Just to add to our comments bellow,
One experiment I did is to use the nounwind/uwtable combination to
achieve what we need.
e.g:
nounwind = emit nothing
nounwind uwtable = emit the .exidx .extab section with the CANTUNWIND
However I gave that up because of tests legacy. e.g
./CodeGen/ARM/ehabi.ll we test :
; (6) .cantunwind directive should be available if the function is
marked with
; nounwind function attribute.
So many tests would need to be rewritten as
; (6) .cantunwind directive should be available if the function is
marked with
; nounwind uwtable function attributes
So I'm not sure in which extend we can change attribute semantic if this
implies adapting the regressions tests ?
But if this is acceptable I'm ready to try this... any other opinion ?
Cheers
Christian
Le 03/21/2017 à 09:11 PM, Jonathan Roelofs a écrit :
On 3/21/17 1:53 PM, Christian Bruel via Phabricator wrote:
chrib added a comment.
In https://reviews.llvm.org/D31140#706411, @jroelofs wrote:
Can you clarify the logic here? It's my understanding that:
`-fno-exceptions` does *not* imply `-fno-unwind-tables`
however:
`-fno-unwind-tables` *does* imply that exceptions cannot be used on
targets that require the tables to do unwinding.
Yes, (bad things might happen or (std::terminate will be called, or
destructors not called.)...
But -f[no]-unwind-tables implies the UWTable attribute, not NoUwind
attribute. To toggle NoUnwind, use -fno-exceptions
And this is getting worse with .canunwind which means DoesNotThrow :)
in my understanding, the logic is as follow:
Since "An exception cannot propagate through a function with a
nounwind table. The exception handling runtime environment terminates
the program if it encounters a nounwind table during exception
processing." (ARM Information Center)
The "nounwind" LLVM attribute, which means "Function does not throw"
translates as the EXIDX_CANTUNWIND value in the exception table index
table which needs to be created for the purpose (for the function)
I think the problem is here, actually. "nounwind" implies "does not
throw", but "does not throw" really should not imply "nounwind". This
is something that ought to be clarified in the langref with the
addition of a "does not throw" attribute. Then the optimizer should be
fixed to deduce "does not throw" instead of "nounwind", and we can let
"nounwind" continue to imply .cantunwind.
And of course without exception runtime environment (the test here)
we don't need this table. So I can see 3 cases:
- nounwind set : Generate .cantunwind directive
and unwind table
- nounwind set but not EH Do not generate the .cantunwind directive
and do not emit the unwind table
- uwtable set Need to generate the unwind
table (even without EH)
The disable-arm-cantunwind flag means: without EH support if the
function does not throw, do dot generate the exception tables and the
EXIDX_CANTUNWIND value.
I'm not a big fan of this workaround flag. I'd rather see this fixed
by clarifying/fixing the semantics of the IR.
Jon
https://reviews.llvm.org/D31140
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