llvm dev - Sep 2011 - [LLVMdev] FW: Need help in converting int to double

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Cheers,

James

-----Original Message-----
From: James Molloy
Sent: 22 September 2011 11:11
To: 'sarath chandra'
Subject: RE: [LLVMdev] Need help in converting int to double

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!!!
    `·.¸.·´

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--
Sairam,

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

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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
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