From: "Michael D Kinney" <michael.d.kinney@intel.com>
To: "Andrew (EFI) Fish" <afish@apple.com>,
edk2-devel-groups-io <devel@edk2.groups.io>
Cc: "Ni, Ray" <ray.ni@intel.com>, Rebecca Cran <rebecca@bsdio.com>,
"Kinney, Michael D" <michael.d.kinney@intel.com>
Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler
Date: Thu, 18 May 2023 17:05:06 +0000 [thread overview]
Message-ID: <CO1PR11MB49292E29E7DFB47B195FB696D27F9@CO1PR11MB4929.namprd11.prod.outlook.com> (raw)
In-Reply-To: <7C9FD4BA-328C-4CFE-AF5A-3A795BB147E4@apple.com>
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Static global will not work for XIP
Mike
From: Andrew (EFI) Fish <afish@apple.com>
Sent: Thursday, May 18, 2023 9:49 AM
To: edk2-devel-groups-io <devel@edk2.groups.io>; Kinney, Michael D <michael.d.kinney@intel.com>
Cc: Ni, Ray <ray.ni@intel.com>; Rebecca Cran <rebecca@bsdio.com>
Subject: Re: [edk2-devel] CpuDeadLoop() is optimized by compiler
Mike,
I pinged some compiler experts to see if our code is correct, or if the compiler has an issue. Seems to be trending compiler issue right now, but I’ve NOT gotten feedback from anyone on the spec committee yet.
If we move Index to a static global that would likely work around the compiler issue.
Thanks,
Andrew Fish
On May 18, 2023, at 8:36 AM, Michael D Kinney <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>> wrote:
Hi Ray,
So the code generated does deadloop, but is just not easy to resume from as we have been able to do in the past.
We use CpuDeadloop() for 2 purposes. One is a terminal condition with no reason to ever continue.
The 2nd is a debug aide for developers to halt the system at a specific location and then continue from that point, usually with a debugger, to step through code to an area to evaluate unexpected behavior.
We may have to do a NASM implementation of CpuDeadloop() to make sure it meets both use cases.
Mike
From: Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>
Sent: Thursday, May 18, 2023 3:00 AM
To: devel@edk2.groups.io<mailto:devel@edk2.groups.io>
Cc: Kinney, Michael D <michael.d.kinney@intel.com<mailto:michael.d.kinney@intel.com>>; Rebecca Cran <rebecca@bsdio.com<mailto:rebecca@bsdio.com>>; Ni, Ray <ray.ni@intel.com<mailto:ray.ni@intel.com>>
Subject: CpuDeadLoop() is optimized by compiler
Hi,
Starting from certain version of Visual Studio C compiler (I don’t have the exact version. I am using VS2019), CpuDeadLoop is now optimized quite well by compiler.
The optimization is so “good” that it becomes harder for developers to break out of the deadloop.
I copied the assembly instructions as below for your reference.
The compiler does not generate instructions that jump out of the loop when the Index is not zero.
So in order to break out of the loop, developers need to:
1. Manually adjust rsp by increasing 40
2. Manually “ret”
I am not sure if anyone has interest to re-write this function so that compiler can be “fooled” again.
Thanks,
Ray
=======================
; Function compile flags: /Ogspy
; File e:\work\edk2\MdePkg\Library\BaseLib\CpuDeadLoop.c
; COMDAT CpuDeadLoop
_TEXT SEGMENT
Index$ = 48
CpuDeadLoop PROC ; COMDAT
; 26 : {
$LN12:
00000 48 83 ec 28 sub rsp, 40 ; 00000028H
; 27 : volatile UINTN Index;
; 28 :
; 29 : for (Index = 0; Index == 0;) {
00004 48 c7 44 24 30
00 00 00 00 mov QWORD PTR Index$[rsp], 0
$LN10@CpuDeadLoo:
; 30 : CpuPause ();
0000d 48 8b 44 24 30 mov rax, QWORD PTR Index$[rsp]
00012 e8 00 00 00 00 call CpuPause
00017 eb f4 jmp SHORT $LN10@CpuDeadLoo
CpuDeadLoop ENDP
_TEXT ENDS
END
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next prev parent reply other threads:[~2023-05-18 17:05 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-18 9:59 CpuDeadLoop() is optimized by compiler Ni, Ray
2023-05-18 13:19 ` [edk2-devel] " Pedro Falcato
2023-05-18 15:36 ` Michael D Kinney
2023-05-18 16:49 ` [edk2-devel] " Andrew Fish
2023-05-18 17:05 ` Michael D Kinney [this message]
2023-05-18 17:08 ` Andrew Fish
2023-05-18 17:19 ` Michael D Kinney
2023-05-18 17:22 ` Andrew Fish
2023-05-18 17:24 ` Andrew Fish
2023-05-18 18:45 ` Andrew Fish
[not found] ` <17605136DCF3E084.26337@groups.io>
2023-05-18 20:45 ` Andrew Fish
2023-05-18 21:42 ` Michael D Kinney
2023-05-19 0:42 ` Andrew Fish
2023-05-19 2:53 ` Ni, Ray
2023-05-19 3:03 ` Jeff Fan
2023-05-19 15:31 ` Rebecca Cran
2023-05-19 16:31 ` Andrew Fish
2023-10-31 2:51 ` Ni, Ray
2023-10-31 3:37 ` Michael D Kinney
2023-10-31 8:30 ` Ni, Ray
2023-10-31 14:19 ` Michael D Kinney
2024-06-05 1:07 ` Michael D Kinney
2024-06-05 16:48 ` Oliver Smith-Denny
2024-06-07 16:57 ` Hernandez Miramontes, Jose Miguel
[not found] ` <1760952DCE55DF8D.29365@groups.io>
2023-05-19 16:09 ` Rebecca Cran
2023-05-18 17:36 ` Rebecca Cran
2023-05-18 18:21 ` Andrew Fish
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