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Re: [Qemu-devel] [PATCH RFC 00/11] AREG0 elimination


From: Aurelien Jarno
Subject: Re: [Qemu-devel] [PATCH RFC 00/11] AREG0 elimination
Date: Sun, 15 May 2011 14:49:42 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Sun, May 15, 2011 at 03:30:17PM +0300, Blue Swirl wrote:
> On Sun, May 15, 2011 at 2:36 PM, Aurelien Jarno <address@hidden> wrote:
> > On Sun, May 15, 2011 at 02:12:27PM +0300, Blue Swirl wrote:
> >> On Sun, May 15, 2011 at 1:19 PM, Aurelien Jarno <address@hidden> wrote:
> >> > I still don't get where are this list of possible changes? Did I miss
> >> > something in another thread?
> >>
> >> I'm referring to the patches I sent.
> >
> > Ok, patches 1, 2 and 4 to 7 looks ok at a first glance, though I think
> > patches 6 and 7 should be done for all hosts or none of them.
> 
> The changes can be done in steps, but of course removing temp_buf from
> CPUState would need all targets to be converted first.
> 
> >> > On the TCG generated code, the env structure is used almost for every
> >> > op, so it really makes sense to keep it in a register instead of having 
> >> > to
> >> > reload the address of env regularly from memory. Given it only affects
> >> > TCG generated code, I don't see the point of portability here.
> >>
> >> For example, maybe the bugs in Sparc glibc could be avoided by using
> >> one of %i set of registers (not accessible from helpers) for AREG0
> >> within generated code instead of %g registers which seem to be
> >> fragile.
> >
> > First of all, but it's a different subject, I am not sure there are
> > sparc glibc bugs, I'd rather says QEMU mis-uses some register. For
> > example the following code is probably wrong:
> >
> > /* Note: must be synced with dyngen-exec.h */
> > #ifdef CONFIG_SOLARIS
> > #define TCG_AREG0 TCG_REG_G2
> > #elif defined(__sparc_v9__)
> > #define TCG_AREG0 TCG_REG_G5
> > #else
> > #define TCG_AREG0 TCG_REG_G6
> > #endif
> >
> > __sparc_v9__ can set on the 32-bit ABI, when the compiler targets V8+,
> > so the condition is probably wrong there. Secondly the SPARC ABI [1] on
> > page 23 that %g5 to %g7 are reserved for system. I don't think QEMU has
> > the right to use this registers.
> 
> Yes, but the situation is not so nice. Please see this post for status
> as of 2010:
> http://article.gmane.org/gmane.comp.emulators.qemu/63610
> 
> This is from Debian glibc 2.11.2-10:
> $ file /lib/libc-2.11.2.so
> /lib/libc-2.11.2.so: ELF 32-bit MSB shared object, SPARC32PLUS,
> version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux
> 2.6.18, stripped
> $ objdump -d /lib/libc.so.6 |grep %g1|wc -l
> 69648
> $ objdump -d /lib/libc.so.6 |grep %g2|wc -l
> 37299
> $ objdump -d /lib/libc.so.6 |grep %g3|wc -l
> 20635
> $ objdump -d /lib/libc.so.6 |grep %g4|wc -l
> 11603
> $ objdump -d /lib/libc.so.6 |grep %g5|wc -l
> 448
> $ objdump -d /lib/libc.so.6 |grep %g6|wc -l
> 150
> $ objdump -d /lib/libc.so.6 |grep %g7|wc -l
> 3052
> 
> Glibc is compiled for Sparc32plus, so it should only use %g6 and %g7,

>From the calling convention point of view, sparc32 and sparc32plus are
the same ABI, so %g5 is also reserved for system use.

> or %g1 and %g5 for scratch purposes. However, it is the application
> registers %g2 to %g4 that are used heaviest. Looking inside the
> objdump it's easy to see that the uses are not for example saving or
> restoring, but actually using them without saving the previous value
> first:

Well, we have to define system and application. System is defined as
library in Chapter 6, and I don't see the libc there, and is probably
considered as part of the application.

