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
