:) thanks for make it perfectly clear.
----- Message d'origine ----
> De : Paolo Bonzini <bonz...@gnu.org>
> À : Mathieu Suen <mathieus...@yahoo.fr>
> Cc : help-smalltalk@gnu.org
> Envoyé le : Mar 12 octobre 2010, 16h 29min 14s
> Objet : Re: [Help-smalltalk] About no_opt macro
>
> On 10/12/2010 03:28 PM, Mathieu Suen wrote:
> > Will reading the vm.def, I cam across the no_opt macro.
> > I guess is to avoid some optimization but but what kind of optimization
> > and
>why.
> >
> > The macro is define as:
> >
> > #define no_opt(x) ({ __typeof__ ((x)) _result; \
> > asm ("" : "=r" (_result) : "0" ((x))); _result; })
> >
> >
> > From what I understand is that it define a variable _result and force to
>move x
> > into _result.
> >
> > The macro is used in tha bytecode PLUS_SPECIAL an MINUS SPECIAL ... :
> >
> > intptr_t iresult = no_opt (iop1 + iop2);
> >
> > Thanks for shading the light on this point.
>
> Here is the context:
>
> intptr_t iop1 = (intptr_t) op1;
> intptr_t iop2 = ((intptr_t) op2) - 1;
> intptr_t iresult = no_opt (iop1 + iop2);
> if (iresult < iop1)
>
> Remember that these snippets are copied in many places. For example, when
>compiling code for a superoperator (compound bytecode) like
>
> PUSH_TEMPORARY(*)
> PUSH_INTEGER(1)
> PLUS_SPECIAL()
>
> GCC would have
>
> intptr_t iop1 = (intptr_t) op1;
> intptr_t iresult = no_opt (iop1 + 2);
> if (iresult < iop1)
>
> Without no_opt, "iop1 + 2 < iop1" would be simplified to "false", thus
> causing
>the overflow check to misbehave.
>
> Paolo
>
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