[llvm-dev] Change in optimisation with UB in mind

[ORiordan, Martin via llvm-dev]

Hi Sanjoy,

Yes these are C tests (from 'gcc.c-torture/execute'), and as I indicated in my original message, the tests are not valid because the behaviour is undefined.  However, it was while investigating these new failures in these tests that I realised that this optimisation existed.

The optimisation itself is perfectly valid, but it does mean that integer underflow will no longer be detected on systems which have hardware for this purpose.

My question is not about the validity of the optimisation - it's perfectly legitimate - but rather about whether a target can configure their TTI so as to not perform this optimisation and fall-back to using ADD instead so as to trigger integer underflow detection.

Thanks,

	MartinO

-----Original Message-----
From: Sanjoy Das [mailto:sanjoy at playingwithpointers.com] 
Sent: Tuesday, October 3, 2017 5:59 AM
To: ORiordan, Martin <martin.oriordan at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Change in optimisation with UB in mind

It isn't just about execution on the target machine -- transforming the Add to an Or makes it easier to infer that the highest bit of the result is always set later in the optimization pipeline.  Are the failing tests unit tests for a C compiler?  If so, I'd say the test cases are broken, unless they're specifically test that e.g. the compiler does not crash when compiling code like this.

-- Sanjoy

On Fri, Sep 29, 2017 at 4:34 AM, ORiordan, Martin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> With LLVM v5.0, I found failures in some of the ‘gcc.c-torture/execute’
> tests due to a change in the optimisation where undefined behaviour is 
> involved.  The tests that fail are the ‘20040409-[123].c’ tests.
>
>
>
> The underlying failure is due to the optimisation of the following:
>
>
>
>    int test2(int x) { return x + INT_MIN; }
>
>
>
> from using an ADD instruction to using an OR instruction.  The 
> optimisation is entirely valid and will produce the correct result for 
> all correct values of ‘x’, but it introduces a change for 
> architectures that have support for detecting integer overflow/underflow (signalling or quiet).
>
>
>
> For many systems the execution cost of ADD versus OR is the same, so 
> there is no real reason to choose one versus the other, and UB is UB either way.
> However, it does impact detection of out-of-range integer arithmetic 
> for systems that have support for this.
>
>
>
> Is there a new trait in the TTI, or a call-back elsewhere that allows 
> the target to choose whether it wants this optimisation to use an ADD 
> or an OR (possibly in the cost-models)?  I would generally prefer to 
> use ADD versus OR if the cost of execution is the same.
>
>
>
> Thanks,
>
>
>
>     MartinO
>
>
>
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