[llvm-commits] patch: codegen non-exiting calls as tail, even if not tail in IR

[Nick Lewycky]

Duncan Sands wrote:
> Hi Nick,
>
> On 15/10/12 05:44, Nick Lewycky wrote:
>> This patch teaches the LLVM codegen layer to try emitting calls to
>> 'terminal' functions as tail calls, even if they aren't marked tail
>> calls in the
>> IR. The motivation is to minimize the amount of extra instructions
>> emitted by
>> clang's -fcatch-undefined-behavior flag (and other tools like
>> asan/msan/tsan, if
>> run in a mode where they don't turn off tail calls due to the loss of
>> stacktrace
>> info).
>>
>> Why not mark them 'tail' in the IR? Because the IR definition states
>> that a tail
>> call is one does not access any allocas or varargs in the caller. This
>> is good
>> because it's defining an IR concept in terms of other IR concepts, and
>> I don't
>> want to change it. BasicAA uses this for example, by assuming that a
>> call with
>> 'tail' doesn't touch any alloca'd memory.
>>
>> What's a terminal function? One that is noreturn + nounwind. A
>> function that
>> calls longjmp() will be marked nounwind by clang, and I can't tell
>> whether this
>> is correct due to a lack of precision in the definition of 'nounwind'
>> in the
>> language reference.
>>
>> At the machine level, it's safe to tail call to a function that
>> accesses its
>> caller's stack, as long as we plan to never return.
>
> is this really true? Imagine that you pass the callee pointers to
> variables on
> the caller's stack, and the callee reads/writes those variables via the
> pointers. Imagine also that the callee declares some stack variables of its
> own, and these get placed on top of the caller's variables. Then writing
> to the
> caller's variables can modify the callee's variables and vice versa,
> causing the
> callee to malfunction. The fact that the callee doesn't return isn't
> relevant
> in this scenario.

Oof, you're right. This patch doesn't quite do what I want.

I want the caller to adjust the stack pointer as it normally would, so 
that the only difference between the tail call and a call is that the 
tail call doesn't push the return address on the stack.

Nick

>>
>> Please review! The two cases that I'm concerned about are when the
>> callee calls
>> longjmp(), and when the call is varargs. I think both of these cases
>> are fine
>> without special handling, but I'd appreciate someone double-checking
>> that.
>>
>> Nick
>>
>>
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