https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67602
Bug ID: 67602
Summary: unsigned left-shift overflow should not be undefined
in c++
Product: gcc
Version: 4.8.5
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: huntting at glarp dot com
Target Milestone: ---
Created attachment 36343
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36343&action=edit
example.cc
For an unsigned E1, E1<<E2 should be equal to E1*2^E2 mod 2^N (where N is the
size of E1). There is no restriction on E2 except that it must be non-negative.
Hence ~0U<<32 should always be 0 if ints are 32bits.
Unfortunately, when I compile the attached example.cc on amd64 with -O0, I get
this:
sizeof(0ULL) = 8
~0ULL = 0xffffffffffffffff
~0ULL << (8*sizeof(0ULL)) = 0xffffffffffffffff
~(~0ULL << (8*sizeof(0ULL))) = 0
sizeof(0U) = 4
~0U = 0xffffffff
~0U << (8*sizeof(0U)) = 0xffffffff
~(~0U << (8*sizeof(0U))) = 0
Curiously, enabling -O1 fixes the problem. I have also seen this behavior when
the expression is not known at compile time but I've not tested under various
-O levels in that case.
It should be noted that the C standard appears to allow for undefined behavior
when E2>=N.
