Re: Question about declaring an array in the stack on runtime

[David Brown <david@westcontrol.com>]

On 15/07/2023 12:43, James R T via Gcc-help wrote:
> Hi folks,
> 
> 2. Regardless of whether this is UB or not, is it possible for GCC to
> also output a warning in `-O0` as in `-O2`? If the behavior changes
> across different optimization levels, it seems that it's worth a
> warning or two. It can be a different warning instead of
> `-Wdangling-pointer` since looking at the produced assembly code, GCC
> seems to simply optimize out the whole conditional assignment block in
> `-O2`. If it is UB, I understand that it is impossible to catch all
> UB, but I am just checking on whether it is possible to catch this
> specific one from GCC's perspective. Just FYI, I have also tried using
> `-fsanitize=address` and `-fsanitize=undefined` and it seems that
> AddressSanitizer would throw a `stack-use-after-scope` error in GCC if
> `-fsanitize=address` is specified for both `-O0` and `-O2`, but not in
> Clang. `-fsanitize=undefined` does not seem to be able to detect
> anything.
> 

I think you've had solid answers to your other points already, so you 
can see why it is UB.

When you use the -O flags, you are not just enabling a selection of 
optimisation passes - you are also changing the code analysis passes. 
With -O0, relatively little analysis is done.  This means that many 
warnings are not active at -O0.  So it is quite normal for warnings like 
this to require -O1 or above to work, and some may need -O2 to work 
fully (I don't have details off-hand - I'm a long-term gcc user, not a 
gcc developer).

Personally, I've never had any use for -O0 - I use -O2 usually and -O1 
at a minimum, even for early checking of code, precisely because it 
allows far better static warnings.