* Bradley Lucier via Gcc-help: > Here's the kind of call I'd like turned into a tail call: > > return (*((___host*)((((long*)((___pc)-(1))) + (1 +((-2)))))))(___ps); > > and it appears that gcc doesn't want to convert it to a tail call. > The calling function has the same argument sequence. > > What should I search for in the .s file to see if this was tail > called? It appears that the assembly code that this is translated > into is: > > <stuff removed> > movq -864(%rbp), %rax > subq $9, %rax > .loc 1 95837 157 > movq (%rax), %rdx > movq -1032(%rbp), %rax > movq %rax, %rdi > call *%rdx > .LVL0: > .L1: > .loc 1 95837 220 > movq -24(%rbp), %rax > subq %fs:40, %rax > je .L17743 > call __stack_chk_fail@PLT > .L17743: > movq -8(%rbp), %rbx > leave > .cfi_def_cfa 7, 8 > ret The stack guard suggests that there addressable variables on the stack. Maybe GCC cannot prove that the arguments in the tail call cannot be used to access those stack variables? > PS: LLVM seems to have a musttail attribute that you use at the call site: > > -#define ___PROPER_TAIL_CALL(call) __attribute__((musttail)) return call > > Does gcc have something like that? There was a discussion about this in April: "musttail" statement attribute for GCC? <https://gcc.gnu.org/pipermail/gcc/2021-April/235882.html> Some ABIs make tail-calls hard, and they are not always an optimization. Thanks, Florian
