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

[Nick Lewycky]

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.

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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