Subject: bug#41357: 28.0.50; GC may miss to mark calle safe register content

From: Paul Eggert <[email-masked]>
Date: Sun, 17 May 2020 14:03:12 -0700

>>> I don't think that platform even has callee-saved registers?

>> Eli's platform is 32-bit Microsoft Windows, and W32 has four callee-save
>> registers (ebx, esi, edi, ebp) not counting esp and eip which are of course
>> callee-save by definition. So the problem could at least in theory be occurring
>> on his platform, depending on the compiler and its options.

> I've seen the same problem on 64-bit Windows as well, in Emacs
> compiled with a different (newer) version of GCC.  I don't think this
> has anything to do with how many registers are there. 

You're right that the number of registers doesn't matter, in the sense that the
problem can occur if any registers are callee-save. I was responding to Pip
Cet's comment, where he said he thought your platform (W32 in the original bug
report) had zero callee-saved registers. That would have meant the problem
couldn't occur on your platform. However, because your platform does have
callee-save registers the problem can occur there.

64-bit Windows also has callee-save registers (rbx, rbp, rdi, rsi, r12, r13,
r14, r15) so it can also have the problem. Most platforms do have callee-save
these days.