Re: Change of assumptions/behavior with -ftree-vrp in gcc 4.8+?

[Andrew Pinski <pinskia at gmail dot com>]

On Wed, Aug 3, 2016 at 12:10 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Wed, Aug 3, 2016 at 12:06 PM, Vikram Mulukutla
> <markivx@codeaurora.org> wrote:
>> Hi,
>>
>> The program listed below seems to invoke optimization behavior that produces
>> different results pre 4.8 and 4.8+ versions of gcc. Using the -fno-tree-vrp
>> option makes things consistent again. I make no claim of an understanding of
>> what this flag really does.
>>
>> The program is badly written since the first operand to && in the while loop
>> conditional will cause an invalid array access. I'm assuming that
>> optimization behavior is unpredictable when the assumption that there will
>> be no invalid array accesses is broken (and thus execution/output is
>> untrustworthy), but I am curious as to why there is a change with 4.8+. The
>> only concern I'd have is if this change may somehow affect other, correctly
>> written code.
>>
>> # gcc 4.8 produces this:
>> $ gcc-4.8 -O2 fk2.c && ./a.out
>> i after loop (array size is 5): 5
>> found at 5, answer is 0
>>
>> # pre gcc 4.8 produces this:
>> $ gcc-4.4 -O2 fk2.c && ./a.out
>> i after loop (array size is 5): 5
>> This should be printed.
>> found at 4, answer is 6
>>
>> $ gcc-4.6 -O2 fk2.c && ./a.out
>> i after loop (array size is 5): 5
>> This should be printed.
>> found at 4, answer is 6
>>
>> Program:
>>
>> #include <stdio.h>
>>
>> #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
>>
>> unsigned int numbers[] = {
>>         10,
>>         9,
>>         8,
>>         7,
>>         6,
>> };
>>
>> /* find the element closest to elem */
>> int main()
>> {
>>         unsigned int elem = 0;
>>         unsigned int i = 0;
>>
>>         while ((elem < numbers[i]) && (i < ARRAY_SIZE(numbers)))
>>                         i++;
>
>
> You are invoking undefined behavior by accessing numbers[5].


That is the defined code will never have wrong behavior happening from VRP.

Thanks,
Andrew Pinski

>
> Thanks,
> Andrew Pinski
>
>>
>>         printf("i after loop (array size is %lu): %d\n",
>> ARRAY_SIZE(numbers), i);
>>
>>         if (i == ARRAY_SIZE(numbers)) {
>>                 printf("This should be printed.\n");
>>                 i--;
>>         }
>>
>>         printf("found at %d, answer is %u\n", i, numbers[i]);
>>
>>         return 0;
>> }
>>
>> Thanks,
>> Vikram