On 01/27/2017 10:53 PM, Chris Metcalf wrote:
On 1/27/2017 4:30 PM, Florian Weimer wrote:
On 01/27/2017 09:34 PM, Chris Metcalf wrote:
I'd like to commit the one-line patch below that just declares the
context
variables for test-setcontext2 as static. I think the consensus is that
this is necessary,
No, I think the test is supposed to work as-is; it is valid.
So if this test is broken, it is a bug in the implementation of the
context switching functions (as they are visible to applications, i.e.
the fix might not be in the functions themselves, but adjusting their
calling conventions, inhibiting GCC optimizations which make incorrect
assumptions, etc.).
If we do not have a fix right now, it is better to let the test keep
failing (perhaps XFAIL it), rather than tweaking it so that it tests
something else.
We would be waiting on functionality from gcc that doesn't currently
exist, correct? The current returns_twice semantics is insufficient to
describe the problem that we currently see with shared stack frames.
I think this could still be a GCC regression in the handling of the returns_twice attribute, introduced when GCC started sharing stack slots more aggressively. When returns_twice was initially implemented and documented, the question of stack slot reuse did not arise because GCC probably did not do it at all.
It's possible that returns_twice could be extended to also have the
semantics of "don't share my arguments' stack space with any other
variables in this function" pretty reasonably, but I don't want to try to
guess what will make most sense to the gcc folks.
GCC already has this behavior on x86_64. This could be an accident.
I can certainly let the test keep failing and just open a bug to reference
it and capture some of this discussion.
Yes, would you please file a GCC bug with a preprocessed input file?