Re: [Qemu-devel] [PATCH] alpha: Fix build error for linux-user

[Laurent Vivier]

Le 16/06/2016 à 21:15, Pranith Kumar a écrit :
> On Thu, Jun 16, 2016 at 3:07 PM, Richard Henderson <address@hidden> wrote:
>> On 06/16/2016 11:56 AM, Pranith Kumar wrote:
>>> Using gcc 6.1 for alpha-linux-user target we see the following build
>>> error:
>>>
>>> /mnt/devops/code/qemu/target-alpha/translate.c: In function ‘in_superpage’:
>>> /mnt/devops/code/qemu/target-alpha/translate.c:454:52: error: 
>>> self-comparison always evaluates to true [-Werror=tautological-compare]
>>>              && addr >> TARGET_VIRT_ADDR_SPACE_BITS == addr >> 63);
>>>
>>> Fix it by replacing (addr >> 63) by '1' which is what it evaluates to
>>> since addr is negative.
>>>
>>> Signed-off-by: Pranith Kumar <address@hidden>
>>> ---
>>>  target-alpha/translate.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/target-alpha/translate.c b/target-alpha/translate.c
>>> index f9b2426..31da6ea 100644
>>> --- a/target-alpha/translate.c
>>> +++ b/target-alpha/translate.c
>>> @@ -451,7 +451,7 @@ static bool in_superpage(DisasContext *ctx, int64_t 
>>> addr)
>>>      return ((ctx->tb->flags & TB_FLAGS_USER_MODE) == 0
>>>              && addr < 0
>>>              && ((addr >> 41) & 3) == 2
>>> -            && addr >> TARGET_VIRT_ADDR_SPACE_BITS == addr >> 63);
>>> +            && addr >> TARGET_VIRT_ADDR_SPACE_BITS == 1);
>>>  }
>>
>> This fix is incorrect.
>>
>>   (1) addr is not always negative.
>>   (2) in_superpage is only relevant for alpha-softmmu, where
>>       TARGET_VIRT_ADDR_SPACE_BITS is not 63.
> 
> If you see line 2 of the condition you check for (addr < 0). Only if
> this evaluates to true do you check the right shift value of addr.
> 
> Reg. (2), I think that is what gcc is erroring out for.
> TARGET_VIRT_ADDR_SPACE_BITS is 63 for linux-user and we are comparing
> (addr >> 63) with itself. GCC rightly says that it is a tautological
> compare and errors out. May be we can have a better work around.

Perhaps something like:

#ifndef CONFIG_USER_ONLY
static bool in_superpage(DisasContext *ctx, int64_t addr)
{
...
}
#endif
...

#ifdef CONFIG_USER_ONLY
    pc_mask = ~TARGET_PAGE_MASK;
#else
    if (in_superpage(&ctx, pc_start)) {
        pc_mask = (1ULL << 41) - 1;
    } else {
       pc_mask = ~TARGET_PAGE_MASK;
    }
#endif

Laurent