[LLVMdev] Update PHINode after extracting code

[Vu Le]

I guess I didn't have a clear question.

Suppose we have BB1 and BB2 both point to BB3.
BB1 has variable x. BB2 also as variable x.
BB3 will have PHINode for x with 2 value from BB1 and BB2.
BB1   BB2
   \       /
     BB3

Now if BB1 and BB2 is extracted into a function
(using ExtractCodeRegion), they will be replaced by
a basic block called codeRepl (which has a call to the extracted
function).
codeRepl
     |
    BB3

The problem is that the PHINode in BB3 still has 2 values x,
both from codeRepl. That's wrong because BB3 has only 1 successors,
which is codeRepl. In fact, there is NO way to update PhiNode,
because all the x are from ONE node, codeRepl.
So the function ExtractCodeRegion doesn't work in this example.

My solution for this is before extracting BB1 and BB2, I split the PHINode
in BB3 into another basic block
BB1   BB2
   \       /
   PhiBB
       |
      BB
And extract BB1, BB2 and PhiBB.
If you have any ideas or if you think this will not work, please let me
know.
Thanks.
Vu


On Tue, Jan 25, 2011 at 12:33 AM, Vu Le <vmle at ucdavis.edu> wrote:

> Hi all,
> I have problem with ExtractCodeRegion (CodeExtractor.cpp).
> My original program is as follows.
> bb:
>   ...
>   %tmp.15 = load %struct.MYSQL_ROWS** %3, align 4
>   ...
> bb1:
>   ...
>   %tmp.1 = load %struct.MYSQL_ROWS** %6, align 4
>   ...
> bb4:                                              ; preds = %bb1, %bb,
> %entry
>   %tmp.0 = phi %struct.MYSQL_ROWS* [ null, %entry ], [ %tmp.15, %bb ], [
> %tmp.1, %bb1 ]
>
> %tmp.0 is the PHINode whose value is from entry, bb and bb1.
> After extracting bb and bb1 into new function, the program becomes
>
> codeRepl:                                         ; preds = %entry
>   call void @mysql_data_seek_bb(%struct.MYSQL_DATA* %1, i64 %row,
> %struct.MYSQL_ROWS** %tmp.15.loc, %struct.MYSQL_ROWS** %tmp.1.loc)
> *  %tmp.15.reload = load %struct.MYSQL_ROWS** %tmp.15.loc
>   %tmp.1.reload = load %struct.MYSQL_ROWS** %tmp.1.loc
> *  br label %bb4
>
> bb4:                                              ; preds = %codeRepl,
> %entry
> *  %tmp.0 = phi %struct.MYSQL_ROWS* [ null, %entry ], [ %tmp.15.reload,
> %codeRepl ], [ %tmp.1.reload, %codeRepl ]
>
> *bb4 now has only 2 predecessors since bb and bb1 are replaced by
> codeRepl.
> The PHINode in bb4, on the other hand, still has 3 incoming values and that
> make the assertion failed.
> I do want to extract this code region into function. Does anyone have a
> solution for this?
> Thanks a lot.
> Vu
>
>
>
>
>
>
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