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

Sanjoy Das via llvm-dev
Tue, 03 Oct 2017 04:58:52 -0000

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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