llvm dev - Oct 2014 - [LLVMdev] Emit a jump instruction to a place inside basicblock

Hi all,

I'm a beginner in LLVM. Currently, I want to implement a pass that
generates a jump table. The entry in that table is a jump to some place
(may be an instruction) in a basic block.

I'm reading the JumpTable code in llvm 3.5, there is a table which contains
jump entries to functions. In AsmPrinter::doFinalization function from file
lib/CodeGen/AsmPrinter/AsmPrinter.cpp, it gets a MCSymbolRefExpr from the
function symbol.

While my question is, is there a way to insert jump to place inside a basic
block?

Thanks!

Xiaoguang
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On Wed, Oct 29, 2014 at 05:24:37PM -0400, Xiaoguang Wang
wrote:
> Hi all,
> 
> I'm a beginner in LLVM. Currently, I want to implement a pass that
generates a
> jump table. The entry in that table is a jump to some place (may be an
> instruction) in a basic block.
> 
> I'm reading the JumpTable code in llvm 3.5, there is a table which
contains
> jump entries to functions. In AsmPrinter::doFinalization function from file
> lib/CodeGen/AsmPrinter/AsmPrinter.cpp, it gets a MCSymbolRefExpr from the
> function symbol.
> 
> While my question is, is there a way to insert jump to place inside a basic
> block?
> 
> Thanks!
ISTM you can split a basic bloc in two using ``llvm::SplitBlock'' then
jump to the begining of the second part?

Hi,
>From my understanding of the LLVM IR, it is impossible to jump to the
middle of a Basic Block, only to its beginning. But there is a
splitBasicBlock function that seems like it might be useful to you, to make
sure a basic block is starting at the exact place you want to jump.

Best regards,
Robin

On Wed Oct 29 2014 at 2:28:09 PM Xiaoguang Wang <xgwang09 at gmail.com>
wrote:
> Hi all,
>
> I'm a beginner in LLVM. Currently, I want to implement a pass that
> generates a jump table. The entry in that table is a jump to some place
> (may be an instruction) in a basic block.
>
> I'm reading the JumpTable code in llvm 3.5, there is a table which
> contains jump entries to functions. In AsmPrinter::doFinalization function
> from file lib/CodeGen/AsmPrinter/AsmPrinter.cpp, it gets a MCSymbolRefExpr
> from the function symbol.
>
> While my question is, is there a way to insert jump to place inside a
> basic block?
>
> Thanks!
>
> Xiaoguang
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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On Thu, Oct 30, 2014 at 2:33 PM, Robin Morisset <morisset at google.com>
wrote:
> Hi,
>
> From my understanding of the LLVM IR, it is impossible to jump to the
> middle of a Basic Block, only to its beginning. But there is a
> splitBasicBlock function that seems like it might be useful to you, to make
> sure a basic block is starting at the exact place you want to jump.
>
> Best regards,
> Robin
>
Thanks you all! I'm looking at splitBasicBlock and trying to use that.
BTW, is there a way at lower level of LLVM that can implement this
functionality? E.g. MCBasicBlock or MCInst?

Sincerely,
Xiaoguang
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On Thu, Oct 30, 2014 at 3:46 AM, serge Guelton <
serge.guelton at enst-bretagne.fr> wrote:
> On Wed, Oct 29, 2014 at 05:24:37PM -0400, Xiaoguang Wang wrote:
> > Hi all,
> >
> > I'm a beginner in LLVM. Currently, I want to implement a pass that
> generates a
> > jump table. The entry in that table is a jump to some place (may be an
> > instruction) in a basic block.
> >
> > I'm reading the JumpTable code in llvm 3.5, there is a table which
> contains
> > jump entries to functions. In AsmPrinter::doFinalization function from
> file
> > lib/CodeGen/AsmPrinter/AsmPrinter.cpp, it gets a MCSymbolRefExpr from
the
> > function symbol.
> >
> > While my question is, is there a way to insert jump to place inside a
> basic
> > block?
> >
> > Thanks!
>
> ISTM you can split a basic bloc in two using ``llvm::SplitBlock''
then
> jump to the begining of the second part?
>
 Hi, I tried splitBasicBlock after the specific place in IR, and saved the
newly created BasicBlock*. This works well to create a basic block with a
LABEL to jump back. Then I modified the doFinalization method in AsmPrinter
to add a jump instruction to the LABEL. The result is it can create the
jump to the LABEL, but the LABEL just disappeared. It is like:

*The original IR before I use splitBasicBlock:*
entry:
 %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([8 x i8]*
@.str, i32 0, i32 0))
  ret i32 0

*The transformed IR code by using splitBasicBlock:*
entry:
  %call = call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([8 x
i8]* @.str, i32 0, i32 0))
  br label %RET_TABLE_0



RET_TABLE_0:                                      ; preds = %entry
  ret i32 0

*The final assembly when I modified AsmPrinter:doFinalization to print the
jump instruction back to LABEL:*
    calll   printf
    movl    %eax, -4(%ebp)          # 4-byte Spill
# BB#1:                                 # %.RET_TABLE_0
    movl    $0, %eax
... ...
    .align  8, 0x90
__LLVM_RET_TABLE_0:
    jmp .RET_TABLE_0

So the LLVM will optimize this branch in the pass? Can I disable this
optimization or is there other way to reserve the LABEL?
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