Re: [PATCH] loongarch: use -mno-check-zero-division as the default for optimized code

[Lulu Cheng <chenglulu@loongson.cn>]

在 2022/6/30 下午2:55, Richard Biener 写道:
> On Thu, Jun 30, 2022 at 5:00 AM Xi Ruoyao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> Hi,
>>
>> We've made a consensus [1] that not to enable trapping for division by
>> zero by default for LLVM, and we think GCC should behave similarly.
>>
>> The main rationales:
>>
>> 1. Division by zero is undefined behavior, so in theory any portable
>> program shall not depend on it.
>> 2. There are already many targets where both the hardware and GCC port
>> do nothing to trap on division by zero.  A list taken from
>> gcc.c-torture/execute/20101011-1.c:  PowerPC, RISC-V, ARM64, MSP430, and
>> many others.  So in practice any portable program cannot depend on this
>> trap.
>> 3. As an ICPC assistant coach, I'm well aware that the main disadvantage
>> not to trap on division by zero is "it breaks expectations of newbies".
>> So, we keep -mcheck-zero-division defaulted for -O0 and -Og.  For other
>> optimization levels, it's well known that UBs are already breaking
>> newbies' expectations [2].
>> 4. GCC is going to optimize more heavily exploiting integer division by
>> zero [3].  So let's stop encouraging people to rely on any integer
>> division by zero behavior from now.
>>
>> [1]: https://reviews.llvm.org/D128572/new/#3612039
>> [2]: http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html
>> [3]: https://gcc.gnu.org/pipermail/gcc-patches/2022-May/595099.html
>>
>> Patch content following.  Bootstrapped and regtested on loongarch64-
>> linux-gnu. Ok for trunk?
> It might be worth backporting this behavioral change to the GCC 12 branch
> as well (so 12.1 is the only release with different default behavior) and
> documenting the change in changes.html
diff --git a/gcc/common/config/loongarch/loongarch-common.cc 
b/gcc/common/config/loongarch/loongarch-common.cc
index b6cbd84b873..f8b4660fabf 100644
--- a/gcc/common/config/loongarch/loongarch-common.cc
+++ b/gcc/common/config/loongarch/loongarch-common.cc
@@ -38,7 +38,4 @@ 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
-

I think this modifications are needed, and there is no problem with the 
rest.

Thanks!

Lulu Cheng