[LLVMdev] passing allocas into tail calls (original) (raw)

Nick Lewycky nicholas at mxc.ca
Mon Oct 27 12:26:28 PDT 2014

I have a LangRef question, what does this code mean:

%A = alloca i32 tail call @external(i32* %A)

The 'tail' marker means that the callee doesn't access the caller's stack frame. So what's the right interpretation of this. Is it:

a) %A = alloca i32 unreachable

b) %A = alloca i32 tail call @external(i32* undef)

c) %A = alloca i32 tail call @external(i32* readnone %A)

d) like (b) except that the 'undef' is really a poison value

I think the question boils down to two other questions, what does it mean to "access" the caller's stack frame? Is it accessing to look at the pointer but not to dereference it? And the second question is what exactly is the penalty for violating this rule. Does it happen at the call site, in which case conditionally ignoring the argument would still be invalid? Or is it only full UB once you dereference it? What about comparisons performed on such a pointer?

Nick

More information about the llvm-dev mailing list