#define NULL ((void *)0)
typedef unsigned long size_t; // Customize by platform.
typedef long intptr_t;
typedef unsigned long uintptr_t;
typedef long scalar_t__; // Either arithmetic or pointer type.
/* By default, we understand bool (as a convenience). */
typedef int bool;
#define false 0
#define true 1

/* Forward declarations */

/* Type definitions */

/* Variables and functions */
char tolower(char const);
char *str_lower_case(char *dest, const char *src) {
  size_t i = 0;
  while (src[i] != '\0') {
    dest[i] = tolower(src[i]);
    ++i;
  }
  dest[i] = '\0';
  return dest;
}


x86 assembly (clang, -O0):
<str_lower_case>:
push   %rbp
mov    %rsp,%rbp
sub    $0x20,%rsp
mov    %rdi,-0x8(%rbp)
mov    %rsi,-0x10(%rbp)
movq   $0x0,-0x18(%rbp)
mov    -0x10(%rbp),%rax
mov    -0x18(%rbp),%rcx
movsbl (%rax,%rcx,1),%eax
cmp    $0x0,%eax
je     116c <str_lower_case+0x5c>
mov    -0x10(%rbp),%rax
mov    -0x18(%rbp),%rcx
movsbl (%rax,%rcx,1),%edi
call   1030 <tolower@plt>
mov    %al,%dl
mov    -0x8(%rbp),%rax
mov    -0x18(%rbp),%rcx
mov    %dl,(%rax,%rcx,1)
mov    -0x18(%rbp),%rax
add    $0x1,%rax
mov    %rax,-0x18(%rbp)
jmp    1128 <str_lower_case+0x18>
mov    -0x8(%rbp),%rax
mov    -0x18(%rbp),%rcx
movb   $0x0,(%rax,%rcx,1)
mov    -0x8(%rbp),%rax
add    $0x20,%rsp
pop    %rbp
ret

x86 assembly (clang, -O2):
<str_lower_case>:
push   %r15
push   %r14
push   %rbx
mov    %rdi,%r14
mov    (%rsi),%al
test   %al,%al
je     114c <str_lower_case+0x3c>
mov    %rsi,%r15
xor    %ebx,%ebx
cs nopw 0x0(%rax,%rax,1)
nopl   (%rax)
movsbl %al,%edi
call   1030 <tolower@plt>
mov    %al,(%r14,%rbx,1)
movzbl 0x1(%r15,%rbx,1),%eax
add    $0x1,%rbx
test   %al,%al
jne    1130 <str_lower_case+0x20>
jmp    114e <str_lower_case+0x3e>
xor    %ebx,%ebx
movb   $0x0,(%r14,%rbx,1)
mov    %r14,%rax
pop    %rbx
pop    %r14
pop    %r15
ret

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

LLVM IR (-O2):
define i8* @str_lower_case(i8* returned writeonly %0, i8* nocapture %1) {
%3 = load i8, i8* %1
%4 = icmp eq i8 %3, 0
br i1 %4, label %14, label %5
5: ; preds = %2, %5
%6 = phi i8 [ %12, %5 ], [ %3, %2 ]
%7 = phi i64 [ %10, %5 ], [ 0, %2 ]
%8 = tail call signext i8 @tolower(i8 signext %6)
%9 = getelementptr inbounds i8, i8* %0, i64 %7
store i8 %8, i8* %9
%10 = add i64 %7, 1
%11 = getelementptr inbounds i8, i8* %1, i64 %10
%12 = load i8, i8* %11
%13 = icmp eq i8 %12, 0
br i1 %13, label %14, label %5
14: ; preds = %5, %2
%15 = phi i64 [ 0, %2 ], [ %10, %5 ]
%16 = getelementptr inbounds i8, i8* %0, i64 %15
store i8 0, i8* %16
ret i8* %0
}