[cfe-dev] [llvm-dev] RFC: Implementing -fno-delete-null-pointer-checks in clang

Manoj Gupta via cfe-dev cfe-dev at lists.llvm.org
Thu Apr 19 14:36:55 PDT 2018


On Thu, Apr 19, 2018 at 11:59 AM Friedman, Eli <efriedma at codeaurora.org>
wrote:

> On 4/19/2018 11:57 AM, Friedman, Eli via cfe-dev wrote:
>
> On 4/19/2018 11:48 AM, Manoj Gupta via llvm-dev wrote:
>
> On Wed, Apr 18, 2018 at 12:54 PM Tim Northover <t.p.northover at gmail.com>
> wrote:
>
>
> On Wed, Apr 18, 2018 at 12:02 PM Friedman, Eli <efriedma at codeaurora.org>
>> wrote:
>
> > Despite the name, the flag actually has rather straightforward semantics
>> > from the compiler's perspective.  From the gcc docs for
>> > -fdelete-null-pointer-checks: "Assume that programs cannot safely
>> > dereference null pointers, and that no code or data element resides at
>> > address zero."  (-fno-delete-null-pointer-checks is the opposite.)
>>
>> Ah, now that's quite a bit more palatable. I really should have read
>> the docs before spouting "my favourite rant #1". Then my main concern
>> is that we end up with a sufficiently clear (and documented)
>> definition that we're not promising more than we intend. I get very
>> grumpy if I can't tell someone with UB that they're Doing It Wrong.
>>
>> Of the two options, I still think the second is a non-starter.
>> Something between the two might be a datalayout flag specifying
>> addrspace(0) behaviour. It's pretty easy to argue that it'd be good if
>> code used some kind of
>> "DataLayout::isPointerIntrinsicallyInvalid(Value *)" for this kind of
>> thing anyway (rename or relocate at will).
>>
>> And the name really is terrible, we should change it if at all feasible
>
>
>
> On Wed, Apr 18, 2018 at 1:46 PM Jon Chesterfield via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I'm working with an embedded architecture that could, in some situations,
>> run faster if code or data could be located at address zero. I don't know
>> whether this applies to other embedded chips.
>>
>> Despite the name, the flag actually has rather straightforward semantics
>>> from the compiler's perspective.  From the gcc docs for
>>> -fdelete-null-pointer-checks: "Assume that programs cannot safely
>>> dereference null pointers, and that no code or data element resides at
>>> address zero."  (-fno-delete-null-pointer-checks is the opposite.)
>>>
>>> -Eli
>>>
>>>
>  Thanks Tim and Eli,
> I should have put the GCC description for the flag in the email.
>
> Regarding the suggestion to DataLayout, It is an interesting idea. I like
> it in particular since it will avoid creating a new llvm optimization flag
> and is auto embedded in IR.
>
> Given that, how do we want to proceed, do we want to add yet another field
> to the DataLayout string?
>
>
> Modifying the datalayout is not a good idea; it doesn't interact with LTO
> correctly, and the frontend and the backend generally need to agree on the
> datalayout.
>
> You could maybe use a module flag, if the address-space thing doesn't work
> for some reason, but we don't like to introduce more of those if we can
> avoid it.
>
>
> Actually, thinking about it a bit more, a function attribute would be
> better than a module flag.  But I'd still like to explore the address-space
> thing first, since we already have the LLVM infrastructure to make that
> work.
>
> -Eli
>

Thanks Eli,

I was looking around for the cases where AddrSpace !=0 are checked. Seems
like there are a bunch of optimizations that will fail to apply for non
zero address spaces.
So I'll start with the function attribute approach.

-Manoj
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