On Mar 8, 2013, at 10:12 AM, "Mcdaniel, Daryl" <[email protected]> wrote:
> A multi-platform compiler should not interfere with dereferencing NULL.
> No general purpose (cross) compiler for X86 should interfere with
> dereferencing NULL.
The point I was making is that dereferencing NULL is undefined behavior as far
as the C standard is concerned, and compilers are free to optimize undefined
behavior away. To dereference NULL in C you must use a volatile pointer to
conform to the language definition.
It looks like clang and GCC will optimize out reads of a known NULL pointer.
For clang a store or call through a null pointer are converted to a
__builtin_trap() aka X86 ud2 instruction, and this is an implementation choice
to help programmers remove undefined behavior from their code. For the edk2 we
tell clang to replace the ud2 with a function call that does not exist so we
get a link failure in this case. But a read to NULL will just get optimized out!
Here is a simple example:
~/work/Compiler>cat null.c
int test ()
{
int *x = (int *)0;
int foo = *x;
if (!x)
return -1;
return foo;
}
~/work/Compiler>clang -S -Os null.c -arch i386
~/work/Compiler>cat null.s
.section __TEXT,__text,regular,pure_instructions
.globl _test
_test: ## @test
## BB#0:
pushl %ebp
movl %esp, %ebp
movl $-1, %eax
popl %ebp
ret
.subsections_via_symbols
Paolo mentioned that this is also true for gcc:
GCC will just optimize the code as it sees
fit. As David pointed out, it means for example that (without the above
option):
int foo = *x;
if (!x)
return ERROR;
return foo;
will be optimized to simply "return *x".
Thanks,
Andrew Fish
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