[Bug c/84101] New: -O3 and -ftree-vectorize trying too hard for function returning trivial pair-of-uint64_t-structure

[gcc at gmch dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84101

            Bug ID: 84101
           Summary: -O3 and -ftree-vectorize trying too hard for function
                    returning trivial pair-of-uint64_t-structure
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at gmch dot uk
  Target Milestone: ---

The following:

  typedef struct uint64_pair uint64_pair_t ;
  struct uint64_pair
  {
    uint64_t  w0 ;
    uint64_t  w1 ;
  } ;

  uint64_pair_t pair(int num)
  {
    uint64_pair_t p ;

    p.w0 = num << 1 ;
    p.w1 = num >> 1 ;

    return p ;
  }

for recent x86_64, under v7.1.0, using "-O3", compiles to:

  pair:
   lea    (%rdi,%rdi,1),%eax
   sar    %edi
   movslq %edi,%rdi
   cltq   
   mov    %rax,-0x18(%rsp)
   movq   -0x18(%rsp),%xmm0
   mov    %rdi,-0x18(%rsp)
   movhps -0x18(%rsp),%xmm0
   movaps %xmm0,-0x18(%rsp)
   mov    -0x18(%rsp),%rax
   mov    -0x10(%rsp),%rdx
   retq

using "-O3 -fno-tree-vectorize", compiles to:

  pair:
   lea    (%rdi,%rdi,1),%eax
   sar    %edi
   movslq %edi,%rdx
   cltq   
   retq

I note that v6.3 produces the shorter code without the "-fno-tree-vectorize".