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<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m trying to understand how __builtin_setjmp/longjmp are supposed to interact with the frame pointer on x86_64. In particular, what is the expected behavior when the compiler chooses not to use rsp or rbp to address local variables?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">When built with Clang, the following program will segfault, but it is fine when built with GCC. The target is x86_64 linux.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">int main(int argc, char *argv[]) {<o:p></o:p></p>
<p class="MsoNormal"> void *buf[20];<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> __attribute__((__aligned__(64))) char q; // realign the stack<o:p></o:p></p>
<p class="MsoNormal"> char *p = __builtin_alloca(argc); // dynamic alloca<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> if (__builtin_setjmp(buf)) {<o:p></o:p></p>
<p class="MsoNormal"> *p = 'p';<o:p></o:p></p>
<p class="MsoNormal"> q = 'q';<o:p></o:p></p>
<p class="MsoNormal"> return 0;<o:p></o:p></p>
<p class="MsoNormal"> }<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> asm("movq $0, %rbx");<o:p></o:p></p>
<p class="MsoNormal"> __builtin_longjmp(buf, 1);<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">LLVM is choosing to use rbx as a base pointer to access p and q, but during builtin_setjmp, rbx is not saved; when the longjmp is executed rbx may have a garbage value. GCC on the other hand, is using rbp, which is saved in the jump buffer.
Is this a bug in LLVM, or am I using __builtin_setjmp/longjmp incorrectly?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Note: I’m explicitly clobbering rbx, but the compiler can clobber it on its own if __builtin_longjmp is called from another function.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Ben<o:p></o:p></p>
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