From: "Gerd Möllmann" <gerd.moellmann@gmail.com>
To: Helmut Eller <eller.helmut@gmail.com>
Cc: Eli Zaretskii <eliz@gnu.org>, Emacs Devel <emacs-devel@gnu.org>
Subject: Re: MPS prstack
Date: Mon, 27 May 2024 11:47:03 +0200 [thread overview]
Message-ID: <m2ttijwprc.fsf@pro2.fritz.box> (raw)
In-Reply-To: <87a5kbobt4.fsf@gmail.com> (Helmut Eller's message of "Mon, 27 May 2024 11:15:35 +0200")
Helmut Eller <eller.helmut@gmail.com> writes:
> On Mon, May 27 2024, Gerd Möllmann wrote:
>
>>> prstack.stack[prstack.sp++] = e;
>>
>> Hm. No idea when the increment is sequenced. The side effect of postfix
>> ++ could be applied anywhere before the next sequence point. But where
>> is that sequence point? There is one at the end of the whole expression
>> I think.
>
> The generated code with O0 looks like this:
>
> ...
> 0x000055555587fcb1 <+28>: call 0x55555587fbd0 <grow_print_stack>
> 0x000055555587fcb6 <+33>: mov 0x8845c3(%rip),%rcx # 0x555556104280 <prstack>
> 0x000055555587fcbd <+40>: mov 0x8845cc(%rip),%rax # 0x555556104290 <prstack+16>
> 0x000055555587fcc4 <+47>: lea 0x1(%rax),%rdx
> 0x000055555587fcc8 <+51>: mov %rdx,0x8845c1(%rip) # 0x555556104290 <prstack+16>
> 0x000055555587fccf <+58>: mov %rax,%rdx
> 0x000055555587fcd2 <+61>: mov %rdx,%rax
> 0x000055555587fcd5 <+64>: shl $0x3,%rax
> 0x000055555587fcd9 <+68>: sub %rdx,%rax
> 0x000055555587fcdc <+71>: shl $0x3,%rax
> 0x000055555587fce0 <+75>: add %rcx,%rax
> 0x000055555587fce3 <+78>: mov 0x10(%rbp),%rcx
> 0x000055555587fce7 <+82>: mov 0x18(%rbp),%rbx
> ...
>
> So it looks like prstack.sp is updated with the move at +51, i.e. before
> the stack slot is initialized.
>
> We could write
>
> prstack.stack[prstack.sp] = e;
> prstack.sp += 1;
>
> Not sure how we then guarantee that e still in a register or the control
> stack at the second line.
>
> But my question was more whether MPS actually does call scan_prstack at
> "any moment" or if this is just a theoretical possibility for some
> possible future version of MPS.
I think it is "at any moment". At least I can't find anything in the
docs hinting at something else.
Which probably means that we can't wholy trust stack points. They may be
off.
next prev parent reply other threads:[~2024-05-27 9:47 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-20 17:54 MPS prstack Helmut Eller
2024-05-20 18:21 ` Gerd Möllmann
2024-05-20 18:32 ` Helmut Eller
2024-05-21 3:30 ` Gerd Möllmann
2024-05-22 16:27 ` Helmut Eller
2024-05-22 17:55 ` Gerd Möllmann
2024-05-24 14:17 ` Helmut Eller
2024-05-24 14:27 ` Gerd Möllmann
2024-05-25 13:45 ` Gerd Möllmann
2024-05-27 7:27 ` Helmut Eller
2024-05-27 8:43 ` Gerd Möllmann
2024-05-27 9:15 ` Helmut Eller
2024-05-27 9:47 ` Gerd Möllmann [this message]
2024-05-27 11:45 ` Eli Zaretskii
2024-05-27 12:39 ` Helmut Eller
2024-05-27 13:12 ` Eli Zaretskii
2024-05-27 14:15 ` Helmut Eller
2024-05-27 14:37 ` Mattias Engdegård
2024-05-27 14:54 ` Helmut Eller
2024-05-27 14:43 ` Po Lu
2024-05-27 15:08 ` Helmut Eller
2024-05-27 11:34 ` Eli Zaretskii
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