RE: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info()

From: Segher Boessenkool

Sent: 06 March 2017 14:18
On Mon, Mar 06, 2017 at 01:03:19PM +0100, Gabriel Paubert wrote:

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The PowerPC divw etc. instructions do not trap by themselves, but=

 recent

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GCC inserts trap instructions on code paths that are always undef=

ined

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behaviour (like, dividing by zero).

Is it systematic or does it depend from, e.g., optimization levels?

In this case it needs -fisolate-erroneous-paths-dereference which is
default at -O2 and higher.

Great, another optimization-dependent behaviour. :-(

=20
It makes the "behaviour" for undefined behaviour *less* surprising.
It does not change anything else: malformed programs stay malformed,
correct programs do exactly what they did before, too.

Yep, 'undefined behaviour' is exactly that.
It doesn't mean 'undefined result', or 'maybe a signal'.
Wiping the disk and targeting the user with an ICBM are both valid.

	David