[LLVMdev] Need help in converting int to double

sarath chandra
Thu, 22 Sep 2011 10:17:31 -0000

On Thu, Sep 22, 2011 at 3:46 PM, sarath chandra <sarathcse19 at
gmail.com>wrote:

> Hi James,
>
> First i converted the void * to int* and then did FPToSI...then did SHL...(
> because CreateShl only accepts integers... i pointer casted it to int64
type
> first)... Below is the code snippet....
>
>
> lhs = mBuilder.CreateStructGEP(firstArg, 0);
>     lhs = mBuilder.CreateLoad(lhs);
>     lhs = mBuilder.CreatePointerCast(lhs, PointerType::get(
>             mBuilder.getInt64Ty(), 0));
>     int typelhs = getValueType(lhs);
>     rhs = mBuilder.CreateStructGEP(secondArg, 0);
>     rhs = mBuilder.CreateLoad(rhs);
>     rhs = mBuilder.CreatePointerCast(rhs, PointerType::get(
>             mBuilder.getInt64Ty(), 0));
>
>     lhs = mBuilder.CreateLoad(lhs);
>     rhs = mBuilder.CreateLoad(rhs);
>
>     lhs = convertDoubleToInt64(lhs); // used FPToSI
>     rhs = convertDoubleToInt64(rhs);//    "         "
>
>     //lhs = mBuilder.CreateLoad(lhs);
>     //rhs = mBuilder.CreateLoad(rhs);
>
>     lhs = mBuilder.CreateShl(lhs, rhs);
>     //lhs = mBuilder.CreatePointerCast(lhs,
> PointerType::get(mBuilder.getDoubleTy(),0));
>     lhs = convertIntToDouble(lhs);
>     typelhs = getValueType(lhs);
>
>
>
>     llvm::Value* returnValue = mBuilder.CreateAlloca(PointerType::get(
>             mModule->getTypeByName("Value"), 0));
>
>     llvm::Value* valueStructSize = getValueStructSize();
>     llvm::Value* memory = insertCallToMalloc(valueStructSize);
>
>     memory = mBuilder.CreatePointerCast(memory, PointerType::get(
>             mModule->getTypeByName("Value"), 0));
>
>     mBuilder.CreateStore(memory, returnValue);
>
>     allocateAndAssignDoubleTypeVar(mBuilder.CreateLoad(returnValue), lhs);
>
>     //return the value
>     mBuilder.CreateRet(mBuilder.CreateLoad(returnValue));
>
>
>     /*
>      * create call to SHL function
>      */
>     mCurrentFunction = previousFunction;
>     mBuilder.SetInsertPoint(previousBlock);
>
>     return mBuilder.CreateCall2(shlFunction, aLHS, aRHS);
>
>
>
> On Thu, Sep 22, 2011 at 3:40 PM, James Molloy <James.Molloy at
arm.com>wrote:
>
>> Hi Sarath,
>>
>> Your example will break.
>>
>> If the values are held internally as doubles casted to void* (which
>> incidentally will only work on 64-bit systems), just casting void*
-> int*
>> will not get you a valid integer value. It will get you the
double's
>> internal representation as an integer. For example (actual values are
made
>> up):
>>
>>    double a = 42.0;
>>    void *v = (void*)&a;
>>    int b = *(int*)v;
>>
>> "b" will not contain 42. It will contain what looks like
random garbage
>> (although it is the IEE754 floating point representation of
"42.0").
>>
>> You need to perform an actual cast:
>>
>>    double a  = 42.0;
>>    void *v = (void*)&a;
>>    double b = *(double*)v;
>>    int c = (int)b;
>>
>> "c" will contain 42.
>>
>> In LLVM speak, you need to LOAD as a double*, perform a fptosi to
create
>> an int64, perform the shift, then sitofp and store:
>>
>> %1 = load bitcast i8* %arg0 to double*  ; Cast the void* to double*,
then
>> load it.
>> %2 = load bitcast i8* %arg1 to double*
>>
>> %3 = fptosi double %1 to i64 ; Perform explicit double -> int
conversion.
>> %4 = fptosi double %2 to i64
>>
>> %5 = shr i64 %3, %4 ; Perform your operation.
>>
>> %6 = sitofp i64 %5 to double ; Convert explicitly back to integer
>> representation.
