Re: [Qemu-devel] [PATCH 1/6] target/arm: Fix SVE signed division vs x86 overflow exception

[Peter Maydell]

On 29 June 2018 at 09:29, Peter Maydell <address@hidden> wrote:
> On 29 June 2018 at 01:15, Richard Henderson
> <address@hidden> wrote:
>> We already check for the same condition within the normal integer
>> sdiv and sdiv64 helpers.  Use a slightly different formation that
>> does not require deducing the expression type.
>>
>> Fixes: f97cfd596ed
>> Signed-off-by: Richard Henderson <address@hidden>
>> ---
>>  target/arm/sve_helper.c | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/target/arm/sve_helper.c b/target/arm/sve_helper.c
>> index 790cbacd14..7d7fc90566 100644
>> --- a/target/arm/sve_helper.c
>> +++ b/target/arm/sve_helper.c
>> @@ -369,7 +369,13 @@ void HELPER(NAME)(void *vd, void *vn, void *vm, void 
>> *vg, uint32_t desc) \
>>  #define DO_MIN(N, M)  ((N) >= (M) ? (M) : (N))
>>  #define DO_ABD(N, M)  ((N) >= (M) ? (N) - (M) : (M) - (N))
>>  #define DO_MUL(N, M)  (N * M)
>> -#define DO_DIV(N, M)  (M ? N / M : 0)
>> +
>> +/* The zero divisor case is architectural; the -1 divisor case works
>> + * around the x86 INT_MIN / -1 overflow exception without having to
>> + * deduce the minimum integer for the type of the expression.
>> + */
>
> It works around INT_MIN / -1 being C undefined behaviour: the
> need to special-case this is not x86-specific... The required
> answer for Arm is just as architectural as the required answer
> for division-by-zero (which is also C UB).

Suggested revised comment text:

/* We must avoid the C undefined behaviour cases: division by
 * zero and signed division of INT_MIN by -1. Both of these
 * have architecturally defined required results for Arm.
 * We special case all signed divisions by -1 to avoid having
 * to deduce the minimum integer for the type involved.
 */

?

thanks
-- PMM