[Bug target/71310] New: Bitfields cause load hit store with smaller store and larger load

["anton at samba dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71310

            Bug ID: 71310
           Summary: Bitfields cause load hit store with smaller store and
                    larger load
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: anton at samba dot org
  Target Milestone: ---

The following testcase built on ppc64le:

typedef unsigned char __u8;

struct sk_buff {
        void    *junk;

        __u8    pkt_type:3;
        __u8    pfmemalloc:1;
        __u8    ignore_df:1;
        __u8    nfctinfo:3;

        __u8    nf_trace:1;
        __u8    ip_summed:2;
        __u8    ooo_okay:1;
        __u8    l4_hash:1;
        __u8    sw_hash:1;
        __u8    wifi_acked_valid:1;
        __u8    wifi_acked:1;

        __u8    no_fcs:1;
        __u8    encapsulation:1;
        __u8    encap_hdr_csum:1;
        __u8    csum_valid:1;
};

int foo(struct sk_buff *skb)
{
        skb->csum_valid = 1;

        if (skb->ip_summed)
                return 1;

        return 0;
}

Results in a LHS, where the load is larger than the previous store:

        lwz 9,8(3)
        ori 9,9,0x1000
        stw 9,8(3)
        ori 2,2,0
        ld 9,8(3)
        rldicr 9,9,9,54
        rldicr 9,9,55,10
        addic 3,9,-1
        subfe 3,3,9
        blr

This may trip up the load hit store forwarding and require the store to hit the
L2 before the load can proceed.