[Webkit-unassigned] [Bug 196533] [META] Undefined behavior bugs

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=196533

--- Comment #12 from Yusuke Suzuki <ysuzuki at apple.com> ---
(In reply to Filip Pizlo from comment #11)
> Simple question: has anyone perf tested JSC with -fwrapv?  I believe both
> clang and gcc support it.
>
> If that is perf-neutral, then we won't have to fix any more signed overflow
> bugs, ever.

I don't think this option causes performance regression.
We also need VC++ support too since we build WebKit with VC++ too.
I've found that undocumented flag `/d2UndefIntOverflow` and I think it should work[1].
Without this option, VC++ leverages UBs of signed integer overflow to aggressively optimize the code.
I agree to enabling this option now to make signed overflow well-defined.

[1]: https://devblogs.microsoft.com/cppblog/new-code-optimizer/

> 
> I'm not sure I understand the argument that says that running a sanitizer
> and fixing those bugs one-off is superior to just simply adding one compiler
> flag.

My main concern is about leaving UBs. If we can avoid UBs by either fixing UBs or removing UBs from compiler behavior, I think either is fine.
Removing UBs can be achieved by either (1) adding a flag to compiler to make some UBs well-defined or (2) fixing the spec.
If we take (1) approach, I think we also need to ensure that all our supported compilers have flags to make UBs well-defined.

> If that is successful, maybe we can just ask for an option in clang to mask
> shift amounts.  I could write an llvm pass to do that in an afternoon.

I think this pass should not cause any performance problem in x86.
I searched and learned that UBs in shift amount is because of some CPU behavior (PowerPC), and having a flag to align the behavior to x86 behavior does not cause any additional code emission in x86 I think. Maybe, it could emit masking ops for PowerPC, but we don't care the performance on PowerPC.

Anyway, I think we need a flag for this to explicitly make this type of UBs well-defined.
But..., we need to add this flag to GCC and VC++ too to remove UBs.

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