https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38534
--- Comment #15 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by H.J. Lu <h...@gcc.gnu.org>: https://gcc.gnu.org/g:7cc9adc62cee0aa91ce834b3dd6296ce38f1d79d commit r14-8470-g7cc9adc62cee0aa91ce834b3dd6296ce38f1d79d Author: H.J. Lu <hjl.to...@gmail.com> Date: Tue Jan 23 06:59:51 2024 -0800 x86: Don't save callee-saved registers in noreturn functions There is no need to save callee-saved registers in noreturn functions if they don't throw nor support exceptions. We can treat them the same as functions with no_callee_saved_registers attribute. Adjust stack-check-17.c for noreturn function which no longer saves any registers. With this change, __libc_start_main in glibc 2.39, which is a noreturn function, is changed from __libc_start_main: endbr64 push %r15 push %r14 mov %rcx,%r14 push %r13 push %r12 push %rbp mov %esi,%ebp push %rbx mov %rdx,%rbx sub $0x28,%rsp mov %rdi,(%rsp) mov %fs:0x28,%rax mov %rax,0x18(%rsp) xor %eax,%eax test %r9,%r9 to __libc_start_main: endbr64 sub $0x28,%rsp mov %esi,%ebp mov %rdx,%rbx mov %rcx,%r14 mov %rdi,(%rsp) mov %fs:0x28,%rax mov %rax,0x18(%rsp) xor %eax,%eax test %r9,%r9 In Linux kernel 6.7.0 on x86-64, do_exit is changed from do_exit: endbr64 call <do_exit+0x9> push %r15 push %r14 push %r13 push %r12 mov %rdi,%r12 push %rbp push %rbx mov %gs:0x0,%rbx sub $0x28,%rsp mov %gs:0x28,%rax mov %rax,0x20(%rsp) xor %eax,%eax call *0x0(%rip) # <do_exit+0x39> test $0x2,%ah je <do_exit+0x8d3> to do_exit: endbr64 call <do_exit+0x9> sub $0x28,%rsp mov %rdi,%r12 mov %gs:0x28,%rax mov %rax,0x20(%rsp) xor %eax,%eax mov %gs:0x0,%rbx call *0x0(%rip) # <do_exit+0x2f> test $0x2,%ah je <do_exit+0x8c9> I compared GCC master branch bootstrap and test times on a slow machine with 6.6 Linux kernels compiled with the original GCC 13 and the GCC 13 with the backported patch. The performance data isn't precise since the measurements were done on different days with different GCC sources under different 6.6 kernel versions. GCC master branch build time in seconds: before after improvement 30043.75user 30013.16user 0% 1274.85system 1243.72system 2.4% GCC master branch test time in seconds (new tests added): before after improvement 216035.90user 216547.51user 0 27365.51system 26658.54system 2.6% gcc/ PR target/38534 * config/i386/i386-options.cc (ix86_set_func_type): Don't save and restore callee saved registers for a noreturn function with nothrow or compiled with -fno-exceptions. gcc/testsuite/ PR target/38534 * gcc.target/i386/pr38534-1.c: New file. * gcc.target/i386/pr38534-2.c: Likewise. * gcc.target/i386/pr38534-3.c: Likewise. * gcc.target/i386/pr38534-4.c: Likewise. * gcc.target/i386/stack-check-17.c: Updated.
