Would it be feasible to implement a "musttail" statement attribute in GCC to get a guarantee that tail call optimization will be performed?
Such an attribute has just landed in the trunk of Clang (https://reviews.llvm.org/D99517). It makes it possible to write algorithms that use arbitrarily long chains of tail calls without risk of blowing the stack. I would love to see something like this land in GCC also (ultimately I'd love to see it standardized). I wrote more about some motivation for guaranteed tail calls here: https://blog.reverberate.org/2021/04/21/musttail-efficient-interpreters.html GCC successfully optimizes tail calls in many cases already. What would it take to provide an actual guarantee, and make it apply to non-optimized builds also? The Clang implementation enforces several rules that must hold for the attribute to be correct, including: - It must be on a function call that is tail position. - Caller and callee must have compatible function signatures (including the implicit "this", if any), differing only in cv qualifiers. - Caller and callee must use the same calling convention. - Caller and callee may not be constructors or destructors. - All arguments, the return type, and any temporaries created must be trivially destructible. - All variables currently in scope must be trivially destructible. - The caller cannot be in a try block, an Objective-C block, or a statement expression. Some of these restrictions may be overly strict for some calling conventions, but they are useful as the "least common denominator" that should be safe in all cases. When implementing this in Clang, we found that we were able to reuse some of the checks around goto and asm goto, since a tail call is sort of like a goto out of the function's scope. Thanks, Josh
