Re: (select-window nil) crash with gcc-8.2.0

[Paul Eggert]

I reproduced the bug with GCC 8.3.1 20190223 (Red Hat 8.3.1-2) on x86-64. It's 
clearly a compiler bug with -O2 -Os. The machine code for select_window starts 
this way:

select_window:
        pushq   %r13
        movl    %edx, %r13d
        pushq   %r12
        pushq   %rbp
        movq    %rsi, %rbp
        pushq   %rbx
        movq    %rdi, %rbx
        pushq   %rcx
        call    WINDOWP
        movq    75(%rbx), %r12
        xorl    %edi, %edi
        testb   %al, %al
        je      .L981

and that last movq dereferences the window pointer in %rbx before the result of 
WINDOWP is checked to verify that the argument (originally in %rdi, now in %rbx) 
is indeed a window.

Could you file a GCC bug report for this? And in the meantime, I wouldn't use 
-Os.