[Qemu-devel] Re: sparc64 lazy conditional codes evaluation

[Blue Swirl]

On 5/8/10, Igor Kovalenko <address@hidden> wrote:
> On Thu, May 6, 2010 at 10:51 PM, Blue Swirl <address@hidden> wrote:
>  > On 5/5/10, Igor Kovalenko <address@hidden> wrote:
>  >> On Wed, May 5, 2010 at 12:21 AM, Blue Swirl <address@hidden> wrote:
>  >>  > On 5/3/10, Igor Kovalenko <address@hidden> wrote:
>  >>  >> On Mon, May 3, 2010 at 11:54 PM, Blue Swirl <address@hidden> wrote:
>  >>  >>  > On 5/3/10, Igor Kovalenko <address@hidden> wrote:
>  >>  >>  >> On Mon, May 3, 2010 at 11:24 PM, Blue Swirl <address@hidden> 
> wrote:
>  >>  >>  >>  > On 5/3/10, Igor Kovalenko <address@hidden> wrote:
>  >>  >>  >>  >> Hi!
>  >>  >>  >>  >>
>  >>  >>  >>  >>  There is an issue with lazy conditional codes evaluation 
> where
>  >>  >>  >>  >>  we return from trap handler with mismatching conditionals.
>  >>  >>  >>  >>
>  >>  >>  >>  >>  I seldom reproduce it here when dragging qemu window while
>  >>  >>  >>  >>  machine is working through silo initialization. I use gentoo 
> minimal cd
>  >>  >>  >>  >>  install-sparc64-minimal-20100322.iso but I think anything 
> with silo boot
>  >>  >>  >>  >>  would experience the same. Once in a while it would report 
> crc error,
>  >>  >>  >>  >>  unable to open cd partition or it would fail to decompress 
> image.
>  >>  >>  >>  >
>  >>  >>  >>  > I think I've also seen this.
>  >>  >>  >>  >
>  >>  >>  >>  >>  Pattern that fails appears to require a sequence of compare 
> insn
>  >>  >>  >>  >>  possibly followed by a few instructions which do not touch 
> conditionals,
>  >>  >>  >>  >>  then conditional branch insn. If it happens that we trap 
> while processing
>  >>  >>  >>  >>  conditional branch insn so it is restarted after return from 
> trap then
>  >>  >>  >>  >>  seldom conditional codes are calculated incorrectly.
>  >>  >>  >>  >>
>  >>  >>  >>  >>  I cannot point to exact cause but it appears that after trap 
> return
>  >>  >>  >>  >>  we may have CC_OP and CC_SRC* mismatch somewhere,
>  >>  >>  >>  >>  since adding more cond evaluation flushes over the code 
> helps.
>  >>  >>  >>  >>
>  >>  >>  >>  >>  We already tried doing flush more frequently and it is still 
> not
>  >>  >>  >>  >>  complete, so the question is how to finally do this once and 
> right :)
>  >>  >>  >>  >>
>  >>  >>  >>  >>  Obviously I do not get the design of lazy evaluation right, 
> but
>  >>  >>  >>  >>  the following list appears to be good start. Plan is to 
> prepare
>  >>  >>  >>  >>  a change to qemu and find a way to test it.
>  >>  >>  >>  >>
>  >>  >>  >>  >>  1. Since SPARC* is a RISC CPU it seems to be not profitable 
> to
>  >>  >>  >>  >>    use DisasContext->cc_op to predict if flags should be not 
> evaluated
>  >>  >>  >>  >>    due to overriding insn. Instead we can drop cc_op from 
> disassembler
>  >>  >>  >>  >>    context and simplify code to only use cc_op from env.
>  >>  >>  >>  >
>  >>  >>  >>  > Not currently, but in the future we may use that to do even 
> lazier
>  >>  >>  >>  > flags computation. For example the sequence 'cmp x, y; bne 
> target'
>  >>  >>  >>  > could be much more optimal by changing the branch to do the
>  >>  >>  >>  > comparison. Here's an old unfinished patch to do some of this.
>  >>
>  >>
>  >> I wonder if it buys anything. Sparc RISC architecture means optimizing 
> compiler
>  >>  would prevent any extra flags computation, right? So it is basically 
> 1-to-1
>  >>  conditional computation and use. Or even worse, if we delay computation
>  >>  until there are two or more consumers, correct?
