https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109943
Bug ID: 109943 Summary: [13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r13-6834-g41ade3399bd Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: theodort at inf dot ethz.ch Target Milestone: --- void foo(void); static int b = 5; static int *c = &b; static char(a)(char d, char g) { ; return d % g; } static void e(unsigned char d, unsigned g) { char h = a(g, d); if (h) if ((d) >= 0) { foo(); }; } static void f(int d) { e(b, d); } int main() { f(*c); *c = 0; } gcc-trunk -O3 generates: main: movl $0, b(%rip) xorl %eax, %eax ret However, including additional info via __builtin_unreachable results to worse code: void foo(void); static int b = 5; static int *c = &b; static char(a)(char d, char g) { if ((g) <= 0) { __builtin_unreachable(); } return d % g; } static void e(unsigned char d, unsigned g) { char h = a(g, d); if (h) if ((d) >= 0) { foo(); }; } static void f(int d) { e(b, d); } int main() { f(*c); *c = 0; } gcc-trunk (and 13) -O3 generates: main: movl b(%rip), %ecx movsbl %cl, %eax movzbl %cl, %ecx cltd idivl %ecx testl %edx, %edx jne .L11 movl $0, b(%rip) xorl %eax, %eax ret .L11: pushq %rax call foo xorl %edx, %edx xorl %eax, %eax movl %edx, b(%rip) popq %rcx ret https://godbolt.org/z/n34Tsr3vq gcc-12 can generate optimized code for both cases Started with r13-6834-g41ade3399bd