On 09/22/2017 01:45 PM, Sylvestre Ledru wrote:
On 22/09/2017 20:27, Hal Finkel wrote:
On 09/22/2017 01:09 PM, Kaylor, Andrew wrote:
The reason I introduced this patch to begin with is that there are
circumstances under which the optimizer will eliminate loads from addresses
that were generated based on the null pointer arithmetic (because clang
previously emitted a null-based GEP and still will in the Firefox case because
it's using subtraction). It would seem that the Firefox case won't ever
dereference the pointer it is creating this way, so it should be safe from the
optimization I was seeing.
On the other hand, what the warning says is true, right? I believe clang will produce an
inbounds GEP in the Firefox case and the LLVM language reference says, "The only in
bounds address for a null pointer in the default address-space is the null pointer
itself." So it's entirely possible that some optimization will interpret the result
of the GEP generated to represent '(((char*)0)-1)' as a poison value.
-Andy
I agree. The warning seems good here. As I recall, doing pointer
arithmetic on the null pointer is UB (even if we never dereference it).
For convenience, it looks like this:
pointer arithmetic on a null pointer has undefined behavior if the offset is
nonzero [-Werror,-Wnull-pointer-arithmetic]
return net_FindCharInSet(str, NET_MAX_ADDRESS, set);
^~~~~~~~~~~~~~~
/data/jenkins/workspace/firefox-clang-last/obj-x86_64-pc-linux-gnu/dist/include/nsURLHelper.h:224:36:
note: expanded from macro 'NET_MAX_ADDRESS'
#define NET_MAX_ADDRESS (((char*)0)-1)
To be clear, I wasn't arguing!
I was just giving feedback about this new warning.
Sounds good :-)
By the way, maybe we should add that to the release notes?
https://clang.llvm.org/docs/ReleaseNotes.html
I agree. That would be a good idea.
-Hal
Sylvestre
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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