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Subject: Re: [PATCH] LoongArch: fix signed overflow in
 loongarch_emit_int_compare
To: Xi Ruoyao <xry111@xry111.site>, gcc-patches@gcc.gnu.org
Cc: Wang Xuerui <i@xen0n.name>, Chenghua Xu <xuchenghua@loongson.cn>
References: <20221104063942.1594844-1-xry111@xry111.site>
From: Lulu Cheng <chenglulu@loongson.cn>
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Date: Sun, 6 Nov 2022 09:46:16 +0800
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在 2022/11/4 下午2:39, Xi Ruoyao 写道:
> Signed overflow is an undefined behavior, so we need to prevent it from
> happening, instead of "checking" the result.
>
> gcc/ChangeLog:
>
> 	* config/loongarch/loongarch.cc (loongarch_emit_int_compare):
> 	Avoid signed overflow.
> ---
>
> Bootstrapped and regtested on loongarch64-linux-gnu.  OK for trunk?
>
>   gcc/config/loongarch/loongarch.cc | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/gcc/config/loongarch/loongarch.cc b/gcc/config/loongarch/loongarch.cc
> index f54c233f90c..032fb1474c7 100644
> --- a/gcc/config/loongarch/loongarch.cc
> +++ b/gcc/config/loongarch/loongarch.cc
> @@ -4175,13 +4175,14 @@ loongarch_emit_int_compare (enum rtx_code *code, rtx *op0, rtx *op1)
>   	      HOST_WIDE_INT new_rhs;
>   	      bool increment = *code == mag_comparisons[i][0];
>   	      bool decrement = *code == mag_comparisons[i][1];
> -	      if (!increment && !decrement)
> +	      if ((!increment && !decrement)
> +		  || (increment && rhs == HOST_WIDE_INT_MAX)
> +		  || (decrement && rhs == HOST_WIDE_INT_MIN))
>   		continue;

I think it should be here:

              if (!increment && !decrement)
                 continue;

+             if ((increment && rhs == HOST_WIDE_INT_MAX)
+                 || (decrement && rhs == HOST_WIDE_INT_MIN))
+               break;
+

It is not necessary to continue when *code matches one of 
mag_comparisons[i].

>   
>   	      new_rhs = rhs + (increment ? 1 : -1);
>   	      if (loongarch_integer_cost (new_rhs)
> -		    < loongarch_integer_cost (rhs)
> -		  && (rhs < 0) == (new_rhs < 0))
> +		    < loongarch_integer_cost (rhs))
>   		{
>   		  *op1 = GEN_INT (new_rhs);
>   		  *code = mag_comparisons[i][increment];