#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 */
int strlen(char *);
char *reverse(char *str) {
  char *end = str + strlen(str) - 1;
  char *start = str;
  while (start < end) {
    char tmp = *start;
    *start = *end;
    *end = tmp;
    end--;
    start++;
  }
  return str;
}


x86 assembly (clang, -O0):
<reverse>:
push   %rbp
mov    %rsp,%rbp
sub    $0x30,%rsp
mov    %rdi,-0x8(%rbp)
mov    -0x8(%rbp),%rax
mov    %rax,-0x28(%rbp)
mov    -0x8(%rbp),%rdi
call   1030 <strlen@plt>
mov    %eax,%ecx
mov    -0x28(%rbp),%rax
movslq %ecx,%rcx
add    %rcx,%rax
add    $0xffffffffffffffff,%rax
mov    %rax,-0x10(%rbp)
mov    -0x8(%rbp),%rax
mov    %rax,-0x18(%rbp)
mov    -0x18(%rbp),%rax
cmp    -0x10(%rbp),%rax
jae    1192 <reverse+0x82>
mov    -0x18(%rbp),%rax
mov    (%rax),%al
mov    %al,-0x19(%rbp)
mov    -0x10(%rbp),%rax
mov    (%rax),%cl
mov    -0x18(%rbp),%rax
mov    %cl,(%rax)
mov    -0x19(%rbp),%cl
mov    -0x10(%rbp),%rax
mov    %cl,(%rax)
mov    -0x10(%rbp),%rax
add    $0xffffffffffffffff,%rax
mov    %rax,-0x10(%rbp)
mov    -0x18(%rbp),%rax
add    $0x1,%rax
mov    %rax,-0x18(%rbp)
jmp    1149 <reverse+0x39>
mov    -0x8(%rbp),%rax
add    $0x30,%rsp
pop    %rbp
ret

x86 assembly (clang, -O2):
<reverse>:
push   %r14
push   %rbx
push   %rax
mov    %rdi,%r14
call   1030 <strlen@plt>
cltq
add    %r14,%rax
add    $0xffffffffffffffff,%rax
cmp    %r14,%rax
jbe    114c <reverse+0x3c>
lea    0x1(%r14),%rcx
xchg   %ax,%ax
movzbl -0x1(%rcx),%edx
movzbl (%rax),%ebx
mov    %bl,-0x1(%rcx)
mov    %dl,(%rax)
add    $0xffffffffffffffff,%rax
lea    0x1(%rcx),%rdx
cmp    %rax,%rcx
mov    %rdx,%rcx
jb     1130 <reverse+0x20>
mov    %r14,%rax
add    $0x8,%rsp
pop    %rbx
pop    %r14
ret

LLVM IR (-O0):
define i8* @reverse(i8* %0) {
%2 = alloca i8*
%3 = alloca i8*
%4 = alloca i8*
%5 = alloca i8
store i8* %0, i8** %2
%6 = load i8*, i8** %2
%7 = load i8*, i8** %2
%8 = call i32 @strlen(i8* %7)
%9 = sext i32 %8 to i64
%10 = getelementptr inbounds i8, i8* %6, i64 %9
%11 = getelementptr inbounds i8, i8* %10, i64 -1
store i8* %11, i8** %3
%12 = load i8*, i8** %2
store i8* %12, i8** %4
br label %13
13: ; preds = %17, %1
%14 = load i8*, i8** %4
%15 = load i8*, i8** %3
%16 = icmp ult i8* %14, %15
br i1 %16, label %17, label %29
17: ; preds = %13
%18 = load i8*, i8** %4
%19 = load i8, i8* %18
store i8 %19, i8* %5
%20 = load i8*, i8** %3
%21 = load i8, i8* %20
%22 = load i8*, i8** %4
store i8 %21, i8* %22
%23 = load i8, i8* %5
%24 = load i8*, i8** %3
store i8 %23, i8* %24
%25 = load i8*, i8** %3
%26 = getelementptr inbounds i8, i8* %25, i32 -1
store i8* %26, i8** %3
%27 = load i8*, i8** %4
%28 = getelementptr inbounds i8, i8* %27, i32 1
store i8* %28, i8** %4
br label %13
29: ; preds = %13
%30 = load i8*, i8** %2
ret i8* %30
}

LLVM IR (-O2):
define i8* @reverse(i8* returned %0) {
%2 = tail call i32 @strlen(i8* %0)
%3 = sext i32 %2 to i64
%4 = getelementptr inbounds i8, i8* %0, i64 %3
%5 = getelementptr inbounds i8, i8* %4, i64 -1
%6 = icmp ugt i8* %5, %0
br i1 %6, label %7, label %15
7: ; preds = %1, %7
%8 = phi i8* [ %13, %7 ], [ %5, %1 ]
%9 = phi i8* [ %12, %7 ], [ %0, %1 ]
%10 = load i8, i8* %9
%11 = load i8, i8* %8
store i8 %11, i8* %9
store i8 %10, i8* %8
%12 = getelementptr inbounds i8, i8* %9, i64 1
%13 = getelementptr inbounds i8, i8* %8, i64 -1
%14 = icmp ult i8* %12, %13
br i1 %14, label %7, label %15
15: ; preds = %7, %1
ret i8* %0
}