llvm dev - Jul 2012 - [LLVMdev] Merge 2 BasicBlocks

Hi!

I'm trying to merge 2 basicblocks. I look in BasicBlockUtils.h but the merge
function does not what I want.

I'm trying to merge basicblock from a switch where the 'case' have
no break, from that:

int a;
switch(c) {
    case 3:
        a=1;
    case 2;
        b=1;
    case 1:
        c=1;
}

to that:

int a;
switch(c) {
    case 3:
        a=1;
        b=1;
        c=1;
        break;
    case 2;
        b=1;
        c=1;
        break;
    case 1:
        c=1;
        break;
}

I use clone() on instruction and insert each cloned instruction onto the right
basicblock.
In that simple case, it works great, but when I have some code with pointers
like:

int a;
int b[5] = {1,2,3,4,5};
switch(c) {
    case 3:
        a=1;
        b[c] = 1;
    case 2;
        b=1;
        b[c] = 1;
    case 1:
        c=1;
        b[c] = 1;
}

I have a lot of 'Instruction does not dominate all uses!' because the
clone() method don't recalculate the dominator tree.

Is there a way to fix that?

Thx,
Cheers

'Instruction does not dominate all uses!' is from the Verifier pass
and usually signifies bad IR instead of a stale dominator tree. If you
dump the IR before and after, you may see what the problem is: usually
one instruction uses another that hasn't yet been defined.

The Verifier pass requests a dominator tree, so if you think it really
is just a stale dominator tree you can make sure that your pass
doesn't claim to preserve that analysis.

On Mon, Jul 16, 2012 at 7:33 AM, Rinaldini Julien
<julien.rinaldini at heig-vd.ch> wrote:
> Hi!
>
> I'm trying to merge 2 basicblocks. I look in BasicBlockUtils.h but the
merge function does not what I want.
>
> I'm trying to merge basicblock from a switch where the 'case'
have no break, from that:
>
> int a;
> switch(c) {
>     case 3:
>         a=1;
>     case 2;
>         b=1;
>     case 1:
>         c=1;
> }
>
> to that:
>
> int a;
> switch(c) {
>     case 3:
>         a=1;
>         b=1;
>         c=1;
>         break;
>     case 2;
>         b=1;
>         c=1;
>         break;
>     case 1:
>         c=1;
>         break;
> }
>
> I use clone() on instruction and insert each cloned instruction onto the
right basicblock.
> In that simple case, it works great, but when I have some code with
pointers like:
>
> int a;
> int b[5] = {1,2,3,4,5};
> switch(c) {
>     case 3:
>         a=1;
>         b[c] = 1;
>     case 2;
>         b=1;
>         b[c] = 1;
>     case 1:
>         c=1;
>         b[c] = 1;
> }
>
> I have a lot of 'Instruction does not dominate all uses!' because
the clone() method don't recalculate the dominator tree.
>
> Is there a way to fix that?
>
> Thx,
> Cheers
>
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