[stackprotector] Add the llvm.stackprotectorcheck intrinsic

[Michael Gottesman]

r188191.

On Aug 9, 2013, at 5:07 PM, Michael Gottesman <mgottesman at apple.com> wrote:

> Any more objections/comments/lack of consensus/etc?
> 
> Michael
> 
> On Aug 9, 2013, at 1:36 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> 
>> On Fri, Aug 9, 2013 at 1:21 PM, Michael Gottesman <mgottesman at apple.com> wrote:
>>> Hey Eli! Sorry for the delay.
>>> 
>>> On Jul 23, 2013, at 4:14 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>> 
>>> What happens if the call satisfies all the platform-independent
>>> requirements, but fails some platform-specific requirement?  Does it
>>> just not matter because the buffers the stack protector is protecting
>>> can't be referenced?
>>> 
>>> 
>>> The key thing about the patch is that something magical is happening. No
>>> matter where the stack protector intrinsic is in the actual basic block, the
>>> stack protector code will be inserted right before the terminator whether or
>>> not it is a return or a tail return. If the call fails the platform specific
>>> requirements, then there will be a call and then the terminator. The stack
>>> protector code will be inserted right before the terminator and thus right
>>> before the call.
>>> 
>>> 
>>> Also, I don't understand why we need to introduce an intrinsic: if
>>> steps 1 and 2 are both in the same pass, can't you just insert the
>>> compare+branch before the tail call?
>>> 
>>> 
>>> I can’t actually insert the compare + branch before the tail call at the IR
>>> level since the branch will be a terminator implying that I would need to
>>> split the basic block at the IR level. Splitting the basic block at the IR
>>> level is exactly what we do not want to do since then we would be required
>>> to move a failed tail call at the MI level from the success basic block to
>>> the parent basic block before the actual stack protector compare happens.
>>> This would require doing platform dependent pattern matching, something that
>>> I am trying to avoid.
>>> 
>>> What the intrinsic is really being used for is delaying the code-gen of the
>>> stack protector check until we know the exact position where we wish to
>>> insert it (which due to the way we represent tail calls at the MI level is
>>> always right before the terminator of the basic block no matter whether we
>>> have a tail call, failed tail call, or normal call). Thus we can stay
>>> platform independent and clean.
>> 
>> That makes sense; thanks.
>> 
>> -Eli
> 
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