[Bug c++/96252] New: mis-optimization where identical functions have very different codegen since gcc 10

["wjwray at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96252

            Bug ID: 96252
           Summary: mis-optimization where identical functions have very
                    different codegen since gcc 10
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wjwray at gmail dot com
  Target Milestone: ---

(Stumbled on this odd effect while examining codegen for operator<=>)

The reduced sample compiles c++11 and up (std version likely irrelevant)
with the different codegen occurring since GCC 10.

Two identical functions:

  bool cmp_x(cmp l, cmp r) noexcept {
      return std::lexicographical_compare(begin(l),end(l)
                                         ,begin(r),end(r)); }
  bool cmp_y(cmp l, cmp r) noexcept {
      return std::lexicographical_compare(begin(l),end(l)
                                         ,begin(r),end(r)); }

generate very different code for cmp = array<int,64> at -O2 and -O3.

The first is looping, the second has much longer unrolled codegen.
My benchmarks show 40% difference in runtime, quick-bench shows 30%. 

Compiler Explorer https://godbolt.org/z/97box6
Quick-bench 1.3x https://quick-bench.com/q/480qkw1sP4OWOH6JBxsm-J_9uOk

(Adding -fno-inline may provide a clue.)