[Bug middle-end/97357] New: Unable to coalesce ssa_names which are marked as MUST COALESCE.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97357

            Bug ID: 97357
           Summary: Unable to coalesce ssa_names  which are marked as MUST
                    COALESCE.
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qinzhao at gcc dot gnu.org
  Target Milestone: ---

the latest gcc10.2.1 failed with SSA corruption on multiple c modules of our
important application on O3. disabling -fsplit-loops cures the failure.

I was able to reduce the huge routine to the following simple one to reproduce
the failure:

#include <setjmp.h>
#include <stdlib.h>

static void * my_malloc (size_t size);

typedef struct glk {
  struct glk *nxt;
} glk;

typedef struct Lock
{
 glk ByteLock;
} Lock;

static Lock *l, *lk;

void bytelocks(glk *rethead, jmp_buf jb)
{
  glk *cur, *cur_lk;

  if (( _setjmp (jb)) == 0) 
    for (cur = &l->ByteLock; cur != ((glk *)0) ; cur = (cur)->nxt)
        for (cur_lk = &lk->ByteLock; cur_lk != ((glk *)0); cur_lk =
cur_lk->nxt)
          {
            glk *retrng;

            if(!rethead)
              rethead = (glk *) my_malloc (sizeof(glk));
            retrng = (glk *) my_malloc (sizeof(glk));

            retrng->nxt = rethead;
          }

 return;
}
/home/qinzhao/Install/latest/bin/gcc -O3 t.c
t.c:4:15: warning: ‘my_malloc’ used but never defined
    4 | static void * my_malloc (size_t size);
      |               ^~~~~~~~~

Unable to coalesce ssa_names 5 and 6 which are marked as MUST COALESCE.
rethead_5(ab) and  rethead_6(ab)
during RTL pass: expand
t.c: In function ‘bytelocks’:
t.c:17:6: internal compiler error: SSA corruption
   17 | void bytelocks(glk *rethead, jmp_buf jb)
      |      ^~~~~~~~~
0xbcca65 fail_abnormal_edge_coalesce
        ../../latest_gcc/gcc/tree-ssa-coalesce.c:1003
0xbcca65 coalesce_partitions
        ../../latest_gcc/gcc/tree-ssa-coalesce.c:1425
0xbcca65 coalesce_ssa_name(_var_map*)
        ../../latest_gcc/gcc/tree-ssa-coalesce.c:1755
0xb7d597 remove_ssa_form
        ../../latest_gcc/gcc/tree-outof-ssa.c:1065
0xb7d597 rewrite_out_of_ssa(ssaexpand*)
        ../../latest_gcc/gcc/tree-outof-ssa.c:1323
0x6fb5d5 execute
        ../../latest_gcc/gcc/cfgexpand.c:6352
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.