[llvm-bugs] [Bug 143397] Incorrect optimization of multiple __builtin_unreachable() conditions leads to logic errors in control flow

[LLVM Bugs via llvm-bugs]

Issue	143397
Summary	Incorrect optimization of multiple __builtin_unreachable() conditions leads to logic errors in control flow
Labels
Assignees
Reporter	hutuhutong

    this problems exist in X86_64 clang 18/20。
clang exhibits incorrect optimization behavior when handling multiple (>2) __builtin_unreachable() statements.

When only a single __builtin_unreachable() is used, or when compiling with -O0, the code executes correctly. However, when two or more __builtin_unreachable() statements are present, the program behaves correctly under -O0, but under -O1/O2/O3/Os, clang incorrectly folds the entire test_builtin_unreachable() function, leading to an infinite loop at runtime.

This suggests that the optimizer does not correctly account for the interactions of multiple unreachable paths during optimization.

========the code========
#include <stdio.h>
#include <stdlib.h>

void test_output() {
    printf("the code is executing\n");
}

void test_builtin_unreachable() {
    int bb = 2;
    if ((bb & ~3) != 0)
        __builtin_unreachable();
    if ((bb & 1) == 0)
        __builtin_unreachable();
    if (bb == 2)
        printf("the value of bb is: %d\n", bb);
}

int main() {
    test_output();
    test_builtin_unreachable();
    return 0;
}

========the  output========
$ clang -O0 test.c -o test
$ ./test
the code is executing
the value of bb is: 2

$ clang -O1 test.c -o test
$ ./test
the code is executing
the code is executing
Segmentation fault (core dumped)

========the assembly code=========
when use the -O1， we can see the function test_builtin_unreachable is none, so the test_output always be executing:

test_output:
        lea     rdi, [rip + .Lstr]
        jmp     puts@PLT

test_builtin_unreachable:

main:
        push    rax
        lea     rdi, [rip + .Lstr]
        call    puts@PLT

.Lstr:
        .asciz  "the code is executing"

_______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs