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2017 Nov 08
2
Is it ok to allocate > half of address space?
...8888888
%b = gep inbounds %a, 1
If %a is within bounds, the "gep inbounds" gives a signed overflow even
though it's just a +1 (since 0x88888888 + 1 overflows).
So GEP inbounds disables large objects outright.
BTW I've always wondered why EmitGEPOffset
(http://llvm.org/doxygen/Local_8h_source.html#l00247) doesn't use 'add nsw'
if the semantics of GEP inbounds allows that (if my reading of LangRef is
correct).
> Assuming we're not dealing with inbounds GEPs (e.g. you pass -fwrapv to
> clang), I don't see any particular reason to disallow allocations more...
2017 Nov 08
5
Is it ok to allocate > half of address space?
Hi,
I was looking into the semantics of GEP inbounds and some BasicAA
rules and I'm wondering if it's valid in LLVM IR to allocate more than
half of the address space with a global variable or an alloca.
If that's a scenario want to consider, then we have problems :)
Consider this C code (32 bits):
#include <string.h>
char obj[0x80000008];
char f() {
char *p = obj +