void string_of_byte(char s[], const char c) {
  for (int i = 0; i < 8; ++i)
    s[7 - i] = (c & (1 << i)) ? '1' : '0';
  s[8] = '\0';
}


x86 assembly (clang, -O0):
<string_of_byte>:
push   %rbp
mov    %rsp,%rbp
mov    %sil,%al
mov    %rdi,-0x8(%rbp)
mov    %al,-0x9(%rbp)
movl   $0x0,-0x10(%rbp)
cmpl   $0x8,-0x10(%rbp)
jge    1161 <string_of_byte+0x61>
movsbl -0x9(%rbp),%edx
mov    -0x10(%rbp),%ecx
mov    $0x1,%eax
shl    %cl,%eax
and    %eax,%edx
mov    $0x30,%eax
mov    $0x31,%ecx
cmp    $0x0,%edx
cmovne %ecx,%eax
mov    %al,%dl
mov    -0x8(%rbp),%rax
mov    $0x7,%ecx
sub    -0x10(%rbp),%ecx
movslq %ecx,%rcx
mov    %dl,(%rax,%rcx,1)
mov    -0x10(%rbp),%eax
add    $0x1,%eax
mov    %eax,-0x10(%rbp)
jmp    1115 <string_of_byte+0x15>
mov    -0x8(%rbp),%rax
movb   $0x0,0x8(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<string_of_byte>:
movd   %esi,%xmm0
pshufd $0x0,%xmm0,%xmm0
movdqa 0xeef(%rip),%xmm1
pand   %xmm0,%xmm1
pand   0xef3(%rip),%xmm0
pxor   %xmm2,%xmm2
pcmpeqd %xmm2,%xmm0
pcmpeqd %xmm2,%xmm1
packssdw %xmm0,%xmm1
packsswb %xmm1,%xmm1
pmovmskb %xmm1,%esi
mov    %esi,%r11d
and    $0x2,%r11b
shr    %r11b
mov    %esi,%r8d
and    $0x4,%r8b
shr    $0x2,%r8b
mov    %esi,%r9d
and    $0x8,%r9b
shr    $0x3,%r9b
mov    %esi,%r10d
and    $0x10,%r10b
shr    $0x4,%r10b
mov    %esi,%ecx
and    $0x20,%cl
shr    $0x5,%cl
mov    %esi,%edx
and    $0x40,%dl
shr    $0x6,%dl
mov    %esi,%eax
sar    $0x7,%al
add    $0x31,%al
mov    %al,0x7(%rdi)
xor    $0x31,%dl
mov    %dl,0x6(%rdi)
xor    $0x31,%cl
mov    %cl,0x5(%rdi)
xor    $0x31,%r10b
mov    %r10b,0x4(%rdi)
xor    $0x31,%r9b
mov    %r9b,0x3(%rdi)
xor    $0x31,%r8b
mov    %r8b,0x2(%rdi)
xor    $0x31,%r11b
mov    %r11b,0x1(%rdi)
and    $0x1,%sil
xor    $0x31,%sil
mov    %sil,(%rdi)
movb   $0x0,0x8(%rdi)
ret

LLVM IR (-O0):
define void @string_of_byte(i8* %0, i8 signext %1) {
%3 = alloca i8*
%4 = alloca i8
%5 = alloca i32
store i8* %0, i8** %3
store i8 %1, i8* %4
store i32 0, i32* %5
br label %6
6: ; preds = %24, %2
%7 = load i32, i32* %5
%8 = icmp slt i32 %7, 8
br i1 %8, label %9, label %27
9: ; preds = %6
%10 = load i8, i8* %4
%11 = sext i8 %10 to i32
%12 = load i32, i32* %5
%13 = shl i32 1, %12
%14 = and i32 %11, %13
%15 = icmp ne i32 %14, 0
%16 = zext i1 %15 to i64
%17 = select i1 %15, i32 49, i32 48
%18 = trunc i32 %17 to i8
%19 = load i8*, i8** %3
%20 = load i32, i32* %5
%21 = sub nsw i32 7, %20
%22 = sext i32 %21 to i64
%23 = getelementptr inbounds i8, i8* %19, i64 %22
store i8 %18, i8* %23
br label %24
24: ; preds = %9
%25 = load i32, i32* %5
%26 = add nsw i32 %25, 1
store i32 %26, i32* %5
br label %6
27: ; preds = %6
%28 = load i8*, i8** %3
%29 = getelementptr inbounds i8, i8* %28, i64 8
store i8 0, i8* %29
ret void
}

LLVM IR (-O2):
define void @string_of_byte(i8* nocapture writeonly %0, i8 signext %1) {
%3 = zext i8 %1 to i32
%4 = insertelement <8 x i32> poison, i32 %3, i64 0
%5 = shufflevector <8 x i32> %4, <8 x i32> poison, <8 x i32> zeroinitializer
%6 = and <8 x i32> %5, <i32 128, i32 64, i32 32, i32 16, i32 8, i32 4, i32 2, i32 1>
%7 = icmp eq <8 x i32> %6, zeroinitializer
%8 = extractelement <8 x i1> %7, i64 7
%9 = select i1 %8, i8 48, i8 49
%10 = getelementptr inbounds i8, i8* %0, i64 7
store i8 %9, i8* %10
%11 = extractelement <8 x i1> %7, i64 6
%12 = select i1 %11, i8 48, i8 49
%13 = getelementptr inbounds i8, i8* %0, i64 6
store i8 %12, i8* %13
%14 = extractelement <8 x i1> %7, i64 5
%15 = select i1 %14, i8 48, i8 49
%16 = getelementptr inbounds i8, i8* %0, i64 5
store i8 %15, i8* %16
%17 = extractelement <8 x i1> %7, i64 4
%18 = select i1 %17, i8 48, i8 49
%19 = getelementptr inbounds i8, i8* %0, i64 4
store i8 %18, i8* %19
%20 = extractelement <8 x i1> %7, i64 3
%21 = select i1 %20, i8 48, i8 49
%22 = getelementptr inbounds i8, i8* %0, i64 3
store i8 %21, i8* %22
%23 = extractelement <8 x i1> %7, i64 2
%24 = select i1 %23, i8 48, i8 49
%25 = getelementptr inbounds i8, i8* %0, i64 2
store i8 %24, i8* %25
%26 = extractelement <8 x i1> %7, i64 1
%27 = select i1 %26, i8 48, i8 49
%28 = getelementptr inbounds i8, i8* %0, i64 1
store i8 %27, i8* %28
%29 = extractelement <8 x i1> %7, i64 0
%30 = select i1 %29, i8 48, i8 49
store i8 %30, i8* %0
%31 = getelementptr inbounds i8, i8* %0, i64 8
store i8 0, i8* %31
ret void
}