> 000211e0 <__divdi3>:
>    211e0:       9d e3 bf a0     save  %sp, -96, %sp
>    211e4:       90 10 00 18     mov  %i0, %o0
>    211e8:       92 10 00 19     mov  %i1, %o1
>    211ec:       94 10 00 1a     mov  %i2, %o2
>    211f0:       96 10 00 1b     mov  %i3, %o3
>    211f4:       80 a6 20 00     cmp  %i0, 0
>    211f8:       06 40 00 10     bl,pn   %icc, 21238 <__divdi3+0x58>
>    211fc:       a0 10 20 00     clr  %l0
>    21200:       80 a2 a0 00     cmp  %o2, 0
>    21204:       26 40 00 13     bl,a,pn   %icc, 21250 <__divdi3+0x70>
>    21208:       a0 38 00 10     xnor  %g0, %l0, %l0
>    2120c:       7f ff fe ed     call  20dc0 <__ashldi3+0x40>
>    21210:       98 10 20 00     clr  %o4
>    21214:       84 10 00 08     mov  %o0, %g2
> 
> ...whoops...
> 
>    21218:       80 a4 20 00     cmp  %l0, 0
>    2121c:       02 40 00 04     be,pn   %icc, 2122c <__divdi3+0x4c>
>    21220:       86 10 00 09     mov  %o1, %g3
> 
> ...whoops...
> 
>    21224:       86 a0 00 09     subcc  %g0, %o1, %g3
>    21228:       84 60 00 02     subc  %g0, %g2, %g2
>    2122c:       b2 10 00 03     mov  %g3, %i1
>    21230:       81 cf e0 08     rett  %i7 + 8
>    21234:       90 10 00 02     mov  %g2, %o0
>    21238:       92 a0 00 19     subcc  %g0, %i1, %o1
>    2123c:       90 60 00 18     subc  %g0, %i0, %o0
>    21240:       80 a2 a0 00     cmp  %o2, 0
>    21244:       16 4f ff f2     bge  %icc, 2120c <__divdi3+0x2c>
>    21248:       a0 10 3f ff     mov  -1, %l0
>    2124c:       a0 38 00 10     xnor  %g0, %l0, %l0
>    21250:       96 a0 00 0b     subcc  %g0, %o3, %o3
>    21254:       10 6f ff ee     b  %xcc, 2120c <__divdi3+0x2c>
>    21258:       94 60 00 0a     subc  %g0, %o2, %o2
>    2125c:       01 00 00 00     nop
> 
> This is libc from OpenBSD/Sparc64 4.9:
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g1|wc -l
>    40562
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g2|wc -l
>    20384
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g3|wc -l
>    10240
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g4|wc -l
>     6606
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g5|wc -l
>     3811
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g6|wc -l
>        4
> $ objdump -d /usr/lib/libc.so.58.0 |grep %g7|wc -l
>       20
> 
> Not so great there either.
> 
> > Anyway, I don't see why keeping TCG_AREG0 inside the TCG generated code
> > would prevent you to use a register from the %i set for it.
> 
> The helpers currently use global env register, but %i registers can't
> be accessed from the next level of function call nesting hierarchy so
> they can't be used for global env.
> 

That's the current situation yes. Using %i registers for TCG_AREG0 does
mean you can't use a global env register in the helpers, but it doesn't
mean that internal TCG code can't use them for TCG_AREG0.

What I am telling you since the beginning is that:
- I have no objection that we stop using a fixed register in GCC
  generated code (that is completely removing HELPER_CFLAGS). However I
  don't really see the point of doing that, though the Sparc issue might
  be an argument.
- I do have objection to remove TCG_AREG0 from inside the TCG generated
  code, this register is used for almost every TCG op, and I don't see
  any real argument for not keeping it.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net



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