[llvm-dev] A strange problem about type i64 for LLVM

[mats petersson via llvm-dev]

I think this requires "complete code" (ideally the smallest possible that
still reproduces the problem), not snippets. It is quite likely that the
problem is somewhere other than in the precise lines quoted.

Using 64-bit values in x86 is far from rare, but it's of course possible
that:
1. you have found a rare bug because your code is doing something unusual.
2. there is a bug somewhere in your own code, that you haven't spotted.

The bits of code you have posted don't look wrong, but they also aren't
showing enough code to determine "yes that should work" or "no, that is
wrong because ..."

--
Mats

On 12 July 2017 at 17:24, Qiuping Yi via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hi Tim Northover,
>
> I just use Mac OS X, Intel Core i5. 'retValue' is initialized, computed,
> and then returned from 'myFunction', so I think this problem is not
> introduced by uninitialized value. In addition, '306205760' is not
> a meaningless value, but the result of truncating '140583176769504' to 32
> bits.
>
> Actually, I instrumented some call instructions for value store and load,
> which can be simplified as follow:
>
> 1. Storing values with function 'myStore':
>        call void myStore(i64 %1);
>
> 2. Loading values with function 'myLoad':
>        %0 = call i64 myLoad();
>
> The problem is that the call on 'myStore' can correctly pass 'i64 %1' to
> the function body of 'myStore'. However, when I use 'myLoad' to return the
> stored value to '%0', the 64bits value is truncated to a 32bits value. For
> example, '140583176769504' is returned in 'myLoad', but actually '306205760'
> is assigned to %0.
>
>
>
>
>
> Best regards,
>
> Qiuping Yi
> Parasol Laboratory
> Department of Computer Science and Engineering
> Texas A&M University
> College Station
> TX 77843
>
> On Wed, Jul 12, 2017 at 10:16 AM, Tim Northover <t.p.northover at gmail.com>
> wrote:
>
>> On 12 July 2017 at 11:10, Qiuping Yi via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > int64_t myFunction() {
>> >      int64_t retValue;
>> >      ...
>> >      std::cout << "retValue: " << retValue << "\n";
>> >      return retValue;
>> > }
>>
>> If retValue isn't initialized then this program has undefined
>> behaviour as C++ so all bets are off (LLVM might be propagating an
>> "undef" and simply not returning a meaningful value).
>>
>> Otherwise we need more detail. I'd expect this to work on all targets
>> (int64_t would in almost every case map to i64, and this straight
>> return should be fine, particularly if it comes from C++). So are you
>> using a custom target and does it have any quirks that might be
>> relevant? The key functions to look at would be LowerCall and
>> LowerReturn in XYZISelLowering.cpp.
>>
>> Cheers.
>>
>> Tim.
>>
>
>
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