On: ISO C became unusable for operating systems develo...

[mzs]

Yeah I know I get it, it's me more being wishful, but I more seriously wish at least compilers could emit a warning when they optimize something after UB:

  % cat foo.c
  #include <stdlib.h>
   
  int
  foo(int *bar)
  {
   int baz = *bar;
  
   if (bar == NULL) exit(2);
  
   return (baz);
  }
  % cc -O3 -Wall -Wextra -c foo.c
  % objdump -dr foo.o            
  
  foo.o: file format Mach-O 64-bit x86-64
  
  
  Disassembly of section __TEXT,__text:
  
  0000000000000000 _foo:
         0: 55                            pushq %rbp
         1: 48 89 e5                      movq %rsp, %rbp
         4: 8b 07                         movl (%rdi), %eax
         6: 5d                            popq %rbp
         7: c3                            retq
  % cc -O0 -Wall -Wextra -c foo.c
  % objdump -dr foo.o            
  
  foo.o: file format Mach-O 64-bit x86-64
  
  
  Disassembly of section __TEXT,__text:
  
  0000000000000000 _foo:
         0: 55                            pushq %rbp
         1: 48 89 e5                      movq %rsp, %rbp
         4: 48 83 ec 10                   subq $16, %rsp
         8: 48 89 7d f8                   movq %rdi, -8(%rbp)
         c: 48 8b 45 f8                   movq -8(%rbp), %rax
        10: 8b 08                         movl (%rax), %ecx
        12: 89 4d f4                      movl %ecx, -12(%rbp)
        15: 48 83 7d f8 00                cmpq $0, -8(%rbp)
        1a: 0f 85 0a 00 00 00             jne 10 <_foo+0x2a>
        20: bf 02 00 00 00                movl $2, %edi
        25: e8 00 00 00 00                callq 0 <_foo+0x2a>
    0000000000000026:  X86_64_RELOC_BRANCH _exit
        2a: 8b 45 f4                      movl -12(%rbp), %eax
        2d: 48 83 c4 10                   addq $16, %rsp
        31: 5d                            popq %rbp
        32: c3                            retq
  % 

That's very similar to something that bit me in embedded except it was with pointer to structure. Compiler realizes I've derefed NULL and that's UB anyway so no need to do the NULL check later and merrily scribble exc vectors or whatever.

[foxfluff → mzs]

That's a nice example. It'd definitely be nice to have a warning for this one.

Fwiw GCC does have a related warning flag (-Wnull-dereference) but I'm not sure it's made exactly for this. I believe it works based on functions being annotated for possibly returning NULL, e.g. malloc. It's also not enabled by -Wall or -W because apparently there were too many false positives: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96554

I imagine patches would be welcome. I'm guessing there are more people who like to wish for more compiler features than there are people who like to develop compilers :)

[mzs → foxfluff]

edit: Thanks, I just checked and the warning doesn't work

https://godbolt.org/z/4TP1hfx4j

But your hint found -fno-delete-null-pointer-checks which does the trick

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

And -fno-delete-null-pointer-checks made it into LLVM too. It's a good to know but a little late for when I needed it, cheers :)

  % cc -O3 -fno-delete-null-pointer-checks -Wall -Wextra -c foo.c
  % objdump -dr foo.o                                            
  
  foo.o: file format Mach-O 64-bit x86-64
  
  
  Disassembly of section __TEXT,__text:
  
  0000000000000000 _foo:
         0: 55                            pushq %rbp
         1: 48 89 e5                      movq %rsp, %rbp
         4: 48 85 ff                      testq %rdi, %rdi
         7: 74 04                         je 4 <_foo+0xd>
         9: 8b 07                         movl (%rdi), %eax
         b: 5d                            popq %rbp
         c: c3                            retq
         d: bf 02 00 00 00                movl $2, %edi
        12: e8 00 00 00 00                callq 0 <_foo+0x17>
    0000000000000013:  X86_64_RELOC_BRANCH _exit

[astrange → mzs]

If you want a general approach you can turn on ubsan in trap-only mode and see what traps have ended up in your output.

[zajio1am → mzs]

But such code might be generated by macros (or some code generator), in which case silent elimination of unnecessary code is expected and wanted behavior.

[Too → zajio1am]

In this particular case the code is wrong and can be fixed easily by swapping the assignment and the comparison.

Likewise, code generators shouldn’t be generating this faulty code.

Raising compiler error here is the only right thing to do.

There are of course more ambiguous examples, though obvious examples like the one above are sadly way too common.

[UncleMeat → Too]

Why can't we say that the original code is wrong then? The whole point of having something be UB rather than implementation defined is because the language committee believes that it represents a bug in your program.

[MauranKilom → zajio1am]

(Or templates, in the C++ case. Think of all the inlined iterator functions...)