>  >
>  > For x86 target, that is the other part of savings from using lazy
>  > condition computation. It's true that it will not benefit RISC targets
>  > much.
>  >
>  > But I think you are missing the other part, where the actual flags are
>  > not calculated but instead the original comparison can be used.
>  >
>  > For example, consider this Sparc64 code:
>  > IN:
>  > 0x000001fff000be58:  cmp  %g2, 0x51
>  >
>  > OUT: [size=82]
>  > 0x40df16b0:  mov    0x10(%r14),%rbp
>  > 0x40df16b4:  mov    %rbp,%rbx
>  > 0x40df16b7:  sub    $0x51,%rbx
>  > 0x40df16bb:  mov    %rbx,%r12
>  > 0x40df16be:  mov    %r12,0x10a60(%r14)
>  > 0x40df16c5:  mov    %rbp,0x60(%r14)
>  > 0x40df16c9:  mov    $0x51,%ebp
>  > 0x40df16ce:  mov    %rbp,0x68(%r14)
>  > 0x40df16d2:  mov    %rbx,0x70(%r14)
>  > 0x40df16d6:  mov    $0x7,%ebp
>  > 0x40df16db:  mov    %ebp,0x78(%r14)
>  > 0x40df16df:  mov    $0x1fff000be5c,%rbp
>  > 0x40df16e9:  mov    %rbp,0x48(%r14)
>  > 0x40df16ed:  mov    $0x1fff000be60,%rbp
>  > 0x40df16f7:  mov    %rbp,0x50(%r14)
>  > 0x40df16fb:  xor    %eax,%eax
>  > 0x40df16fd:  jmpq   0xbfface
>  >
>  > --------------
>  > IN:
>  > 0x000001fff000be5c:  bne  0x1fff000c268
>  >
>  > OUT: [size=95]
>  > 0x40df1710:  callq  0x518260
>  > 0x40df1715:  mov    0x98(%r14),%ebp
>  > 0x40df171c:  mov    %ebp,%ebx
>  > 0x40df171e:  shr    $0x16,%rbx
>  > 0x40df1722:  and    $0x1,%ebx
>  > 0x40df1725:  xor    $0x1,%rbx
>  > 0x40df1729:  mov    %rbx,0x90(%r14)
>  > 0x40df1730:  mov    $0x1fff000be60,%rbp
>  > 0x40df173a:  mov    %rbp,0x48(%r14)
>  > 0x40df173e:  test   %rbx,%rbx
>  > 0x40df1741:  je     0x40df175a
>  > 0x40df1747:  mov    $0x1fff000c268,%rbp
>  > 0x40df1751:  mov    %rbp,0x50(%r14)
>  > 0x40df1755:  jmpq   0x40df1768
>  > 0x40df175a:  mov    $0x1fff000be64,%rbp
>  > 0x40df1764:  mov    %rbp,0x50(%r14)
>  > 0x40df1768:  xor    %eax,%eax
>  > 0x40df176a:  jmpq   0xbfface
>  >
>  > Instead of calculating any flags, we should generate for combined
>  > 'cmp/btest + branch' sequences something like:
>  > mov    0x10(%r14),%rbp
>  > mov    %rbp,%rbx
>  > cmp    $0x51,%rbx
>  > je     0x40ac275a
>  > mov    $0x1fff000c268,%rbp
>  > mov    %rbp,0x50(%r14)
>  > jmpq   0x40ac2768
>  > mov    $0x1fff000be64,%rbp
>  > mov    %rbp,0x50(%r14)
>  > xor    %eax,%eax
>  > jmpq   0xbfface
>  >
>  > But I fully agree that correct code generation is the most important issue.
>  >
>  >>  >>  >>  >
>  >>  >>  >>  >>    Another point is that we always write to env->cc_op when
>  >>  >>  >>  >>  translating *cc insns
>  >>  >>  >>  >>    This should solve any issue with dc->cc_op prediction going
>  >>  >>  >>  >>    out of sync with env->cc_op and cpu_cc_src*
>  >>  >>  >>  >
>  >>  >>  >>  > I think this is what is happening now.
>  >>  >>  >>  >
>  >>  >>  >>  >>  2. We must flush lazy evaluation back to CC_OP_FLAGS in a 
> few cases when
>  >>  >>  >>  >>    a. conditional code is required by insn (like addc, cond 
> branch etc.)
