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

[ORiordan, Martin via llvm-dev]

Yes, the hairy-edges of undefined behaviour - UB is UB.

It does mean that given:

  __attribute__((noinline)) int foo(int a, int b) { return a + b; }

  int bar1(int x) { return foo(x, INT_MIN); }
  int bar2(int x) { return x + INT_MIN; }

'bar1' and 'bar2' have different outcomes.

However, I think that the new optimisation is neat and valid and I am not suggesting that it should be changed, but it would be useful if a target could override this if they choose to do so.

	MartinO

-----Original Message-----
From: Dr D. Chisnall [mailto:dc552 at hermes.cam.ac.uk] On Behalf Of David Chisnall
Sent: Tuesday, October 3, 2017 12:20 PM
To: ORiordan, Martin <martin.oriordan at intel.com>
Cc: Sanjoy Das <sanjoy at playingwithpointers.com>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Change in optimisation with UB in mind

On 3 Oct 2017, at 11:37, ORiordan, Martin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 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.

I don’t believe that this is the correct approach.  You are asking for behaviour of integer overflow in C to be well-defined as trapping (which is a valid implementation of undefined behaviour).  In LLVM, this would be represented as using the overflow-checking add intrinsic followed by a branch to a trap in the overflow case.  The back end should then lower this to a trapping add instruction.  Clang already has -ftrapv that implements almost this behaviour.

David

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