no debug info for stack guard checks

[nickdesaulniers]

Consider the following:

// clang -fstack-protector-strong -O2 -mstack-protector-guard-reg=gs -mstack-protector-guard-symbol=__stack_chk_guard -fno-pie -g
void foo (int*);

void bar (int x) {
    int y [x];
    foo(y);
}

It looks like the following assembler is lacking debug info:

        movq    __stack_chk_guard@GOTPCREL(%rip), %rbx
        movq    %gs:(%rbx), %rax
        movq    %rax, -16(%rbp)
...
        callq   __stack_chk_fail@PLT

This is more easily visible in Godbolt: https://godbolt.org/z/cnozfa15E

[llvmbot]

@llvm/issue-subscribers-debuginfo

[jmorse]

What would be your preferred source location for the stack check setup -- presumably the same as the prologue?

Putting a source location on the call to __stack_chk_fail is slightly more tricky seeing how multiple return paths presumably branch to the call. Possibly the closing brace of the function would be most appropriate, however I'm not sure if that's available so late in compilation.

[nickdesaulniers]

What would be your preferred source location for the stack check setup -- presumably the same as the prologue?

SGTM

Perhaps the epilog can be reused for __stack_chk_fail if available?

[nickdesaulniers]

Also, it looks like -g1 changes the debug info associated? (The coverage looks better with -g1 than -g2).