[LLVMdev] Why int variable get promoted to i64
Joe Abbey
joe.abbey at gmail.com
Fri Aug 19 06:16:07 PDT 2011
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;
>> }
>
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