[Bug target/67808] New: LRA ICEs on simple double to long double conversion test case

["bergner at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67808

            Bug ID: 67808
           Summary: LRA ICEs on simple double to long double conversion
                    test case
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bergner at gcc dot gnu.org
  Target Milestone: ---

During a conversion from double to long double, LRA ends up going into a
infinite loop spilling things until it hits a threshold and quits with an ICE. 
The simplest test case I have at the moment is:

bergner@genoa:~/gcc/BUGS/LRA/20011123-1$ cat bug2.i
void
foo (long double *ldb1, double *db1)
{
  *ldb1 = *db1;
}
bergner@genoa:~/gcc/BUGS/LRA/20011123-1$
/home/bergner/gcc/build/gcc-fsf-mainline-bootstrap-lra-default-debug/gcc/xgcc
-B/home/bergner/gcc/build/gcc-fsf-mainline-bootstrap-lra-default-debug/gcc/ -S
-O1 -mvsx -S bug2.i
bug2.i: In function ÃfooÃ:
bug2.i:5:1: internal compiler error: Max. number of generated reload insns per
insn is achieved (90)

 }
 ^
0x10962903 lra_constraints(bool)
       
/home/bergner/gcc/gcc-fsf-mainline-bootstrap-lra-default/gcc/lra-constraints.c:4351
0x10942af7 lra(_IO_FILE*)
        /home/bergner/gcc/gcc-fsf-mainline-bootstrap-lra-default/gcc/lra.c:2298
0x108c0ac7 do_reload
        /home/bergner/gcc/gcc-fsf-mainline-bootstrap-lra-default/gcc/ira.c:5391
0x108c1183 execute
        /home/bergner/gcc/gcc-fsf-mainline-bootstrap-lra-default/gcc/ira.c:5562


After IRA, things are pretty simple, with just the following one insn which
needs
a reload/spill, since we don't have memory to memory ops on POWER:

(insn 7 4 10 2 (parallel [
            (set (mem:TF (reg:DI 3 3 [ ldb1 ]) [0 *ldb1_5(D)+0 S16 A128])
                (float_extend:TF (mem:DF (reg:DI 4 4 [ db1 ]) [0 *db1_2(D)+0 S8
A64])))
            (use (const_double:DF 0.0 [0x0.0p+0]))
        ]) bug2.i:4 445 {*extenddftf2_internal}
     (expr_list:REG_DEAD (reg:DI 4 4 [ db1 ])
        (expr_list:REG_DEAD (reg:DI 3 3 [ ldb1 ])
            (nil))))

In LRA, comes along and gives us the following which looks good:

(insn 7 4 11 2 (parallel [
            (set (reg:TF 159)
                (float_extend:TF (mem:DF (reg:DI 4 4 [ db1 ]) [0 *db1_2(D)+0 S8
A64])))
            (use (const_double:DF 0.0 [0x0.0p+0]))
        ]) bug2.i:4 445 {*extenddftf2_internal}
     (expr_list:REG_DEAD (reg:DI 4 4 [ db1 ])
        (expr_list:REG_DEAD (reg:DI 3 3 [ ldb1 ])
            (nil))))

(insn 11 7 10 2 (set (mem:TF (reg:DI 3 3 [ ldb1 ]) [0 *ldb1_5(D)+0 S16 A128])
        (reg:TF 159)) bug2.i:4 435 {*movtf_64bit_dm}
     (nil))

But reg 159 was allocated to a VSX_REG and we don't have a TFmode mov* pattern
for vsx_reg -> fp_reg copy so we try to copy thru memory which is where we
started, so we loop forever until we hit the reload limit and ICE.