>>
>> %7 = store bitcast i8* %dest to double*, double %6 ; Store back,
>> reinterpreting your void* (which is i8* in LLVM) as a double*.
>>
>>
>> Hopefully this makes sense, especially if you know how you would have
to
>> do it in C.
>>
>> Cheers,
>>
>> James
>>
>> From: sarath chandra [mailto:sarathcse19 at gmail.com]
>> Sent: 22 September 2011 10:49
>> To: James Molloy
>> Subject: Re: [LLVMdev] Need help in converting int to double
>>
>> Hi James,
>>
>>    CreateShl() accepts only integers or vectors as arguments. At the
>> starting my arguments , let us suppose LHS,RHS, are double values. To
pass
>> them as arguments to CreateShl(), i used Pointer Casting to convert the
>> Void* to Int*. Now the problem is after getting the result is, the
result
>> variable which captures the output is Int*.. i wanted to make it
>> Double*..(so that i can accomadate it in value structure)
>> On Thu, Sep 22, 2011 at 3:12 PM, James Molloy <James.Molloy at
arm.com>
>> wrote:
>> Hi Sarath,
>>
>> If you can only hold doubles (not integers), and you originally
converted
>> the doubles to integers to do an integer shift, why can you not just
convert
>> the result back to a double using CreateFPToSI ?
>>
>> CreateFPToSI(CreateShr(CreateSIToFP(arg0), CreateSIToFP(arg1)))
>>
>> Cheers,
>>
>> James
>>
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] On
>> Behalf Of sarath chandra
>> Sent: 22 September 2011 10:37
>> To: llvmdev at cs.uiuc.edu
>> Subject: [LLVMdev] Need help in converting int to double
>>
>> Hi,
>>
>>     I'm pursuing M.Tech course. As a part of the project work
i'm using
>> LLVM as back-end. My project area is "Enhancing the performance of
V8
>> javascript engine using LLVM as a back-end".
>>
>>    Now i'm writing code for shift left(SHL) operator. I had my own
Value
>> Structure .. it's like this
>>
>> Struct Value
>> {
>> void *val  ;
>> char type;
>> }
>>
>>  The "char type" holds
DoubleType,DoubleConst,StringType,StringConst...
>>
>>  when i'm executing the IrBuilder.CreateShl(LHS,RHS) instruction it
is
>> returning an integer value as output.. i'm unable to store the
value in my
>> structure....(because my structure can hold Doubles,Strings).
>>
>>  Is there any way to store the integer output in my structure( i used
>> CreateSIToFP() to change int to double)........
>>
>> Thanks in advance
>>
>> Regards,
>>
>>  (¨`·.·´¨)
>>   `·.¸(¨`·.·´¨)
>>  (¨`·.·´¨)¸.·´ Sarath!!!
>>    `·.¸.·´
>>
>> -- IMPORTANT NOTICE: The contents of this email and any attachments are
>> confidential and may also be privileged. If you are not the intended
>> recipient, please notify the sender immediately and do not disclose the
>> contents to any other person, use it for any purpose, or store or copy
the
>> information in any medium. Thank you.
>>
>>
>>
>> --
>> Sairam,
>>
>>  (¨`·.·´¨)
>>   `·.¸(¨`·.·´¨)
>>  (¨`·.·´¨)¸.·´ Sarath!!!
>>    `·.¸.·´
>>
>> -- IMPORTANT NOTICE: The contents of this email and any attachments are
>> confidential and may also be privileged. If you are not the intended
>> recipient, please notify the sender immediately and do not disclose the
>> contents to any other person, use it for any purpose, or store or copy
the
>> information in any medium.  Thank you.
>>
>>
>
>
> --
> Sairam,
>
>   (¨`·.·´¨)
>    `·.¸(¨`·.·´¨)
>   (¨`·.·´¨)¸.·´ Sarath!!!
>     `·.¸.·´
>



--
Sairam,

  (¨`·.·´¨)
   `·.¸(¨`·.·´¨)
  (¨`·.·´¨)¸.·´ Sarath!!!
    `·.¸.·´
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