[fpc-devel] Dangerous optimization in CASE..OF

[Florian Klämpfl]

Am 16.07.2017 um 20:25 schrieb Ondrej Pokorny:
> For now, Pascal enumerated types work as aliases for underlying ordinal values - a concept that is
> exactly the same as C enums:
>

Very good point:

florian at ubuntu64:~$ cat test.cc
#include <stdio.h>

enum tenum { e1,e2,e3,e4,e5,e6,e7,e8 };

int f(tenum e)
{
  switch (e)
  {
    case e1:
      printf("Hello 1 %d\n",e1);
      return 1;
    case e2:
      return 354;
    case e3:
      return 351;
    case e4:
      return 315;
    case e5:
      return 35;
    case e6:
      printf("Hello asdf\n");
      return 1;
    case e7:
      printf("Hello \n");
      return 2;
    case e8:
      printf("Hello\n");
      return 3;
  }
}

int main()
{
  f(tenum(12));
}
florian at ubuntu64:~$ clang test.cc
florian at ubuntu64:~$ ./a.out
Ungültiger Maschinenbefehl (Speicherabzug geschrieben)
florian at ubuntu64:~$ clang test.cc -O3
florian at ubuntu64:~$ ./a.out
florian at ubuntu64:~$ clang --version
clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

"Ungültiger Maschinenbefehl (Speicherabzug geschrieben)" = Invalid opcode (memory dump written).
Why? Because it does not range check before entering the jump table.

Funnily enough clang does not create crashing code with -O3 as it removes all code :), to get a
crash, compile probably both function separately, the assembler code for f() suggests this.