[llvm-dev] Inst->replaceAllUsesWith and uses in ConstantExpr

[(IIIT) Siddharth Bhat via llvm-dev]

Sure. Essentially, I'm trying to replace malloc and free with custom
variants of these calls. However, in certain situations, we receive these
calls within bitcasts, for example,

call void bitcast (void (i8*)* @free to void (%struct.fson_string*)*)
    (%struct.fson_string* %13) #2


A simple replaceAllUsesWith is not going to work, because the @free is
within a bitcast, which is a ConstantExpr. Unless I did something wrong,
this particular occurence of @free was not replaced even when I called
replaceAllUsesOf on the @free object.

I currently have a really hacky way of achieving this: I expand every
ConstantExpr that uses this Value into Instructions, on which I then call
replaceUsesOfWith.

However, a neater solution would be appreciated.

Cheers,
Siddharth.
On Thu, 17 Aug 2017 at 10:22 Nemanja Ivanovic via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Siddarth,
> Perhaps to provide a short explanation why you want to RAUW
> ConstantExpr's. Then others are likely to be able to provide you with more
> useful feedback for how you can accomplish your goal.
>
> On Thu, Aug 17, 2017 at 8:53 AM, (IIIT) Siddharth Bhat via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I see. Is there a pre-existing way to do this in LLVM?
>>
>> Cheers,
>> ~Siddharth.
>>
>> On Thu, 17 Aug 2017 at 02:12 Craig Topper <craig.topper at gmail.com> wrote:
>>
>>> ConstantExprs are immutable, they can't be changed once they are
>>> created. And a ConstantExpr can reference other ConstantExprs. So replacing
>>> all uses of a Value in a ConstantExpr would require creating a new
>>> immutable object for each ConstantExpr that references the one you're
>>> changing. And that would continue rippling outward.
>>>
>>> ~Craig
>>>
>>> On Wed, Aug 16, 2017 at 5:01 PM, (IIIT) Siddharth Bhat via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Whoops, sorry, I meant "value->replaceAllUsesWith".
>>>>
>>>> Should I create a new post with an updated title?
>>>>
>>>> Thanks
>>>> Siddharth
>>>>
>>>> On Thu 17 Aug, 2017, 01:05 Tim Northover <t.p.northover at gmail.com>
>>>> wrote:
>>>>
>>>>> On 16 August 2017 at 15:39, (IIIT) Siddharth Bhat via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>> > From what I have observed, using `Inst->replaceAllUsesWith` does not
>>>>> replace
>>>>> > uses of the `Inst` in `ConstantExpr`s. Is there some way to have a
>>>>> universal
>>>>> > replaceAllUsesWith?
>>>>>
>>>>> Where are you seeing instructions used in ConstantExprs? That sounds
>>>>> really dodgy to me. Instructions are horribly non-Constant so I'd have
>>>>> thought it would be banned (even for technically plausible things like
>>>>> "and(i32 %inst, 0)").
>>>>>
>>>>> Cheers.
>>>>>
>>>>> Tim.
>>>>>
>>>> --
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>>>>
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>>>>
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