GNU bug report logs - #41357
28.0.50; GC may miss to mark calle safe register content

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Message #74 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: bug-gnu-emacs <at> gnu.org, akrl <at> sdf.org
Subject: Re: 28.0.50; GC may miss to mark calle safe register content
Date: Sun, 17 May 2020 14:21:49 -0700

On 5/17/20 12:26 PM, Eli Zaretskii wrote:
> And where's GC in this picture?  If it's called directly from 'f', can
> you show me such code in Emacs?  Then we could disassembly it and see
> what we've got.
> 
> Usually the code that calls GC is much deeper, and thus the chance of
> that temporary to stay in a register is very small, to say the least.

The probability is not that small, unfortunately. Compilers often have a habit
of running through the same set of callee-save registers in the same order.
Let's say you're on the x86 and your compiler consumes the four callee-save
registers in the order ebx, esi, edi, ebp. Then if we call f which calls g which
calls h which calls the GC, it's likely that f will save just ebx, then g will
save just ebx, esi, edi, then h will save just ebx and esi. Hence if the caller
has assigned a local variable to ebp, the GC won't see the variable's contents.

We should give Andrea a big round of applause for catching this bug.

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