Re: [Tarantool-patches] [PATCH] jit: fix cdatanum addressing for GC64 mode on x86

[Igor Munkin <imun@tarantool.org>]

Sergos,

On 14.10.20, Sergey Ostanevich wrote:
> 
> 
> Thanks! 
> 
> LGTM.

Added your tag:
| Reviewed-by: Sergey Ostanevich <sergos@tarantool.org>

> 
> 
> On 14 окт 21:31, Igor Munkin wrote:
> > Sergos,
> > 
> > Thanks for you review!
> > 
> > On 14.10.20, Sergey Ostanevich wrote:
> > > Hi!
> > > 
> > > Thanks for the patch, I got one question below.
> > > 
> > > Regards,
> > > Sergos
> > > 
> > > On 14 окт 16:53, Igor Munkin wrote:
> > > > This patch fixes the regression introduced in scope of
> > > > 5f6775ae0e141422193ad9b492806834064027ca ('core: introduce various
> > > > platform metrics'). As a result of the patch <cdatanum> displacement is
> > > > misencoded when GC64 mode is enabled.
> > > > 
> > > > In X86 long mode 32-bit displacement is encoded either via SIB byte or
> > > > is addressed relatively to RIP register value. The first approach is
> > > > used in JIT for 32-bit addresses (i.e. when GC64 mode is disabled), but
> > > > doesn't work for 64-bit ones. As a result all addresses to GG_State
> > > > contents to be "hardcoded" on the trace are encoded relatively to
> > > > RID_DISPATCH register (i.e. callee-safe R14 register) containing global
> > > > dispatch table. For this purpose this register is not used by the JIT
> > > > register allocator in GC64 build and not spoiled throughout LuaJIT VM
> > > > cycle (and therefore trace execution).
> > > > 
> > > > NB: Since R14 is the additional GRP, the <add> instruction ought to be
> > > > REX-prefixed.
> > > > 
> > > > Follows up tarantool/tarantool#5187
> > > > 
> > > > Reported-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
> > > > Signed-off-by: Igor Munkin <imun@tarantool.org>
> > > > ---
> > > > 
> > > > Branch: https://github.com/tarantool/luajit/compare/imun/gh-5187-fix-disp-encoding-on-gc64
> > > > 
> > > > Unforunately, CI is red, but those failures relates to the known build
> > > > issues. Nevertheless I tested the patch manually on tntmac04 and faced
> > > > no failures.
> > > > 
> > > >  src/lj_asm_x86.h | 5 +++++
> > > >  1 file changed, 5 insertions(+)
> > > > 
> > > > diff --git a/src/lj_asm_x86.h b/src/lj_asm_x86.h
> > > > index 959fc2d..767bf6f 100644
> > > > --- a/src/lj_asm_x86.h
> > > > +++ b/src/lj_asm_x86.h
> > > > @@ -1837,8 +1837,13 @@ static void asm_cnew(ASMState *as, IRIns *ir)
> > > >  
> > > >    /* Increment cdatanum counter by address directly. */
> > > >    emit_i8(as, 1);
> > > > +#if LJ_GC64
> > > > +  emit_rmro(as, XO_ARITHi8, XOg_ADD|REX_64, RID_DISPATCH,
> > > > +	    dispofs(as, &J2G(as->J)->gc.cdatanum));
> > > 
> > > Should we cast the disp to 32bit? Here
> > 
> > IIRC, in function calls, the arguments are converted to the types of the
> > corresponding parameters. <ofs> parameter in <emit_rmro> is int32_t
> > type, so I guess an explicit cast is not obligatory here, isn't it?
> > 
> > > https://wiki.osdev.org/X86-64_Instruction_Encoding#Displacement
> > > I see only a disp32. 
> > 
> > However, as you've already mentioned offline the *valid* dispofs values
> > fit 32-bit integers since the size of GG_State equals to 6344 bytes.
> 
> I thought it should cover the difference from assembly placement to the
> _G, but now I got it should cover only the one from dispatch table to 
> your gc stuff. 

Precisely.

> 
> > 
> > I surmise, these explicit casts around relate to the old dark times when
> > various compilers were not so good, so Mike had to add such casts
> > everywhere. I checked the machine code generated by GCC on my machine
> > and see no difference between two versions: with or without the cast.
> > 
> > > 
> > > > +#else
> > > >    emit_rmro(as, XO_ARITHi8, XOg_ADD, RID_NONE,
> > > >  	    ptr2addr(&J2G(as->J)->gc.cdatanum));
> > > > +#endif
> > > >    /* Combine initialization of marked, gct and ctypeid. */
> > > >    emit_movtomro(as, RID_ECX, RID_RET, offsetof(GCcdata, marked));
> > > >    emit_gri(as, XG_ARITHi(XOg_OR), RID_ECX,
> > > > -- 
> > > > 2.25.0
> > > > 
> > 
> > -- 
> > Best regards,
> > IM

-- 
Best regards,
IM