llvm dev - Jun 2010 - [LLVMdev] How to prevent an instruction to be executed more than once?

I need to mark the beginning of some regions of code, that I want to patch at
runtime.
In this case, I want to mark the beginning of BB4 and then to patch the
jumps, because I want to temporarily change the flow at runtime and then to
restore the previous version.

In order to patch, I need to know the exact structure of the generated code.
So, I might have a BasicBlock like:

BB4:
 call void asm "label_BB4:\0A", ""()
 call void asm "jmp label_BB5\0A", ""()
 br label %BB5

and in the generated code the br label %BB5 will actually never be reached.
But I introduce the jmp in inline asm in hexa code with .byte in order to
control if it is a long or a short jump and to know how many bytes to patch.

I tried to keep it short, please let me know if it is not clear enough.
Mainly, what I need to know statically is the beginning of the code
contained in the BasicBlock BB4 and the size of jump, in the generated code.

Alexandra
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On Jun 24, 2010, at 7:00 AMPDT, Xinfinity wrote:
>
> I need to mark the beginning of some regions of code, that I want to  
> patch at
> runtime.
> In this case, I want to mark the beginning of BB4 and then to patch  
> the
> jumps, because I want to temporarily change the flow at runtime and  
> then to
> restore the previous version.
>
> In order to patch, I need to know the exact structure of the  
> generated code.
> So, I might have a BasicBlock like:
>
> BB4:
> call void asm "label_BB4:\0A", ""()
> call void asm "jmp label_BB5\0A", ""()
> br label %BB5
You can't prevent various optimizations from making multiple copies of  
asm's.  Also, branches from one asm to another, or into or out of an  
asm, do not work, in general.   All of this is the same as gcc.

I'm not sure if a branch within the same asm is powerful enough to do  
what you want, but the way to avoid the duplicate-label problem in  
that case is to use local labels, "jmp 0f" etc.  See gcc description  
of asm for examples.
> and in the generated code the br label %BB5 will actually never be  
> reached.
> But I introduce the jmp in inline asm in hexa code with .byte in  
> order to
> control if it is a long or a short jump and to know how many bytes  
> to patch.
>
> I tried to keep it short, please let me know if it is not clear  
> enough.
> Mainly, what I need to know statically is the beginning of the code
> contained in the BasicBlock BB4 and the size of jump, in the  
> generated code.
>
> Alexandra
> -- 
> View this message in context:
http://old.nabble.com/How-to-prevent-an-instruction-to-be-executed-more-than-once--tp28981639p28982790.html
> Sent from the LLVM - Dev mailing list archive at Nabble.com.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

As I have seen in the CodeGen phase there is the tail duplication step which
duplicates all my inlined asm. However, tail duplication checks for each
machine instruction if the description allows duplication or not. 

/// isNotDuplicable - Return true if this instruction cannot be safely
00363   /// duplicated.  For example, if the instruction has a unique labels
attached
00364   /// to it, duplicating it would cause multiple definition errors.
00365   bool isNotDuplicable() const {
00366     return Flags & (1 << TID::NotDuplicable);
00367   }

Is there a way to mark my instructions as not duplicable and to propagate
this description till the machine instruction level? What is the "unique
label" attached to an instruction?

Thanks.

Alexandra.
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