[Vm-dev] Strange error while building stack vm

Eliot Miranda eliot.miranda at gmail.com
Thu Dec 30 21:35:16 UTC 2010

Hi Igor,

On Thu, Dec 30, 2010 at 1:37 AM, Igor Stasenko <siguctua at gmail.com> wrote:

> (warnings are supressed)
> In file included from
> /Users/sig/projects/cog/sig-cog/build/../platforms/Mac
> OS/vm/Developer/sqGnu.h:46,
>                 from
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c:20:
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c: At top level:
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c: At top level:
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c:15490: note:
> ‘theIP’ was declared here
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c:34730: note:
> ‘hdrTypeBits’ was declared here
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c:14690: note:
> ‘hdrTypeBits’ was declared here
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c:35424: note:
> ‘freeChunkSize’ was declared here
> /Users/sig/projects/cog/sig-cog/src/vm/gcc3x-interp.c:35311: note:
> ‘freeChunkSize’ was declared here
> /var/folders/kE/kEXZB7BWHXWwxswd6aL4XU+++TM/-Tmp-//ccIxqx9C.s:143250:FATAL:Symbol
> L0bereaveAllMarriedContexts already defined.

So two things here.  One is what these labels are.  The labels that are
added by the code generator simply for performance measurement.  See the
define of VM_LABEL in the various sqPlatformSpecific.h files.  So this is
not necessary but essential if using the VMProfiler to profile the stack
interpreter.  If a method is marked with an <asmLabel: true> pragma the code
generator outputs a label for it and one can ten identify the particular
code sequence (e.g. a bytecode) in the VMProfiler.

The other is what causes the error.  The C compiler can choose to inline
code, but both gcc and icc appear not to examine the code they're inlining
for inserted labels and end up duplicating the labels inserted by VM_LABEL.
 Since this duplication depends on the exact source and compiler
optimization level in force there's no easy way to predict when a label will
be duplicated.  The fix is to add <asmLabel: false> to the relevant method.
 A better fix might be to add a <C: #noinline> pragma that would result in
turning off inlining for the relevant function (for example I don't think it
appropriate at all that the C compiler is
inlining bereaveAllMarriedContexts; it doesn't happen all that often -
basically at snapshot or voiding machien code - and inlining it is a silly
decision on the C compiler's part).  A hack fix is of course to simply
define VM_LABEL as 0, but that criples the VMProfiler.


> i found the only use of it in generated code:
> voidVMStateForSnapshot(void) {
>    sqInt activeContext;
>    sqInt header;
>    sqInt oop;
>    sqInt sz;
>        activeContext = divorceAllFrames();
>        /* begin bereaveAllMarriedContexts */
>        VM_LABEL(0bereaveAllMarriedContexts);
> but gcc insists that there are another one???
> --
> Best regards,
> Igor Stasenko AKA sig.
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