Hi,
I was looking at the implementation of mstackalign in gcc (O2/O3) and it looks like we generate code for mstackalign (i.e. generate instructions for stack alignment) before pushing $rbp to stack/setting $rbp. In the epilogue, we do the same in reverse order. some_routine: 0x0000000000400860 <+0>: lea 0x8(%rsp),%r10 0x0000000000400865 <+5>: and $0xfffffffffffffff0,%rsp 0x0000000000400869 <+9>: pushq -0x8(%r10) 0x000000000040086d <+13>: push %rbp 0x000000000040086e <+14>: mov %rsp,%rbp : 0x0000000000400983 <+291>: pop %rbp >> 0x0000000000400984 <+292>: lea -0x8(%r10),%rsp 0x0000000000400988 <+296>: retq In gdb, when I single-step to IP (Instruction Pointer) 0x0000000000400984 (shown above) and try to get my stack trace using ‘gdb - where’, I do not get my full stack trace, this is what I get: (gdb) where #0 0x00000000004009e3 in backtrace() () #1 0x0000000000400b18 in fac(int) () Backtrace stopped: previous frame inner to this frame (corrupt stack?) (gdb) And of course, if I just do one more ‘single stepi’ and stop at the next instruction retq / IP: 0x0000000000400988, then everything works fine. (gdb) si 0x00000000004009e7 in backtrace() () (gdb) where #0 0x00000000004009e7 in backtrace() () #1 0x0000000000400b18 in fac(int) () #2 0x0000000000400b49 in func1(int) () #3 0x0000000000400b69 in func2(int) () #4 0x000000000040078a in main () (gdb) It looks like this issue is showing up due to the order in which ‘%rbp’ is saved/restored w.r.t the ‘stack alignment code’ execution. In CLANG, %rbp is pushed/set first, then the stack is aligned, here is the CLANG generated code: Some_func: 0x0000000000400850 <+0>: push %rbp 0x0000000000400851 <+1>: mov %rsp,%rbp : 0x000000000040085d <+13>: and $0xfffffffffffffff0,%rsp 0x0000000000400861 <+17>: sub $0x8d0,%rsp 0x0000000000400868 <+24>: lea 0x520(%rsp),%r14 0x0000000000400870 <+32>: mov %r14,%rdi : 0x000000000040093a <+234>: lea -0x28(%rbp),%rsp : 0x0000000000400947 <+247>: pop %rbp 0x0000000000400948 <+248>: retq And with the CLANG generated code, ‘gdb – where’ works fine at every instruction pointer. Whereas, in GCC, the stack is aligned first, then the %rbp is pushed/set and that seems to be opening up a few instruction holes where 'gdb - where' wouldn't work. Would you folks consider this as a GCC bug? Do you folks see any issue in the CLANG approach? Is there a way (any switch) in gcc which would reverse the order of ‘setting %rbp’ vs ‘stack alignment code’ (like CLANG)? I used -mstackrealign with -O3 and -fomit-frame-pointer for generating my code.
