[PATCH] Avoid negative bitpos in expand_expr_real_1 (PR 86984)

[Jeff Law]

On 08/20/2018 07:28 AM, Richard Biener wrote:
> On Mon, 20 Aug 2018, Bernd Edlinger wrote:
> 
>> On 08/20/18 12:41, Richard Biener wrote:
>>> On Sun, 19 Aug 2018, Bernd Edlinger wrote:
>>>
>>>> Hi!
>>>>
>>>>
>>>> This fixes a wrong code issue in expand_expr_real_1 which happens because
>>>> a negative bitpos is actually able to reach extract_bit_field which
>>>> does all computations with poly_uint64, thus the offset 0x1ffffffffffffff0.
>>>>
>>>> To avoid that I propose to use Jakub's r204444 patch from the expand_assignment
>>>> also in the expand_expr_real_1.
>>>>
>>>> This is a rather unlikely thing to happen, as there are lots of checks that are
>>>> of course all target dependent between the get_inner_reference and the
>>>> actual extract_bit_field call, and all other code paths may or may not have a problem
>>>> with negative bit offsets.  Most don't have a problem, but the bitpos needs to be
>>>> folded into offset before it is used, therefore it is necessary to handle the negative
>>>> bitpos very far away from the extract_bit_field call.  Doing that later is IMHO not
>>>> possible.
>>>>
>>>> The fix in CONSTANT_ADDRESS_P is actually unrelated, and I only spotted it because
>>>> this macro is used in alpha_legitimize_address which is of course what one looks
>>>> at first if something like that happens.
>>>>
>>>> I think even with this bogus offset it should not have caused a linker error, so there
>>>> is probably a second problem in the *movdi code pattern of the alpha.md, because it
>>>> should be split into instructions that don't cause a link error.
>>>>
>>>> Once the target is fixed to split the impossible assembler instruction, the test case
>>>> will probably no longer be able to detect the pattern in the assembly.
>>>>
>>>> Therefore the test case looks both at the assembler output and the expand rtl dump
>>>> to spot the bogus offset.  I only check the first 12 digits of the bogus constant,
>>>> because it is actually dependent on the target configuration:
>>>>
>>>> I built first a cross-compiler without binutils, and it did used a slightly different
>>>> offset of 0x2000000000000000, (configured with: --target=alpha-linux-gnu --enable-languages=c
>>>> --disable-libgcc --disable-libssp --disable-libquadmath --disable-libgomp --disable-libatomic)
>>>> when the binutils are installed at where --prefix points, the offset is 0x1ffffffffffffff0.
>>>>
>>>> Regarding the alpha target, I could not do more than build a cross compiler and run
>>>> make check-gcc-c RUNTESTFLAGS="alpha.exp=pr86984.c".
>>>>
>>>>
>>>> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
>>>> Is it OK for trunk?
>>>
>>> Hmm, I don't remember why we are inconsistent in get_inner_reference
>>> with respect to negative bitpos.  I think we should be consistent
>>> here and may not be only by accident?  That is, does
>>>
>>> diff --git a/gcc/expr.c b/gcc/expr.c
>>> index c071be67783..9dc78587136 100644
>>> --- a/gcc/expr.c
>>> +++ b/gcc/expr.c
>>> @@ -7272,7 +7272,7 @@ get_inner_reference (tree exp, poly_int64_pod
>>> *pbitsize,
>>>                                        TYPE_PRECISION (sizetype));
>>>         tem <<= LOG2_BITS_PER_UNIT;
>>>         tem += bit_offset;
>>> -      if (tem.to_shwi (pbitpos))
>>> +      if (tem.to_shwi (pbitpos) && !maybe_lt (*pbitpos, 0))
>>>          *poffset = offset = NULL_TREE;
>>>       }
>>>   
>>> fix the issue?
>>>
>>
>> Yes, at first sight, however, I was involved at PR 58970,
>> see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58970
>>
>> and I proposed a similar patch, which was objected by Jakub:
>>
>> see comment #25 of PR 58970:
>> "Jakub Jelinek 2013-11-05 19:41:12 UTC
>>
>> (In reply to Bernd Edlinger from comment #24)
>>> Created attachment 31169 [details] https://gcc.gnu.org/bugzilla/attachment.cgi?id=31169
>>> Another (better) attempt at fixing this ICE.
>>>
>>> This avoids any negative bitpos from get_inner_reference.
>>> These negative bitpos values are just _very_ dangerous all the
>>> way down to expmed.c
>>
>> I disagree that it is better, you are forgetting get_inner_reference has other > 20 callers outside of expansion and there is no reason why negative bitpos would be a problem in those cases."
>>
>> So that is what Jakub said at that time, and with the
>> suggested change in get_inner_reference,
>> this part of the r204444 change would be effectively become superfluous:
>>
>> @@ -4721,6 +4721,15 @@ expand_assignment (tree to, tree from, bool nontem
>>         tem = get_inner_reference (to, &bitsize, &bitpos, &offset, &mode1,
>>                                   &unsignedp, &volatilep, true);
>>   
>> +      /* Make sure bitpos is not negative, it can wreak havoc later.  */
>> +      if (bitpos < 0)
>> +       {
>> +         gcc_assert (offset == NULL_TREE);
>> +         offset = size_int (bitpos >> (BITS_PER_UNIT == 8
>> +                                       ? 3 : exact_log2 (BITS_PER_UNIT)));
>> +         bitpos &= BITS_PER_UNIT - 1;
>> +       }
>> +
>>         if (TREE_CODE (to) == COMPONENT_REF
>>            && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1)))
>>          get_bit_range (&bitregion_start, &bitregion_end, to, &bitpos, &offset);
>>
>> and should be reverted.  I did not really like it then, but I'd respect Jakub's advice.
> 
> Hmm.  I agree that there are other callers and yes, replicating Jakubs
> fix is certainly more safe.  Still it's somewhat inconsistent in that
> get_inner_reference already has code to mitigate negative bitpos, but
> only in the case 'offset' wasn't just an INTEGER_CST ...
> 
> So your patch is OK (please change the gcc_asserts to 
> gcc_checking_asserts though to avoid ICEing for release compilers).
> 
> We still might want to revisit get_inner_reference (and make its
> bitpos unsigned then!)
Given this is keeping my tester from running on alpha, I'm going to make
the adjustment to Bernd's patch and commit it momentarily.

jeff