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

[Friedman, Eli via cfe-dev]

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 <mailto:t.p.northover at gmail.com>> wrote:
>>
>>
>>     On Wed, Apr 18, 2018 at 12:02 PM Friedman, Eli
>>     <efriedma at codeaurora.org <mailto: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 <mailto: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

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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