loongarch: use -mno-check-zero-division as the default for optimized code

Integer division by zero is undefined behavior anyway, and there are
already many platforms where neither the GCC port and the hardware do
anything to trap on division by zero.  So any portable program shall not
rely on SIGFPE on division by zero, in both theory and practice.  As the
result, there is no real reason to cost two additional instructions just
for the trap on division by zero with a new ISA.

One remaining reason to trap on division by zero may be debugging,
especially while -fsanitize=integer-divide-by-zero is not implemented
for LoongArch yet.  To make debugging easier, keep -mcheck-zero-division
as the default for -O0 and -Og, but use -mno-check-zero-division as the
default for all other optimization levels.

Backport this behavior change for 12.2, so 12.1 will be the only release
with a different default.

Co-authored-by: Lulu Cheng <chenglulu@loongson.cn>

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_check_zero_div_p):
	New static function.
	(loongarch_idiv_insns): Use loongarch_check_zero_div_p instead
	of TARGET_CHECK_ZERO_DIV.
	(loongarch_output_division): Likewise.
	* common/config/loongarch/loongarch-common.cc
	(TARGET_DEFAULT_TARGET_FLAGS): Remove unneeded hook.
	* doc/invoke.texi: Update to match the new behavior.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/execute/20101011-1.c (dg-additional-options):
	add -mcheck-zero-division for LoongArch targets.

(cherry picked from commit f150dc1bd1)

gcc/common/config/loongarch/loongarch-common.cc

@@ -37,7 +37,4 @@ static const struct default_options loongarch_option_optimization_table[] =
   { OPT_LEVELS_NONE, 0, NULL, 0 }
 };
 
-#undef TARGET_DEFAULT_TARGET_FLAGS
-#define TARGET_DEFAULT_TARGET_FLAGS	MASK_CHECK_ZERO_DIV
-
 struct gcc_targetm_common targetm_common = TARGETM_COMMON_INITIALIZER;

gcc/config/loongarch/loongarch.cc

@@ -2102,6 +2102,19 @@ loongarch_load_store_insns (rtx mem, rtx_insn *insn)
   return loongarch_address_insns (XEXP (mem, 0), mode, might_split_p);
 }
 
+/* Return true if we need to trap on division by zero.  */
+
+static bool
+loongarch_check_zero_div_p (void)
+{
+  /* if -m[no-]check-zero-division is given explicitly.  */
+  if (target_flags_explicit & MASK_CHECK_ZERO_DIV)
+    return TARGET_CHECK_ZERO_DIV;
+
+  /* if not, don't trap for optimized code except -Og.  */
+  return !optimize || optimize_debug;
+}
+
 /* Return the number of instructions needed for an integer division.  */
 
 int
@@ -2110,7 +2123,7 @@ loongarch_idiv_insns (machine_mode mode ATTRIBUTE_UNUSED)
   int count;
 
   count = 1;
-  if (TARGET_CHECK_ZERO_DIV)
+  if (loongarch_check_zero_div_p ())
     count += 2;
 
   return count;
@@ -4051,7 +4064,6 @@ loongarch_do_optimize_block_move_p (void)
   return !optimize_size;
 }
 
-
 /* Expand a QI or HI mode atomic memory operation.
 
    GENERATOR contains a pointer to the gen_* function that generates
@@ -5263,7 +5275,7 @@ loongarch_output_division (const char *division, rtx *operands)
   const char *s;
 
   s = division;
-  if (TARGET_CHECK_ZERO_DIV)
+  if (loongarch_check_zero_div_p ())
     {
       output_asm_insn (s, operands);
       s = "bne\t%2,%.,1f\n\tbreak\t7\n1:";

gcc/doc/invoke.texi

@@ -24527,7 +24527,8 @@ Set the cost of branches to roughly @var{n} instructions.
 @itemx -mno-check-zero-divison
 @opindex -mcheck-zero-division
 Trap (do not trap) on integer division by zero.  The default is
-@option{-mcheck-zero-division}.
+@option{-mcheck-zero-division} for @option{-O0} or @option{-Og}, and
+@option{-mno-check-zero-division} for other optimization levels.
 
 @item -mcond-move-int
 @itemx -mno-cond-move-int

gcc/testsuite/gcc.c-torture/execute/20101011-1.c

@@ -1,6 +1,7 @@
 /* { dg-options "-fnon-call-exceptions" } */
 /* With -fnon-call-exceptions 0 / 0 should not be eliminated.  */
 /* { dg-additional-options "-DSIGNAL_SUPPRESS" { target { ! signal } } } */
+/* { dg-additional-options "-mcheck-zero-division" { target { loongarch*-*-* } } } */
 
 #ifdef SIGNAL_SUPPRESS
 # define DO_TEST 0