llvm dev - Apr 2010 - [LLVMdev] Number of Bytes a GetElementPtr .

Hi,

  I have the value of a getelementptr instruction. How can i find how many
bytes it is accessing from the base address. I need this to call the alias
method.

Thanks,
Rohith.
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hi,

You could get it from the ScalarEvolution analysis(target data must
available):
c code:
int f(int a[][4], int i, int j) {
 return a[i][j];
}
llvm ir:
define i32 @f([4 x i32]* nocapture %a, i32 %i, i32 %j) nounwind readonly {
entry:
  %0 = sext i32 %i to i64                         ; <i64> [#uses=1]
  %1 = sext i32 %j to i64                         ; <i64> [#uses=1]
  %2 = getelementptr inbounds [4 x i32]* %a, i64 %0, i64 %1 ; <i32*>
[#uses=1]
  %3 = load i32* %2, align 4                      ; <i32> [#uses=1]
  ret i32 %3
}

SCEV expression of %2:
%2 = getelementptr inbounds [4 x i32]* %a, i64 %0, i64 %1 ; <i32*>
[#uses=1]
 -->  ((4 * (sext i32 %j to i64)) + (16 * (sext i32 %i to i64)) + %a)


--best regards
ether
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Hi Rohith,
>    I have the value of a getelementptr instruction. How can i find how
> many bytes it is accessing from the base address. I need this to call
> the alias method.
you can find code doing this in lots of places.  For example, take a
look at lines 205-220 in lib/Analysis/ConstantFolding.cpp, starting
     // Otherwise, add any offset that our operands provide.

Ciao,

Duncan.