[Bug tree-optimization/53991] _mm_popcnt_u64 fails with -O3 -fgnu-tm

[torvald at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53991

--- Comment #7 from torvald at gcc dot gnu.org ---
A piece of code is tm_pure if, roughly, it doesn't need any instrumentation
(e.g., in contrast to memory loads/stores).  In the test case, I suppose that
the compiler detects that it is tm_pure, but we also allow programmers to
declare it.

Ideally, tm_pure should be a property of a region of code that is preserved
across optimizations (but where we don't move code into or out of tm_pure
regions).  That may require too much implementation effort (but perhaps we
could reuse the TM regions for that, as a "no-TM" region?)

Alternatively, we could not automatically mark always_inline functions also as
tm_pure, and warn if always_inline is also annotated as tm_pure by the
programmer.

Other thoughts?