unsigned long __get_user(const void *y, int size) {
  unsigned long result;
  __asm__ __volatile__("" : : : "memory");
  switch (size) {
  case 1:
    result = *(unsigned char *)y;
    break;
  case 2:
    result = *(unsigned short *)y;
    break;
  case 4:
    result = *(unsigned int *)y;
    break;
  case 8:
    result = *(unsigned long *)y;
    break;
  default:
    result = bad_user_access_length();
  }
  __asm__ __volatile__("" : : : "memory");
  return result;
}


x86 assembly (clang, -O0):
<__get_user>:
push   %rbp
mov    %rsp,%rbp
sub    $0x20,%rsp
mov    %rdi,-0x8(%rbp)
mov    %esi,-0xc(%rbp)
mov    -0xc(%rbp),%eax
add    $0xffffffff,%eax
mov    %eax,%ecx
mov    %rcx,-0x20(%rbp)
sub    $0x7,%eax
ja     1187 <__get_user+0x77>
mov    -0x20(%rbp),%rax
lea    0xec1(%rip),%rcx
movslq (%rcx,%rax,4),%rax
add    %rcx,%rax
jmp    *%rax
mov    -0x8(%rbp),%rax
movzbl (%rax),%eax
mov    %rax,-0x18(%rbp)
jmp    1194 <__get_user+0x84>
mov    -0x8(%rbp),%rax
movzwl (%rax),%eax
mov    %rax,-0x18(%rbp)
jmp    1194 <__get_user+0x84>
mov    -0x8(%rbp),%rax
mov    (%rax),%eax
mov    %rax,-0x18(%rbp)
jmp    1194 <__get_user+0x84>
mov    -0x8(%rbp),%rax
mov    (%rax),%rax
mov    %rax,-0x18(%rbp)
jmp    1194 <__get_user+0x84>
mov    $0x0,%al
call   1030 <bad_user_access_length@plt>
cltq
mov    %rax,-0x18(%rbp)
mov    -0x18(%rbp),%rax
add    $0x20,%rsp
pop    %rbp
ret

x86 assembly (clang, -O2):
<__get_user>:
push   %rax
add    $0xffffffff,%esi
cmp    $0x7,%esi
ja     1133 <__get_user+0x23>
lea    0xee0(%rip),%rax
movslq (%rax,%rsi,4),%rcx
add    %rax,%rcx
jmp    *%rcx
movzbl (%rdi),%eax
jmp    1145 <__get_user+0x35>
movzwl (%rdi),%eax
jmp    1145 <__get_user+0x35>
xor    %eax,%eax
call   1030 <bad_user_access_length@plt>
cltq
jmp    1145 <__get_user+0x35>
mov    (%rdi),%eax
jmp    1145 <__get_user+0x35>
mov    (%rdi),%rax
pop    %rcx
ret

LLVM IR (-O0):
define i64 @__get_user(i8* %0, i32 %1) {
%3 = alloca i8*
%4 = alloca i32
%5 = alloca i64
store i8* %0, i8** %3
store i32 %1, i32* %4
%6 = load i32, i32* %4
switch i32 %6, label %25 [
i32 1, label %7
i32 2, label %11
i32 4, label %16
i32 8, label %21
]
7: ; preds = %2
%8 = load i8*, i8** %3
%9 = load i8, i8* %8
%10 = zext i8 %9 to i64
store i64 %10, i64* %5
br label %28
11: ; preds = %2
%12 = load i8*, i8** %3
%13 = bitcast i8* %12 to i16*
%14 = load i16, i16* %13
%15 = zext i16 %14 to i64
store i64 %15, i64* %5
br label %28
16: ; preds = %2
%17 = load i8*, i8** %3
%18 = bitcast i8* %17 to i32*
%19 = load i32, i32* %18
%20 = zext i32 %19 to i64
store i64 %20, i64* %5
br label %28
21: ; preds = %2
%22 = load i8*, i8** %3
%23 = bitcast i8* %22 to i64*
%24 = load i64, i64* %23
store i64 %24, i64* %5
br label %28
25: ; preds = %2
%26 = call i32 (...) @bad_user_access_length()
%27 = sext i32 %26 to i64
store i64 %27, i64* %5
br label %28
28: ; preds = %25, %21, %16, %11, %7
%29 = load i64, i64* %5
ret i64 %29
}

LLVM IR (-O2):
define i64 @__get_user(i8* nocapture %0, i32 %1) {
tail call void asm sideeffect "", "~{memory},~{dirflag},~{fpsr},~{flags}"()
switch i32 %1, label %17 [
i32 1, label %3
i32 2, label %6
i32 4, label %10
i32 8, label %14
]
3: ; preds = %2
%4 = load i8, i8* %0
%5 = zext i8 %4 to i64
br label %20
6: ; preds = %2
%7 = bitcast i8* %0 to i16*
%8 = load i16, i16* %7
%9 = zext i16 %8 to i64
br label %20
10: ; preds = %2
%11 = bitcast i8* %0 to i32*
%12 = load i32, i32* %11
%13 = zext i32 %12 to i64
br label %20
14: ; preds = %2
%15 = bitcast i8* %0 to i64*
%16 = load i64, i64* %15
br label %20
17: ; preds = %2
%18 = tail call i32 (...) @bad_user_access_length()
%19 = sext i32 %18 to i64
br label %20
20: ; preds = %17, %14, %10, %6, %3
%21 = phi i64 [ %19, %17 ], [ %16, %14 ], [ %13, %10 ], [ %9, %6 ], [ %5, %3 ]
tail call void asm sideeffect "", "~{memory},~{dirflag},~{fpsr},~{flags}"()
ret i64 %21
}