#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 *s) {
  register char t, *p = s, *q = (s + (strlen(s) - 1));
  while (p < q) {
    t = *p;
    *p++ = *q;
    *q-- = t;
  }
  return (s);
}


x86 assembly (clang, -O0):
<reverse>:
push   %rbp
mov    %rsp,%rbp
sub    $0x30,%rsp
mov    %rdi,-0x8(%rbp)
mov    -0x8(%rbp),%rax
mov    %rax,-0x18(%rbp)
mov    -0x8(%rbp),%rax
mov    %rax,-0x28(%rbp)
mov    -0x8(%rbp),%rdi
call   1030 <strlen@plt>
mov    %eax,%ecx
mov    -0x28(%rbp),%rax
sub    $0x1,%ecx
movslq %ecx,%rcx
add    %rcx,%rax
mov    %rax,-0x20(%rbp)
mov    -0x18(%rbp),%rax
cmp    -0x20(%rbp),%rax
jae    118f <reverse+0x7f>
mov    -0x18(%rbp),%rax
mov    (%rax),%al
mov    %al,-0x9(%rbp)
mov    -0x20(%rbp),%rax
mov    (%rax),%cl
mov    -0x18(%rbp),%rax
mov    %rax,%rdx
add    $0x1,%rdx
mov    %rdx,-0x18(%rbp)
mov    %cl,(%rax)
mov    -0x9(%rbp),%cl
mov    -0x20(%rbp),%rax
mov    %rax,%rdx
add    $0xffffffffffffffff,%rdx
mov    %rdx,-0x20(%rbp)
mov    %cl,(%rax)
jmp    1148 <reverse+0x38>
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>
cmp    $0x2,%eax
jl     114c <reverse+0x3c>
add    $0xffffffff,%eax
add    %r14,%rax
lea    0x1(%r14),%rcx
nopl   0x0(%rax,%rax,1)
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
store i8* %6, i8** %4
%7 = load i8*, i8** %2
%8 = load i8*, i8** %2
%9 = call i32 @strlen(i8* %8)
%10 = sub nsw i32 %9, 1
%11 = sext i32 %10 to i64
%12 = getelementptr inbounds i8, i8* %7, i64 %11
store i8* %12, i8** %5
br label %13
13: ; preds = %17, %1
%14 = load i8*, i8** %4
%15 = load i8*, i8** %5
%16 = icmp ult i8* %14, %15
br i1 %16, label %17, label %27
17: ; preds = %13
%18 = load i8*, i8** %4
%19 = load i8, i8* %18
store i8 %19, i8* %3
%20 = load i8*, i8** %5
%21 = load i8, i8* %20
%22 = load i8*, i8** %4
%23 = getelementptr inbounds i8, i8* %22, i32 1
store i8* %23, i8** %4
store i8 %21, i8* %22
%24 = load i8, i8* %3
%25 = load i8*, i8** %5
%26 = getelementptr inbounds i8, i8* %25, i32 -1
store i8* %26, i8** %5
store i8 %24, i8* %25
br label %13
27: ; preds = %13
%28 = load i8*, i8** %2
ret i8* %28
}

LLVM IR (-O2):
define i8* @reverse(i8* returned %0) {
%2 = tail call i32 @strlen(i8* %0)
%3 = icmp sgt i32 %2, 1
br i1 %3, label %4, label %16
4: ; preds = %1
%5 = add nsw i32 %2, -1
%6 = zext i32 %5 to i64
%7 = getelementptr inbounds i8, i8* %0, i64 %6
br label %8
8: ; preds = %4, %8
%9 = phi i8* [ %14, %8 ], [ %7, %4 ]
%10 = phi i8* [ %13, %8 ], [ %0, %4 ]
%11 = load i8, i8* %10
%12 = load i8, i8* %9
%13 = getelementptr inbounds i8, i8* %10, i64 1
store i8 %12, i8* %10
%14 = getelementptr inbounds i8, i8* %9, i64 -1
store i8 %11, i8* %9
%15 = icmp ult i8* %13, %14
br i1 %15, label %8, label %16
16: ; preds = %8, %1
ret i8* %0
}