[Bug target/77759] New: internal compiler error: in function_arg_record_value

["jrtc27 at jrtc27 dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77759

            Bug ID: 77759
           Summary: internal compiler error: in function_arg_record_value
           Product: gcc
           Version: 6.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jrtc27 at jrtc27 dot com
  Target Milestone: ---

Source:

    struct empty {};

    struct pair_empty {
        struct empty a;
        struct empty b;
    };

    void f(int slot0, int slot1, int slot2, int slot3, int slot4, int slot5,
struct pair_empty pair) {
    }

    int main(int argc, char **argv) {
        struct pair_empty pair;
        f(0, 0, 0, 0, 0, 0, pair);
        return 0;
    }

With g++ 6.2.0 in Debian unstable:

    main.cpp: In function ‘void f(int, int, int, int, int, int, pair_empty)’:
    main.cpp:8:6: internal compiler error: in function_arg_record_value, at
config/sparc/sparc.c:6729
     void f(int slot0, int slot1, int slot2, int slot3, int slot4, int slot5,
struct pair_empty pair) {
          ^
    0xa3bf7f function_arg_record_value
            ../../src/gcc/config/sparc/sparc.c:6729
    0xa3c5bb sparc_function_arg_1
            ../../src/gcc/config/sparc/sparc.c:6879
    0x58ea7b assign_parm_find_entry_rtl
            ../../src/gcc/function.c:2534
    0x58ea7b assign_parms
            ../../src/gcc/function.c:3723
    0x590537 expand_function_start(tree_node*)
            ../../src/gcc/function.c:5207
    0x465243 execute
            ../../src/gcc/cfgexpand.c:6225
    Please submit a full bug report,
    with preprocessed source if appropriate.
    Please include the complete backtrace with any bug report.
    See <file:///usr/share/doc/gcc-6/README.Bugs> for instructions.

With g++ 7.0.0 20160927 from master (7469707ce8cf1d74002118cdca992650998d4acd):

    main.cpp: In function ‘void f(int, int, int, int, int, int, pair_empty)’:
    main.cpp:8:6: internal compiler error: in function_arg_record_value, at
config/sparc/sparc.c:6739
     void f(int slot0, int slot1, int slot2, int slot3, int slot4, int slot5,
struct pair_empty pair) {
          ^
    0xdd8f4f function_arg_record_value
            ../../upstream/gcc/config/sparc/sparc.c:6739
    0xdd9dbb sparc_function_arg_1
            ../../upstream/gcc/config/sparc/sparc.c:6889
    0x79fe93 assign_parm_find_entry_rtl
            ../../upstream/gcc/function.c:2532
    0x79fe93 assign_parms
            ../../upstream/gcc/function.c:3725
    0x7a2ae3 expand_function_start(tree_node*)
            ../../upstream/gcc/function.c:5209
    0x61c5cf execute
            ../../upstream/gcc/cfgexpand.c:6256
    Please submit a full bug report,
    with preprocessed source if appropriate.
    Please include the complete backtrace with any bug report.
    See <http://gcc.gnu.org/bugs.html> for instructions.

function_arg_slotno gets to the MODE_RANDOM case, and follows the
`TARGET_ARCH64 && type` branch. traverse_record_type walks over the the
pair_empty type, but as there are no actual fields (only other records which
are traversed), classify_registers is never called, so data remains { false,
false, false }. Thus, neither of the conditions for returning -1 for records
are satisfied, and so slotno (which is 6) is returned.

Then, in function_arg_record_value, it takes the `nregs == 0` branch (since
traverse_record_type never calls count_registers for the same reason as above),
tries nregs = 1, but then hits the `nregs + slotno > SPARC_INT_ARG_MAX` branch,
and so nregs gets set to `SPARC_INT_ARG_MAX - slotno`, which is 0 in this case
(and would be negative if more ints were added to f to fill up the slots).
Then, the gcc_assert(nregs > 0) fails and gives the above backtrace.

The following patch fixes this; thoughts?

--- a/gcc/config/sparc/sparc.c
+++ b/gcc/config/sparc/sparc.c
@@ -6450,10 +6450,9 @@ function_arg_slotno (const struct sparc_args *cum,
machine_mode mode,
                  && slotno >= SPARC_FP_ARG_MAX - 1)
                return -1;

-             /* If there are only int args and all int slots are filled,
-                then must pass on stack.  */
+             /* If there are only int args or this is an empty record type,
+                and all int slots are filled, then must pass on stack.  */
              if (!data.fp_regs
-                 && data.int_regs
                  && slotno >= SPARC_INT_ARG_MAX)
                return -1;
            }