[llvm-dev] Which assumptions do llvm.memcpy/memmove/memset.* make when the count is 0?

Jeremy Lakeman via llvm-dev llvm-dev at lists.llvm.org
Thu Jul 20 20:46:34 PDT 2017

>From https://www.quora.com/Is-memcpy-0-0-0-undefined-behaviour-in-C

Yes, the C standard explicitly addresses this in §7.24.1/2, which applies
to memcpy and all other functions from string.h

Where an argument declared as size_t n specifies the length of the array
for a
function, n can have the value zero on a call to that function. Unless
explicitly stated otherwise in the description of a particular function in
this subclause, pointer arguments on such a call shall still have valid
values, as described in 7.1.4. On such a call, a function that locates a
character finds no occurrence, a function that compares two character
sequences returns zero, and a function that copies characters copies zero

The description of memcpy in § does not "explicitly state
otherwise", and §7.1.4 defines invalid pointer as

a pointer outside the address space of the program, or a null pointer, or a
pointer to non-modifiable storage when the corresponding parameter is not

So, the pointer arguments of memcpy *shall* (a violation of a shall clause
is UB, per §4/2) have valid values, even though the function will copy zero

On Fri, Jul 21, 2017 at 7:05 AM, Ralf Jung via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
> when I call the llvm.memcpy/memmove/memset.* intrinsics, typically I
> have to pass in valid (non-dangling, non-NULL pointers) of the given
> alignment.  However, to what extent to these rules apply when the count
> is 0?  Concretely (for any variant of the three aforementioned
> intrinsics): Is it UB to call them on a dangling pointer when count is
> 0?  On a pointer of less than the given alignment?
> The actual operation will of course not do anything, but I am worried
> about some analysis seeing a pointer being used as an argument to one of
> these intrinsics, and then assuming the pointer is valid and aligned
> without proving that the count is > 0.
> E.g., Rust's HashMap indirectly calls memset(0x0, 0, 0, ..., false).
> Vec calls memcpy(..., 0x1, 0, 4, false).  Is that a problem?
> Kind regards,
> Ralf
> PS: I'm not on the list, so please keep me in Cc.
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