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

[Pranith Kumar <bobby.prani@gmail.com>]

On Thu, Jun 16, 2016 at 3:07 PM, Richard Henderson <rth@twiddle.net> 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 <bobby.prani@gmail.com>
>> ---
>>  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.

Thanks!
-- 
Pranith