[Bug c/110743] New: Unexpected -ftrivial-auto-var-init=pattern behavior with partial bitfields

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110743

            Bug ID: 110743
           Summary: Unexpected -ftrivial-auto-var-init=pattern behavior
                    with partial bitfields
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arnd at linaro dot org
  Target Milestone: ---

A warning showed up in Linux kernel builds with code that has a data structure
with sub-byte holes in it, making it appear as though the structure was
uninitialized even though there is no user before the intialization:

struct spi_mem_op {
  struct {
    int a : 1;
  };
  struct {
    char b : 1;
    long c;
  };
};
void spi_nor_read_any_reg(struct spi_mem_op *);
void s25fs256t_post_bfpt_fixup_nor(void) {
  struct spi_mem_op op;
  spi_nor_read_any_reg(&op);
}

$ x86_64-linux-gnu-gcc-12 -O2 -ftrivial-auto-var-init=pattern -Wuninitialized
     x86-64 gcc 12.3 (Editor #1)
x86-64 gcc 12.3 - 905ms (5966B) ~392 lines filtered

    Output of x86-64 gcc 12.3 (Compiler #1)

<source>: In function 's25fs256t_post_bfpt_fixup_nor':
<source>:12:21: warning: 'op' is used uninitialized [-Wuninitialized]
   12 |   struct spi_mem_op op;
      |                     ^~
<source>:12:21: note: 'op' declared here
   12 |   struct spi_mem_op op;
      |                     ^~

See also https://godbolt.org/z/o96GfTaaT

gcc-11 and earlier don't show this behavior because they do not support
-ftrivial-auto-var-init=pattern. I notice that in the example above, the first
16 bytes of the structure are intialized to zero with an x86 SSE instruction
since they do not contain any actual data bytes, just single bits.

This seems to be the same underlying problem as in bug #104550, but that one is
marked fixed and I cannot reproduce it on gcc-12 or gcc-13.