#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 /*<<< orphan*/ gets(char *);
int /*<<< orphan*/ printf(char *, ...);
int strlen(char *);
int main(void) {
  char str[50];
  int i;
  printf("Enter a string please: ");
  gets(str);
  int l = strlen(str);
  char *ptr = &str[l - 1];
  for (i = 0; i < l; i++) {
    printf("%c", *(ptr - i));
  }
  return 0;
}


x86 assembly (clang, -O0):
<main>:
push   %rbp
mov    %rsp,%rbp
sub    $0x50,%rsp
movl   $0x0,-0x4(%rbp)
lea    0xeba(%rip),%rdi
mov    $0x0,%al
call   1040 <printf@plt>
lea    -0x40(%rbp),%rdi
call   1050 <gets@plt>
lea    -0x40(%rbp),%rdi
call   1030 <strlen@plt>
mov    %eax,-0x48(%rbp)
mov    -0x48(%rbp),%eax
sub    $0x1,%eax
movslq %eax,%rcx
lea    -0x40(%rbp),%rax
add    %rcx,%rax
mov    %rax,-0x50(%rbp)
movl   $0x0,-0x44(%rbp)
mov    -0x44(%rbp),%eax
cmp    -0x48(%rbp),%eax
jge    11b6 <main+0x86>
mov    -0x50(%rbp),%rax
movslq -0x44(%rbp),%rdx
xor    %ecx,%ecx
sub    %rdx,%rcx
movsbl (%rax,%rcx,1),%esi
lea    0xe77(%rip),%rdi
mov    $0x0,%al
call   1040 <printf@plt>
mov    -0x44(%rbp),%eax
add    $0x1,%eax
mov    %eax,-0x44(%rbp)
jmp    117d <main+0x4d>
xor    %eax,%eax
add    $0x50,%rsp
pop    %rbp
ret

x86 assembly (clang, -O2):
<main>:
push   %r15
push   %r14
push   %rbx
sub    $0x40,%rsp
lea    0xeb0(%rip),%rdi
xor    %eax,%eax
call   1050 <printf@plt>
mov    %rsp,%rbx
mov    %rbx,%rdi
call   1060 <gets@plt>
mov    %rbx,%rdi
call   1040 <strlen@plt>
test   %eax,%eax
jle    11a3 <main+0x63>
movslq %eax,%rcx
add    $0xffffffffffffffff,%rcx
mov    %eax,%r15d
lea    (%rsp,%rcx,1),%r14
neg    %r15
xor    %ebx,%ebx
cs nopw 0x0(%rax,%rax,1)
nopl   0x0(%rax,%rax,1)
movsbl (%r14,%rbx,1),%edi
call   1030 <putchar@plt>
add    $0xffffffffffffffff,%rbx
cmp    %rbx,%r15
jne    1190 <main+0x50>
xor    %eax,%eax
add    $0x40,%rsp
pop    %rbx
pop    %r14
pop    %r15
ret

LLVM IR (-O0):
@.str = [24 x i8] c"Enter a string please: \00"
@.str.1 = [3 x i8] c"%c\00"
define i32 @main() {
%1 = alloca i32
%2 = alloca [50 x i8]
%3 = alloca i32
%4 = alloca i32
%5 = alloca i8*
store i32 0, i32* %1
%6 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([24 x i8], [24 x i8]* @.str, i64 0, i64 0))
%7 = getelementptr inbounds [50 x i8], [50 x i8]* %2, i64 0, i64 0
%8 = call i32 @gets(i8* %7)
%9 = getelementptr inbounds [50 x i8], [50 x i8]* %2, i64 0, i64 0
%10 = call i32 @strlen(i8* %9)
store i32 %10, i32* %4
%11 = load i32, i32* %4
%12 = sub nsw i32 %11, 1
%13 = sext i32 %12 to i64
%14 = getelementptr inbounds [50 x i8], [50 x i8]* %2, i64 0, i64 %13
store i8* %14, i8** %5
store i32 0, i32* %3
br label %15
15: ; preds = %28, %0
%16 = load i32, i32* %3
%17 = load i32, i32* %4
%18 = icmp slt i32 %16, %17
br i1 %18, label %19, label %31
19: ; preds = %15
%20 = load i8*, i8** %5
%21 = load i32, i32* %3
%22 = sext i32 %21 to i64
%23 = sub i64 0, %22
%24 = getelementptr inbounds i8, i8* %20, i64 %23
%25 = load i8, i8* %24
%26 = sext i8 %25 to i32
%27 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([3 x i8], [3 x i8]* @.str.1, i64 0, i64 0), i32 %26)
br label %28
28: ; preds = %19
%29 = load i32, i32* %3
%30 = add nsw i32 %29, 1
store i32 %30, i32* %3
br label %15
31: ; preds = %15
ret i32 0
}

LLVM IR (-O2):
@.str = [24 x i8] c"Enter a string please: \00"
define i32 @main() {
%1 = alloca [50 x i8]
%2 = getelementptr inbounds [50 x i8], [50 x i8]* %1, i64 0, i64 0
%3 = tail call i32 (i8*, ...) @printf(i8* dereferenceable(1) getelementptr inbounds ([24 x i8], [24 x i8]* @.str, i64 0, i64 0))
%4 = call i32 @gets(i8* %2)
%5 = call i32 @strlen(i8* %2)
%6 = add nsw i32 %5, -1
%7 = sext i32 %6 to i64
%8 = getelementptr inbounds [50 x i8], [50 x i8]* %1, i64 0, i64 %7
%9 = icmp sgt i32 %5, 0
br i1 %9, label %10, label %21
10: ; preds = %0
%11 = zext i32 %5 to i64
br label %12
12: ; preds = %10, %12
%13 = phi i64 [ 0, %10 ], [ %19, %12 ]
%14 = sub nsw i64 0, %13
%15 = getelementptr inbounds i8, i8* %8, i64 %14
%16 = load i8, i8* %15
%17 = sext i8 %16 to i32
%18 = call i32 @putchar(i32 %17)
%19 = add nuw nsw i64 %13, 1
%20 = icmp eq i64 %19, %11
br i1 %20, label %21, label %12
21: ; preds = %12, %0
ret i32 0
}