target-mips: Fix warning about shifting negative value

With latest clang you get the following warning:

warning: shifting a negative signed value is undefined [-Wshift-negative-value]

Fix the warning by changing negative shift to inverse 0 shift.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 target-mips/translate.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

On 07/08/2016 03:15, Pranith Kumar wrote:
> With latest clang you get the following warning:
> 
> warning: shifting a negative signed value is undefined [-Wshift-negative-value]
> 
> Fix the warning by changing negative shift to inverse 0 shift.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  target-mips/translate.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/target-mips/translate.c b/target-mips/translate.c
> index bab52cb..d224de0 100644
> --- a/target-mips/translate.c
> +++ b/target-mips/translate.c
> @@ -3282,7 +3282,7 @@ static void gen_r6_muldiv(DisasContext *ctx, int opc, int rd, int rs, int rt)
>          {
>              TCGv t2 = tcg_temp_new();
>              TCGv t3 = tcg_temp_new();
> -            tcg_gen_setcondi_tl(TCG_COND_EQ, t2, t0, -1LL << 63);
> +            tcg_gen_setcondi_tl(TCG_COND_EQ, t2, t0, ~0ULL << 63);
>              tcg_gen_setcondi_tl(TCG_COND_EQ, t3, t1, -1LL);
>              tcg_gen_and_tl(t2, t2, t3);
>              tcg_gen_setcondi_tl(TCG_COND_EQ, t3, t1, 0);
> @@ -3298,7 +3298,7 @@ static void gen_r6_muldiv(DisasContext *ctx, int opc, int rd, int rs, int rt)
>          {
>              TCGv t2 = tcg_temp_new();
>              TCGv t3 = tcg_temp_new();
> -            tcg_gen_setcondi_tl(TCG_COND_EQ, t2, t0, -1LL << 63);
> +            tcg_gen_setcondi_tl(TCG_COND_EQ, t2, t0, ~0ULL << 63);
>              tcg_gen_setcondi_tl(TCG_COND_EQ, t3, t1, -1LL);
>              tcg_gen_and_tl(t2, t2, t3);
>              tcg_gen_setcondi_tl(TCG_COND_EQ, t3, t1, 0);
> @@ -3444,7 +3444,7 @@ static void gen_muldiv(DisasContext *ctx, uint32_t opc,
>          {
>              TCGv t2 = tcg_temp_new();
>              TCGv t3 = tcg_temp_new();
> -            tcg_gen_setcondi_tl(TCG_COND_EQ, t2, t0, -1LL << 63);
> +            tcg_gen_setcondi_tl(TCG_COND_EQ, t2, t0, ~0ULL << 63);
>              tcg_gen_setcondi_tl(TCG_COND_EQ, t3, t1, -1LL);
>              tcg_gen_and_tl(t2, t2, t3);
>              tcg_gen_setcondi_tl(TCG_COND_EQ, t3, t1, 0);
> @@ -3791,7 +3791,7 @@ static void gen_loongson_integer(DisasContext *ctx, uint32_t opc,
>              tcg_gen_movi_tl(cpu_gpr[rd], 0);
>              tcg_gen_br(l3);
>              gen_set_label(l1);
> -            tcg_gen_brcondi_tl(TCG_COND_NE, t0, -1LL << 63, l2);
> +            tcg_gen_brcondi_tl(TCG_COND_NE, t0, ~0ULL << 63, l2);
>              tcg_gen_brcondi_tl(TCG_COND_NE, t1, -1LL, l2);
>              tcg_gen_mov_tl(cpu_gpr[rd], t0);
>              tcg_gen_br(l3);
> @@ -3820,7 +3820,7 @@ static void gen_loongson_integer(DisasContext *ctx, uint32_t opc,
>              TCGLabel *l2 = gen_new_label();
>              TCGLabel *l3 = gen_new_label();
>              tcg_gen_brcondi_tl(TCG_COND_EQ, t1, 0, l1);
> -            tcg_gen_brcondi_tl(TCG_COND_NE, t0, -1LL << 63, l2);
> +            tcg_gen_brcondi_tl(TCG_COND_NE, t0, ~0ULL << 63, l2);
>              tcg_gen_brcondi_tl(TCG_COND_NE, t1, -1LL, l2);
>              gen_set_label(l1);
>              tcg_gen_movi_tl(cpu_gpr[rd], 0);
> 

No, please no.

clang is wrong in putting this under -Wall.

Paolo

Hi Paolo,

On Mon, Aug 8, 2016 at 5:11 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 07/08/2016 03:15, Pranith Kumar wrote:
>> With latest clang you get the following warning:
>>
>> warning: shifting a negative signed value is undefined [-Wshift-negative-value]
>>
>> Fix the warning by changing negative shift to inverse 0 shift.
>>
>
> No, please no.
>
> clang is wrong in putting this under -Wall.
>

Can you please explain why this warning is spurious? Isn't this what
the standard says?

Thanks,

On 08/08/2016 15:56, Pranith Kumar wrote:
> Hi Paolo,
> 
> On Mon, Aug 8, 2016 at 5:11 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 07/08/2016 03:15, Pranith Kumar wrote:
>>> With latest clang you get the following warning:
>>>
>>> warning: shifting a negative signed value is undefined [-Wshift-negative-value]
>>>
>>> Fix the warning by changing negative shift to inverse 0 shift.
>>>
>>
>> No, please no.
>>
>> clang is wrong in putting this under -Wall.
>>
> 
> Can you please explain why this warning is spurious? Isn't this what
> the standard says?

Yes, but GCC promises to never exploit this kind of undefined behavior.

Furthermore (though it's not the case for this patch) it's easy to get
this "fix" wrong.  For example if you change -1 << 3 to -1u << 3, and
then the value is extended to 64-bit, the results will be different.

Paolo