https://bugs.llvm.org/show_bug.cgi?id=46260
Bug ID: 46260
Summary: Using builtin_assume to optimize pointer "ptr + size /
sizeof(Struct)" does not work
Product: clang
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: mv...@google.com
CC: blitzrak...@gmail.com, dgre...@apple.com,
erik.pilking...@gmail.com, llvm-bugs@lists.llvm.org,
richard-l...@metafoo.co.uk
Background: for some vector logic, I attempted to minimize the code for keeping
the distance for some 'begin' vs 'end' span.
i.e.:
void SomeFunction() {
size_type size = byte_size(begin_, end_);
begin_ = grow_or_something(begin_, ....);
end_ = add_byte_size(begin_, size);
}
auto byte_size(S* begin, S* end) {
return (end - begin) * sizeof(S);
}
auto add_byte_size(S* begin, size_t n) {
__builtin_assume((n % sizeof(S)) == 0);
return begin + n / sizeof(S);
}
The 'add_byte_size()' method refuses to do the raw add, despite my 'assume this
is a multiple of sizeof(S)'. The only think that works (reliably) is cast it
through intptr, which I'd prefer not to. (especially as the real code works
with pointer traits / pointer types making it even more verbose / boilerplate.
See examples / attempts / what works at https://gcc.godbolt.org/z/94Upor
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