Re: RFA: new pass to warn on questionable uses of alloca() and VLAs

[Manuel López-Ibáñez <lopezibanez at gmail dot com>]

> +Walloca
> +LangEnabledBy(C ObjC C++ ObjC++)
> +; in common.opt
> +
> +Walloca=
> +LangEnabledBy(C ObjC C++ ObjC++)
> +; in common.opt
> +

I'm not sure what you think the above means, but this is an invalid use of 
LangEnabledBy(). (It would be nice if the .awk scripts were able to catch it 
and give an error.)


> +;; warn_vla == 0 for -Wno-vla
> +;; warn_vla == -1 for nothing passed.
> +;; warn_vla == -2 for -Wvla passed
> +;; warn_vla == NUM for -Wvla=NUM
>  Wvla
>  C ObjC C++ ObjC++ Var(warn_vla) Init(-1) Warning
>  Warn if a variable length array is used.
>
> +Wvla=
> +C ObjC C++ ObjC++ Var(warn_vla) Warning Joined RejectNegative UInteger
> +-Wvla=<number> Warn on unbounded uses of variable-length arrays, and
> +warn on bounded uses of variable-length arrays whose bound can be
> +larger than <number> bytes.
> +

This overloading of warn_vla seems confusing (as shown by all the places that 
require updating). Why not call it Wvla-larger-than= and use a different 
warn_vla_larger_than variable? We already have -Wlarger-than= and 
-Wframe-larger-than=.

Using warn_vla_larger_than (even if you wish to keep -Wvla= as the option name) 
as a variable distinct from warn_vla would make things simpler:

-Wvla => warn_vla == 1
-Wno-vla => warn_vla == 0
-Wvla=N => warn_vla_larger_than == N (where N == 0 means disable).

If you wish that -Wno-vla implies -Wvla=0 then you'll need to handle that 
manually. I don't think we have support for that in the .opt files. But that 
seems far less complex than having a single shared Var().

Cheers,
	Manuel.