void put4digs(char *at, int what) {
  at[0] = (char)('0' + ((what / 1000) % 10));
  at[1] = (char)('0' + ((what / 100) % 10));
  at[2] = (char)('0' + ((what / 10) % 10));
  at[3] = (char)('0' + (what % 10));
}


x86 assembly (clang, -O0):
<put4digs>:
push   %rbp
mov    %rsp,%rbp
mov    %rdi,-0x8(%rbp)
mov    %esi,-0xc(%rbp)
mov    -0xc(%rbp),%eax
mov    $0x3e8,%ecx
cltd
idiv   %ecx
mov    $0xa,%ecx
cltd
idiv   %ecx
add    $0x30,%edx
mov    %dl,%cl
mov    -0x8(%rbp),%rax
mov    %cl,(%rax)
mov    -0xc(%rbp),%eax
mov    $0x64,%ecx
cltd
idiv   %ecx
mov    $0xa,%ecx
cltd
idiv   %ecx
add    $0x30,%edx
mov    %dl,%cl
mov    -0x8(%rbp),%rax
mov    %cl,0x1(%rax)
mov    -0xc(%rbp),%eax
mov    $0xa,%ecx
cltd
idiv   %ecx
mov    $0xa,%ecx
cltd
idiv   %ecx
add    $0x30,%edx
mov    %dl,%cl
mov    -0x8(%rbp),%rax
mov    %cl,0x2(%rax)
mov    -0xc(%rbp),%eax
mov    $0xa,%ecx
cltd
idiv   %ecx
add    $0x30,%edx
mov    %dl,%cl
mov    -0x8(%rbp),%rax
mov    %cl,0x3(%rax)
pop    %rbp
ret

x86 assembly (clang, -O2):
<put4digs>:
movslq %esi,%rax
imul   $0x10624dd3,%rax,%rcx
mov    %rcx,%rdx
shr    $0x3f,%rdx
sar    $0x26,%rcx
add    %edx,%ecx
movslq %ecx,%rcx
imul   $0x66666667,%rcx,%rdx
mov    %rdx,%rsi
shr    $0x3f,%rsi
shr    $0x22,%rdx
add    %esi,%edx
add    %edx,%edx
lea    (%rdx,%rdx,4),%edx
sub    %edx,%ecx
add    $0x30,%cl
mov    %cl,(%rdi)
imul   $0x51eb851f,%rax,%rcx
mov    %rcx,%rdx
shr    $0x3f,%rdx
sar    $0x25,%rcx
add    %edx,%ecx
movslq %ecx,%rcx
imul   $0x66666667,%rcx,%rdx
mov    %rdx,%rsi
shr    $0x3f,%rsi
shr    $0x22,%rdx
add    %esi,%edx
add    %edx,%edx
lea    (%rdx,%rdx,4),%edx
sub    %edx,%ecx
add    $0x30,%cl
mov    %cl,0x1(%rdi)
imul   $0x66666667,%rax,%rcx
mov    %rcx,%rdx
shr    $0x3f,%rdx
sar    $0x22,%rcx
add    %edx,%ecx
movslq %ecx,%rcx
imul   $0x66666667,%rcx,%rdx
mov    %rdx,%rsi
shr    $0x3f,%rsi
shr    $0x22,%rdx
add    %esi,%edx
add    %edx,%edx
lea    (%rdx,%rdx,4),%edx
mov    %ecx,%esi
sub    %edx,%esi
add    $0x30,%sil
mov    %sil,0x2(%rdi)
add    %ecx,%ecx
lea    (%rcx,%rcx,4),%ecx
sub    %ecx,%eax
add    $0x30,%al
mov    %al,0x3(%rdi)
ret

LLVM IR (-O0):
define void @put4digs(i8* %0, i32 %1) {
%3 = alloca i8*
%4 = alloca i32
store i8* %0, i8** %3
store i32 %1, i32* %4
%5 = load i32, i32* %4
%6 = sdiv i32 %5, 1000
%7 = srem i32 %6, 10
%8 = add nsw i32 48, %7
%9 = trunc i32 %8 to i8
%10 = load i8*, i8** %3
%11 = getelementptr inbounds i8, i8* %10, i64 0
store i8 %9, i8* %11
%12 = load i32, i32* %4
%13 = sdiv i32 %12, 100
%14 = srem i32 %13, 10
%15 = add nsw i32 48, %14
%16 = trunc i32 %15 to i8
%17 = load i8*, i8** %3
%18 = getelementptr inbounds i8, i8* %17, i64 1
store i8 %16, i8* %18
%19 = load i32, i32* %4
%20 = sdiv i32 %19, 10
%21 = srem i32 %20, 10
%22 = add nsw i32 48, %21
%23 = trunc i32 %22 to i8
%24 = load i8*, i8** %3
%25 = getelementptr inbounds i8, i8* %24, i64 2
store i8 %23, i8* %25
%26 = load i32, i32* %4
%27 = srem i32 %26, 10
%28 = add nsw i32 48, %27
%29 = trunc i32 %28 to i8
%30 = load i8*, i8** %3
%31 = getelementptr inbounds i8, i8* %30, i64 3
store i8 %29, i8* %31
ret void
}

LLVM IR (-O2):
define void @put4digs(i8* nocapture writeonly %0, i32 %1) {
%3 = sdiv i32 %1, 1000
%4 = srem i32 %3, 10
%5 = trunc i32 %4 to i8
%6 = add nsw i8 %5, 48
store i8 %6, i8* %0
%7 = sdiv i32 %1, 100
%8 = srem i32 %7, 10
%9 = trunc i32 %8 to i8
%10 = add nsw i8 %9, 48
%11 = getelementptr inbounds i8, i8* %0, i64 1
store i8 %10, i8* %11
%12 = sdiv i32 %1, 10
%13 = srem i32 %12, 10
%14 = trunc i32 %13 to i8
%15 = add nsw i8 %14, 48
%16 = getelementptr inbounds i8, i8* %0, i64 2
store i8 %15, i8* %16
%17 = srem i32 %1, 10
%18 = trunc i32 %17 to i8
%19 = add nsw i8 %18, 48
%20 = getelementptr inbounds i8, i8* %0, i64 3
store i8 %19, i8* %20
ret void
}