>  >>  >>  >>  >>       - here we can optimize by evaluating specific bits 
> (carry?)
>  >>  >>  >>  >>       - not sure if it works in case we have two cond 
> consuming insns,
>  >>  >>  >>  >>         where first needs carry another needs the rest of 
> flags
>  >>  >>  >>  >
>  >>  >>  >>  > Here's another patch to optimize C flag handling. It doesn't 
> pass my
>  >>  >>  >>  > tests though.
>  >>  >>  >>  >
>  >>  >>  >>  >>    b. CCR is read by rdccr (helper_rdccr)
>  >>  >>  >>  >>       - have to compute all flags
>  >>  >>  >>  >>    c. trap occurs and we prepare trap level context (saving 
> pstate)
>  >>  >>  >>  >>       - have to compute all flags
>  >>  >>  >>  >>    d. control goes out of tcg runtime (so gdbstub reads 
> correct value from env)
>  >>  >>  >>  >>       - have to compute all flags
>  >>  >>  >>  >
>  >>  >>  >>  > Fully agree.
>  >>  >>  >>
>  >>  >>  >>
>  >>  >>  >> Cool
>  >>  >>  >>
>  >>  >>  >>  Still I'd propose to kill dc->cc_op, find a reliable way to test 
> it
>  >>  >>  >>  and then add it back possibly with more optimizations.
>  >>  >>  >>  I'm lost in the code up to the point where I believe we need to
>  >>  >>  >>  save/restore cc_op and cpu_cc* while switching trap levels.
>  >>  >>  >
>  >>  >>  > I'd think this should do the trick:
>  >>  >>  >
>  >>  >>  > diff --git a/target-sparc/op_helper.c b/target-sparc/op_helper.c
>  >>  >>  > index b27778b..94921cd 100644
>  >>  >>  > --- a/target-sparc/op_helper.c
>  >>  >>  > +++ b/target-sparc/op_helper.c
>  >>  >>  > @@ -3506,6 +3506,8 @@ void do_interrupt(CPUState *env)
>  >>  >>  >     }
>  >>  >>  >     tsptr = cpu_tsptr(env);
>  >>  >>  >
>  >>  >>  > +    helper_compute_psr();
>  >>  >>  > +
>  >>  >>  >     tsptr->tstate = ((uint64_t)GET_CCR(env) << 32) |
>  >>  >>  >         ((env->asi & 0xff) << 24) | ((env->pstate & 0xf3f) << 8) |
>  >>  >>  >         GET_CWP64(env);
>  >>  >>  >
>  >>  >>
>  >>  >>
>  >>  >> Thanks, this change seems to work here for silo issue.
>  >>  >>
>  >>  >>  Another change would be to flush for gdbstub use of GET_CCR and for
>  >>  >>  helper_rdccr.
>  >>  >>  I tried to embed flush into GET_CCR but the code looks ugly since we
>  >>  >>  need to proxy a call to helper_compute_psr from gdbstub passing
>  >>  >>  available env pointer.
>  >>  >>
>  >>  >>  Not really tested with your changes, but still what is the breakage 
> you see?
>  >>  >
>  >>  > Aurora 2.0 
> (http://distro.ibiblio.org/pub/linux/distributions/aurora/build-2.0/sparc/iso/)
>  >>  > breaks.
>  >>  >
>  >>  > This is what I get with git HEAD, having pressed enter key twice:
>  >>  > Welcome to Aurora SPARC Linux
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >                   +--------------+ CD Found +--------------+
>  >>  >                   |                                        |
>  >>  >                   | To begin testing the CD media before   |
>  >>  >                   | installation press OK.                 |
>  >>  >                   |                                        |
>  >>  >                   | Choose Skip to skip the media test     |
>  >>  >                   | and start the installation.            |
>  >>  >                   |                                        |
>  >>  >                   |      +----+             +------+       |
>  >>  >                   |      | OK |             | Skip |       |
>  >>  >                   |      +----+             +------+       |
>  >>  >                   |                                        |
>  >>  >                   |                                        |
>  >>  >                   +----------------------------------------+
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >  <Tab>/<Alt-Tab> between elements  | <Space> selects | <F12> next 
> screen
>  >>  >
>  >>  > This is what I get with the C flag patch applied:
>  >>  > Welcome to Aurora SPARC Linux
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >                    +--------------+ Error +---------------+
>  >>  >                    |                                      |
>  >>  >                    | failed to read keymap information:   |
>  >>  >                    | Success                              |
>  >>  >                    |                                      |
>  >>  >                    |               +----+                 |
>  >>  >                    |               | OK |                 |
>  >>  >                    |               +----+                 |
>  >>  >                    |                                      |
>  >>  >                    |                                      |
>  >>  >                    +--------------------------------------+
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >
>  >>  >  <Tab>/<Alt-Tab> between elements  | <Space> selects | <F12> next 
> screen
>  >>  >
>  >>
>  >>
>  >> I do reproduce the issue here with 0001-Convert-C-flag-input-BROKEN.patch
>  >
>  > The patch seems harmless, so maybe it uncovers some hideous bug.
