[PATCH] remove %p handling from gimple-ssa-sprintf (pr78512)

[Jeff Law]

On 11/28/2016 07:57 PM, Martin Sebor wrote:
> PR 78512 - r242674 miscompiles Linux kernel observes that the Linux
> kernel fails to boot as a result of enabling the -fprintf-return-value
> optimization in GCC.  This is likely because the kernel has its own
> sprintf with a large set of extensions to the %p directive that
> conflict with the optimization.  Ordinarily, programs that define
> their own versions of C library functions that differ from what C
> specifies are expected to disable GCC's built-ins (e.g., by
> -fno-builtin, or for freestanding environments like the Linux kernel,
> by -ffreestanding).  But the Linux kernel doesn't do that and hence
> the conflict.
>
> After discussing a few possible options (handling the kernel extensions
> in GCC, providing a new GCC option to disable the %p handling, and
> disabling both the optimization and the warning for calls involving
> the %p directive, the last was viewed as the best alternative).  The
> attached patch removes the %p handling from GCC.
And just to give a little more information here.

The fundamental issue is that %p handling is implementation defined and 
the implementations can (of course) change over time.   Handling of %p 
essentially turns into coding GCC to an implementation rather than a 
real specification.

The details of those implementations would have to be baked into GCC 
itself.  That wasn't terrible when we were just trying to support the 
rather limited cases found in glibc, uClibc, aix & solaris.  But when we 
add the linux kernel and its extensions into the mix, it didn't seem 
wise to continue bake that knowledge into GCC.   The need to support 
multiple %p implementations from a single compiler just makes things 
even worse.

After deliberating those issues, Jakub, Martin and myself ultimately 
decided that supporting %p for warnings and optimization was unwise.

It's unfortunate because the kernel makes extensive use of %p.  I guess 
one could create a plug-in to check %p for the kernel if they wanted to 
take advantage of the checking capabilities.

Approved for the trunk.

Thanks,

jeff