[Bug tree-optimization/104600] New: VCE(vector){} should be converted (or expanded) into BIT_INSERT_EXPR

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104600

            Bug ID: 104600
           Summary: VCE<integer_type>(vector){} should be converted (or
                    expanded) into BIT_INSERT_EXPR
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

When I looked at PR 104582, I Noticed that we had:

 _14 = {_1, _5};
  _8 = VIEW_CONVERT_EXPR<__int128>(_14);

Which can be converted into (with the ordering corrected for endianness):
t1 = (__128)_1
_8 = BIT_INSERT_EXPR(t1, 64, _5);

You can see this by taking the following testcases:

#define vector __attribute__((vector_size(16)))

__int128 f(long a, long b)
{
  vector long t = {a, b};
  return (__int128)t;
}

void f1(__int128 *t1, long a, long b)
{
  vector long t = {a, b};
  *t1 =  (__int128)t;
}

void f2(__int128 *t1, long a, long b)
{
  vector long t = {a, b};
  *t1 =  ((__int128)t) + 1;
}


f2 is really bad for x86_64 as GCC does a store to the stack and then loads
back.

Note if you use | instead of +, GCC does the right thing even.