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

ORiordan, Martin via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 29 04:34:25 PDT 2017

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.



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