llvm dev - Aug 2011 - [LLVMdev] Why int variable get promoted to i64

Hi, all

  I found in some cases the int variable get promoted to i64, 
although I think it should i32. I use the online demo
(http://llvm.org/demo). And below is the test case. 

------------- test case -------------
int test(int x[], int y[], int n) {
  int i = 0;
  int sum = 0;
  for ( ; i < n; i++) {
    sum += x[i] * y[i];
  }
  return sum;
}
-------------------------------------

  No matter the optimization level is standard or none, int
varible always get promoted to i64. After some discussions
on IRC, Duncan found GCC also does such promotion. So it's
not LLVM-only optimization.

  Anyone knows why? Thanks.

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667

Because you are compiling for a 64-bit system which uses LP64.  As such pointers
are 64-bit in length... while integers remain defined as 32-bits.

So why 64-bits for i?  The compiler is treating index variables special.  Index
variables need to be able to index 64-bits of space and in fact you'll see
the promotion here:

for.body:                                         ; preds = %for.cond
  %tmp2 = load i32* %i                            ; <i32> [#uses=1]
  %tmp3 = load i32** %x.addr                      ; <i32*> [#uses=1]
  %idxprom = sext i32 %tmp2 to i64                ; <i64> [#uses=1]
  %arrayidx = getelementptr inbounds i32* %tmp3, i64 %idxprom ; <i32*>
[#uses=1]

Cheers,

Joe

On Aug 19, 2011, at 5:32 AM, 陳韋任 wrote:
> int test(int x[], int y[], int n) {
>  int i = 0;
>  int sum = 0;
>  for ( ; i < n; i++) {
>    sum += x[i] * y[i];
>  }
>  return sum;
> }
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110819/c64b2680/attachment.html>

The code in clang that performs this promotion for array subscript expressions
is in CGExpr.cpp:

LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E) {
   
  ...snip...

  // Extend or truncate the index type to 32 or 64-bits.
  if (Idx->getType() != IntPtrTy)
    Idx = Builder.CreateIntCast(Idx, IntPtrTy, IdxSigned, "idxprom");
 
Cheers,

Joe

On Aug 19, 2011, at 8:16 AM, Joe Abbey wrote:
> Because you are compiling for a 64-bit system which uses LP64.  As such
pointers are 64-bit in length... while integers remain defined as 32-bits.
> 
> So why 64-bits for i?  The compiler is treating index variables special. 
Index variables need to be able to index 64-bits of space and in fact you'll
see the promotion here:
> 
> for.body:                                         ; preds = %for.cond
>   %tmp2 = load i32* %i                            ; <i32> [#uses=1]
>   %tmp3 = load i32** %x.addr                      ; <i32*> [#uses=1]
>   %idxprom = sext i32 %tmp2 to i64                ; <i64> [#uses=1]
>   %arrayidx = getelementptr inbounds i32* %tmp3, i64 %idxprom ;
<i32*> [#uses=1]
> 
> Cheers,
> 
> Joe
> 
> On Aug 19, 2011, at 5:32 AM, 陳韋任 wrote:
> 
>> int test(int x[], int y[], int n) {
>>  int i = 0;
>>  int sum = 0;
>>  for ( ; i < n; i++) {
>>    sum += x[i] * y[i];
>>  }
>>  return sum;
>> }
> 
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20110819/feced595/attachment.html>

> Because you are compiling for a 64-bit system which uses LP64.  As such
pointers are 64-bit in length... while integers remain defined as 32-bits.
> 
> So why 64-bits for i?  The compiler is treating index variables special. 
Index variables need to be able to index 64-bits of space and in fact you'll
see the promotion here:
> 
> for.body:                                         ; preds = %for.cond
>   %tmp2 = load i32* %i                            ; <i32> [#uses=1]
>   %tmp3 = load i32** %x.addr                      ; <i32*> [#uses=1]
>   %idxprom = sext i32 %tmp2 to i64                ; <i64> [#uses=1]
>   %arrayidx = getelementptr inbounds i32* %tmp3, i64 %idxprom ;
<i32*> [#uses=1]
  Indeed, the index variable "int i" won't be promoted if it 
is not used in the loop body.

------------- test case --------------
int test(int x[], int y[], int n) {
  int i = 0;
  int sum = 0;
  for( ; i < n; i++) {
    sum += x[2] + y[2];
  }
  return sum;
}
--------------------------------------

  But I don't see why the index variable need to be able to index
64-bits of space. Is compiler required to treat index variable
special?

  Thanks!

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667