llvm dev - Apr 2018 - [cfe-dev] RFC: Implementing -fno-delete-null-pointer-checks in clang

On Wed, Apr 18, 2018 at 3:54 PM Tim Northover via llvm-dev <
llvm-dev at lists.llvm.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

+1 for implementing the feature.

> 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)

Whether or not this is put into DataLayout, moving all the null-related
addrSpace != 0 checks into an accessor seems like a great idea. Besides
this feature request, presumably there's also other uses of non-zero
address spaces for which the pointer 0 could usefully be treated as
invalid...

And the name really is terrible, we should change it if at all feasible


Yep. "-fnull-pointer-is-valid" has been suggested before.
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On 4/20/18, James Y Knight wrote:
>
>
> Yep. "-fnull-pointer-is-valid" has been suggested before.
>
-fplacate-linux-kernel-developers ?

Csaba
-- 
You can get very substantial performance improvements by not doing the
right thing.
   - Scott Meyers, An Effective C++11/14 Sampler
So if you're looking for a completely portable, 100% standards-conformat way
to get the wrong information: this is what you want. - Scott Meyers
(C++TDaWYK)

On 20 Apr 2018, at 10:06, Csaba Raduly via cfe-dev <cfe-dev at
lists.llvm.org> wrote:
> 
> On 4/20/18, James Y Knight wrote:
>> 
>> 
>> Yep. "-fnull-pointer-is-valid" has been suggested before.
>> 
> 
> -fplacate-linux-kernel-developers ?
-std=vaguely-like-c99?

David

> On Apr 20, 2018, at 2:06 AM, Csaba Raduly via cfe-dev <cfe-dev at
lists.llvm.org> wrote:
> 
> On 4/20/18, James Y Knight wrote:
>> 
>> 
>> Yep. "-fnull-pointer-is-valid" has been suggested before.
>> 
> 
> -fplacate-linux-kernel-developers ?
Please, lets keep this discussion on topic and productive.  The semantics
described upstream of (the inverse of) "Assume that programs cannot safely
dereference null pointers, and that no code or data element resides at address
zero.” is entirely reasonable.

-Chris