanks for the review!  This problem on LoongArch was first noticed in a
> failed libphobos test case, and the fix is partially borrowed from i386,
> which seemed to be the only architecture without this issue.
> 
> So despite the extra braces (which I'd say I prefer to have because of the
> new block of comment inserted on top of the if statement :P), I am going to
> ask Lulu for pushing this.

I understand and I don't think adding {} is wrong.  The problem is the
indent change causes a large chunk of diff and it makes reviewing more
difficult.  Thus generally we should not mix real code change and format
change in a commit.

i. e. it would be better to separate it into two patches, the first adds
{} and changes the indent, and the second changes the logic.  But now I
don't think it's needed to make a V4, just pushing this should be fine.


-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University

Re: [PATCH v3] LoongArch: Fix eh_return epilogue for normal returns

[Yang Yujie]

On Thu, Dec 07, 2023 at 04:13:51PM +0800, Xi Ruoyao wrote:
> 
> I understand and I don't think adding {} is wrong.  The problem is the
> indent change causes a large chunk of diff and it makes reviewing more
> difficult.  Thus generally we should not mix real code change and format
> change in a commit.
> 
> i. e. it would be better to separate it into two patches, the first adds
> {} and changes the indent, and the second changes the logic.  But now I
> don't think it's needed to make a V4, just pushing this should be fine.
> 

Thanks for the suggestion.  It is clearer that way.

Yujie