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

[Martin Sebor]

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.

Martin

The kernel extensions are documented below:
   https://www.kernel.org/doc/Documentation/printk-formats.txt

and an implementation is here:
   https://github.com/torvalds/linux/blob/master/lib/vsprintf.c
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