llvm dev - Aug 2011 - [LLVMdev] SwitchInst::addCase with BlockAddress

I'm trying to figure out how to feed a blockaddress to a switch 
condition AND destination (basically emulating an indirectbr via a 
switch; I know it's not a good approach, I'm just experimenting).
Suppose I have the following:

SwitchInst *s = SwitchInst::Create(...);
BasicBlock *bb = ...;
PtrToIntInst k = new PtrToIntInst(BlockAddress::get(bb), <TYPE>,
"", s);
s->addCase(k, bb);
...

is this reasonable? what should I put in place of <TYPE> (the type to 
cast to)?
-- 
Carlo Alberto Ferraris <cafxx at strayorange.com 
<mailto:cafxx at strayorange.com>>
website/blog <http://cafxx.strayorange.com> - +39 333 7643 235
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Nella citazione domenica 31 luglio 2011 16:36:14, Carlo Alberto 
Ferraris ha scritto:
> I'm trying to figure out how to feed a blockaddress to a switch 
> condition AND destination (basically emulating an indirectbr via a 
> switch; I know it's not a good approach, I'm just experimenting).
> 
Basically the question boils down to: how do I get a ConstantInt out of 
a BlockAddress? I was able to get a ConstantExpr via 
ConstantExpr::getPointerCast but now I don't know how to get a 
ConstantInt out of a ConstantExpr...



-- 
Carlo Alberto Ferraris <cafxx at strayorange.com 
<mailto:cafxx at strayorange.com>>
website/blog <http://cafxx.strayorange.com> - +39 333 7643 235

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On Sun, Jul 31, 2011 at 7:36 AM, Carlo Alberto Ferraris
<cafxx at strayorange.com> wrote:
> I'm trying to figure out how to feed a blockaddress to a switch
condition
> AND destination (basically emulating an indirectbr via a switch; I know
it's
> not a good approach, I'm just experimenting).
> Suppose I have the following:
>
> SwitchInst *s = SwitchInst::Create(...);
> BasicBlock *bb = ...;
> PtrToIntInst k = new PtrToIntInst(BlockAddress::get(bb), <TYPE>,
"", s);
> s->addCase(k, bb);
> ...
>
> is this reasonable? what should I put in place of <TYPE> (the type to
cast
> to)?
You can't do that; switch cases are required to be constant integers,
and a blockaddress are not constant integers (at least not at
compile-time).

You can use an explicit comparison+branch, though.

-Eli

Nella citazione lunedì 1 agosto 2011 08:13:12, Eli Friedman ha
scritto:
> On Sun, Jul 31, 2011 at 7:36 AM, Carlo Alberto Ferraris
> <cafxx at strayorange.com>  wrote:
>> I'm trying to figure out how to feed a blockaddress to a switch
condition
>> AND destination (basically emulating an indirectbr via a switch; I know
it's
>> not a good approach, I'm just experimenting).
>> Suppose I have the following:
>>
>> SwitchInst *s = SwitchInst::Create(...);
>> BasicBlock *bb = ...;
>> PtrToIntInst k = new PtrToIntInst(BlockAddress::get(bb),<TYPE>,
"", s);
>> s->addCase(k, bb);
>> ...
>>
>> is this reasonable? what should I put in place of<TYPE>  (the
type to cast
>> to)?
> 
> You can't do that; switch cases are required to be constant integers,
> and a blockaddress are not constant integers (at least not at
> compile-time).
> 
> You can use an explicit comparison+branch, though.
BlockAddress is a class derived from Constant, so I thought it was 
meant to be, well, constant.
Besides, as
 I wrote, it is possible to get a ConstantExpr from a 
BlockAddress via ConstantExpr::getPointerCast (that in turn calls 
ConstantExpr::getPtrToInt).
Unfortunately, getPointerCast returns a Constant, not a ConstantInt; 
moreover, dyn_cast<ConstantInt> on that Constant returns null.
What I'm missing is, therefore, how do I get a ConstantInt out of a 
Constant obtained via ConstantExpr::getPointerCast?


-- 
Carlo Alberto Ferraris <cafxx at strayorange.com 
<mailto:cafxx at strayorange.com>>
website/blog <http://cafxx.strayorange.com> - +39 333 7643 235

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