I built the current mainline gcc instrumented with --enable-coverage: heine:~/programs/gambit/gambit> /pkgs/gcc-mainline/bin/gcc -v Using built-in specs. COLLECT_GCC=/pkgs/gcc-mainline/bin/gcc COLLECT_LTO_WRAPPER=/pkgs/gcc-mainline/libexec/gcc/x86_64-pc-linux-gnu/14.0.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: ../../gcc-mainline/configure --enable-coverage --enable-languages=c --disable-multilib --prefix=/pkgs/gcc-mainline --disable-werror Thread model: posix Supported LTO compression algorithms: zlib gcc version 14.0.0 20230802 (experimental) (GCC) and built Gambit with heine:~/programs/gambit/gambit> gsi/gsi -v v4.9.5-3-ge059fffd 20230730151945 x86_64-pc-linux-gnu "./configure 'CC=/pkgs/gcc-mainline/bin/gcc' '--enable-single-host' '--enable-trust-c-tco'" The code to "optimize" sibling calls is split between gcc/tree-tailcall.cc (suitable_for_tail_opt_p and suitable_for_tail_call_opt_p) and gcc/calls.cc (dealt with in maybe_complain_about_tail_call). I think tree-tailcall.cc deals with target-independent transformations, while calls.cc deals with low-level, target-dependent transformations. Because of this split, and having three places where this optimization can be interrupted/reported, it's not clear to me where to place a warning. By examining the .gcov files, for gcc/tree-tailcall.cc and gcc/calls.cc I can see that every tail call in the Gambit source tree was "optimized". Brad