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Re: out of bounds access in insn-automata.c


On Thu, 24 Mar 2016, Bernd Schmidt wrote:
> On 03/24/2016 11:17 AM, Aldy Hernandez wrote:
> > On 03/23/2016 10:25 AM, Bernd Schmidt wrote:
> > > It looks like this block of code is written by a helper function that is
> > > really intended for other purposes than for maximal_insn_latency. Might
> > > be worth changing to
> > >   int insn_code = dfa_insn_code (as_a <rtx_insn *> (insn));
> > >   gcc_assert (insn_code <= DFA__ADVANCE_CYCLE);
> >
> > dfa_insn_code_* and friends can return > DFA__ADVANCE_CYCLE so I can't
> > put that assert on the helper function.
> So don't use the helper function? Just emit the block above directly.

Let me chime in :)  The function under scrutiny, maximal_insn_latency, was
added as part of selective scheduling merge; at the same time,
output_default_latencies was factored out of
output_internal_insn_latency_func, and the pair of new functions
tried to mirror existing pair of

In particular, output_insn_latency_func also invokes
output_internal_insn_code_evaluation (twice, for each argument).  This means
that generated 'insn_latency' can also call 'internal_insn_latency' with
DFA__ADVANCE_CYCLE in arguments.  However, 'internal_insn_latency' then has a
specially emitted 'if' statement that checks if either of the arguments is 
' >= DFA__ADVANCE_CYCLE', and returns 0 in that case.

So ultimately pre-existing code was checking ' > DFA__ADVANCE_CYCLE' first and
' >= DFA_ADVANCE_CYCLE' second (for no good reason as far as I can see), and
when the new '_maximal_' functions were introduced, the second check was not
duplicated in the new copy.

So as long we are not looking for hacking it up further, I'd like to clean up
both functions at the same time.  If calling the 'internal_' variants with
DFA__ADVANCE_CYCLE is rare, extending 'default_insn_latencies' by 1 zero
element corresponding to DFA__ADVANCE_CYCLE is a simple suitable fix. If
either DFA__ADVANCE_CYCLE is not guaranteed to be rare, or extending the table
in that style is undesired, I suggest creating a variant of
'output_internal_insn_code_evaluation' that performs a '>=' rather than '>'
test in the first place, and use it in both output_insn_latency_func and
output_maximal_insn_latency_func.  If acknowledged, I volunteer to regstrap on
x86_64 and submit that in stage1.



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