>  >
>
>
> Right, the bug is inside the gen_op_*cc helpers - we need to compute C
>  using current flag source, whereas current implementation clobbers source
>  then computes C.
>
>  I think I now get the lazy evaluation design right :)
>
>  Something along these changes to your 0001-Convert-C-flag-input-BROKEN.patch
>  appears to be good for aurora 2.0 test:
>
>  diff --git a/target-sparc/translate.c b/target-sparc/translate.c
>  index 94c343d..ea7c71b 100644
>  --- a/target-sparc/translate.c
>  +++ b/target-sparc/translate.c
>  @@ -334,9 +334,9 @@ static inline void gen_op_add_cc(TCGv dst, TCGv
>  src1, TCGv src2)
>
>   static inline void gen_op_addxi_cc(TCGv dst, TCGv src1, target_long src2)
>   {
>  +    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_mov_tl(cpu_cc_src, src1);
>      tcg_gen_movi_tl(cpu_cc_src2, src2);
>  -    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_add_tl(cpu_cc_dst, cpu_cc_src, cpu_tmp0);
>      tcg_gen_addi_tl(cpu_cc_dst, cpu_cc_dst, src2);
>      tcg_gen_mov_tl(dst, cpu_cc_dst);
>  @@ -344,9 +344,9 @@ static inline void gen_op_addxi_cc(TCGv dst, TCGv
>  src1, target_long src2)
>
>   static inline void gen_op_addx_cc(TCGv dst, TCGv src1, TCGv src2)
>   {
>  +    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_mov_tl(cpu_cc_src, src1);
>      tcg_gen_mov_tl(cpu_cc_src2, src2);
>  -    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_add_tl(cpu_cc_dst, cpu_cc_src, cpu_tmp0);
>      tcg_gen_add_tl(cpu_cc_dst, cpu_cc_dst, cpu_cc_src2);
>      tcg_gen_mov_tl(dst, cpu_cc_dst);
>  @@ -417,9 +417,9 @@ static inline void gen_op_sub_cc(TCGv dst, TCGv
>  src1, TCGv src2)
>
>   static inline void gen_op_subxi_cc(TCGv dst, TCGv src1, target_long src2)
>   {
>  +    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_mov_tl(cpu_cc_src, src1);
>      tcg_gen_movi_tl(cpu_cc_src2, src2);
>  -    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_sub_tl(cpu_cc_dst, cpu_cc_src, cpu_tmp0);
>      tcg_gen_subi_tl(cpu_cc_dst, cpu_cc_dst, src2);
>      tcg_gen_mov_tl(dst, cpu_cc_dst);
>  @@ -427,9 +427,9 @@ static inline void gen_op_subxi_cc(TCGv dst, TCGv
>  src1, target_long src2)
>
>   static inline void gen_op_subx_cc(TCGv dst, TCGv src1, TCGv src2)
>   {
>  +    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_mov_tl(cpu_cc_src, src1);
>      tcg_gen_mov_tl(cpu_cc_src2, src2);
>  -    gen_helper_compute_C_icc(cpu_tmp0);
>      tcg_gen_sub_tl(cpu_cc_dst, cpu_cc_src, cpu_tmp0);
>      tcg_gen_sub_tl(cpu_cc_dst, cpu_cc_dst, cpu_cc_src2);
>      tcg_gen_mov_tl(dst, cpu_cc_dst);

Thanks a lot, with this patch my tests passed! I applied the combined patch.

I also did a bit of refactoring to get the original Sparc64 issue fixed.