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