LoongArch: Fix eh_return epilogue for normal returns.

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

@@ -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);
 
 extern bool loongarch_symbolic_constant_p (rtx, enum loongarch_symbol_type *);

gcc/config/loongarch/loongarch.cc

@@ -1015,7 +1015,8 @@ 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;
 
@@ -1024,7 +1025,14 @@ loongarch_for_each_saved_reg (HOST_WIDE_INT 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])
+	/* 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.  */
+
+	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;
@@ -1297,7 +1305,7 @@ 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 +1390,13 @@ 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 using the following insn patterns:
+   "epilogue"	      (style == NORMAL_RETURN)
+   "sibcall_epilogue" (style == SIBCALL_RETURN)
+   "eh_return"	      (style == EXCEPTION_RETURN) */
 
 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 == NORMAL_RETURN && loongarch_can_use_return_insn ())
     {
       emit_jump_insn (gen_return ());
       return;
@@ -1479,7 +1490,9 @@ 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 != EXCEPTION_RETURN);
 
   if (need_barrier_p)
     loongarch_emit_stack_tie ();
@@ -1500,11 +1513,12 @@ loongarch_expand_epilogue (bool sibcall_p)
     }
 
   /* Add in the __builtin_eh_return stack adjustment.  */
-  if (crtl->calls_eh_return)
+  if (crtl->calls_eh_return && style == EXCEPTION_RETURN)
     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 != SIBCALL_RETURN)
     emit_jump_insn (gen_simple_return_internal (ra));
 }
 

gcc/config/loongarch/loongarch.md

@@ -125,6 +125,11 @@
    (T1_REGNUM			13)
    (S0_REGNUM			23)
 
+   ;; Return path styles
+   (NORMAL_RETURN		0)
+   (SIBCALL_RETURN		1)
+   (EXCEPTION_RETURN		2)
+
    ;; PIC long branch sequences are never longer than 100 bytes.
    (MAX_PIC_BRANCH_LENGTH	100)
 ])
@@ -3276,7 +3281,7 @@
   [(const_int 2)]
   ""
 {
-  loongarch_expand_epilogue (false);
+  loongarch_expand_epilogue (NORMAL_RETURN);
   DONE;
 })
 
@@ -3284,7 +3289,7 @@
   [(const_int 2)]
   ""
 {
-  loongarch_expand_epilogue (true);
+  loongarch_expand_epilogue (SIBCALL_RETURN);
   DONE;
 })
 
@@ -3341,6 +3346,20 @@
     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 (EXCEPTION_RETURN);
   DONE;
 })
 

gcc/testsuite/gcc.target/loongarch/eh_return-normal-return.c

@@ -0,0 +1,38 @@
+/* { 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 if (t == 1)
+    /* return here */
+    return 202312;
+
+  else
+    /* never reached: prevent vrp optimization in main */
+    return 0;
+}
+
+int
+main ()
+{
+  if (foo() == 202312)
+    return 0; 
+  else
+    abort ();
+}