[Bug rtl-optimization/66391] New: suboptimal code for assignment of SImode struct with bitfields

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66391

            Bug ID: 66391
           Summary: suboptimal code for assignment of SImode struct with
                    bitfields
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bonzini at gnu dot org
  Target Milestone: ---

This is caused by early SRA splitting elem's assignment into separate per-field
assignments.

struct x {
        unsigned a : 6;
        unsigned b : 26;
};      

int f(struct x *x, unsigned a, unsigned b)
{       
        struct x elem = { .a = a, .b = b };
        int i;

        for (i = 0; i < 512; i++)
                x[i] = elem;
}       

Generated code:

.LFB0:
        .cfi_startproc
        leaq    2048(%rdi), %rcx
        andl    $63, %esi
        sall    $6, %edx
        .p2align 4,,10
        .p2align 3
.L2:
        movzbl  (%rdi), %eax
        addq    $4, %rdi
        andl    $-64, %eax
        orl     %esi, %eax
        movb    %al, -4(%rdi)
        movl    -4(%rdi), %eax
        andl    $63, %eax
        orl     %edx, %eax
        movl    %eax, -4(%rdi)
        cmpq    %rcx, %rdi
        jne     .L2
        rep ret
        .cfi_endproc