From owner-freebsd-hackers@FreeBSD.ORG  Thu Dec  8 09:20:45 2011
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Date: Thu, 8 Dec 2011 10:01:59 +0100
From: Piotr Nowak <pn@semihalf.com>
To: Rafal Jaworowski <raj@semihalf.com>
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Cc: freebsd-hackers@freebsd.org, Gleb Kurtsou <gleb.kurtsou@gmail.com>,
	Arnaud Lacombe <lacombar@gmail.com>, mdf@freebsd.org
Subject: Re: gcc 4.2 miscompilation with -O2 -fno-omit-frame-pointer on amd64
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We're working on PowerPC target using GCC 4.2.1
and FreeBSD 6.1. It seems like we have similar
problem. In our case GCC sometimes very unfortunately
optimize code with -fno-omit-frame-pointer.

Example shown below covers file sys/powerc/booke/pmap.c
and function pmap_kenter. If we disassemble kernel binary
we have:

c019998c:   4b ec 6a ed     bl      c0060478 <_mtx_unlock_spin_flags>
c0199990:   81 61 00 00     lwz     r11,0(r1)
c0199994:   80 0b 00 04     lwz     r0,4(r11)
c0199998:   7d 61 5b 78     mr      r1,r11
c019999c:   82 ab ff d4     lwz     r21,-44(r11)
c01999a0:   7c 08 03 a6     mtlr    r0
c01999a4:   82 cb ff d8     lwz     r22,-40(r11)
c01999a8:   82 eb ff dc     lwz     r23,-36(r11)
c01999ac:   83 0b ff e0     lwz     r24,-32(r11)
c01999b0:   83 2b ff e4     lwz     r25,-28(r11)
c01999b4:   83 4b ff e8     lwz     r26,-24(r11)
c01999b8:   83 6b ff ec     lwz     r27,-20(r11)

As you can see stack pointer on R1 is being updated
before stashed data were pulled off stack. (mr r1,r11)
As a result of this we have chance to get crash when 
any interrupt hit shortly after stack pointer update. 
The interrupt prologue will override not yet pulled off 
pmap_kenter function data.

The problem occures only with -fno-omit-frame-pointer
and not every branch returns are beeing corrupted.

Do you think this issue may be somehow related to yours?
Are there any patches/solutions to fix it?

Regards,
Piotr