Hi everyone,
I am learning llvm IR from official documents, especially the GEP instruction.
Now I have a problem about the GEP instructions generated by llvm.
As the example in https://llvm.org/docs/LangRef.html#getelementptr-instruction say, for the function "foo" in the following code example:
struct RT {
char A;
int B[10][20];
char C;
};
struct ST {
int X;
double Y;
struct RT Z;
};
int *foo(struct ST *s) {
return &s[1].Z.B[5][13];
}
the IR could be:
define i32* @foo(%struct.ST* %s) nounwind uwtable readnone optsize ssp {
entry:
%arrayidx = getelementptr inbounds %struct.ST, %struct.ST* %s, i64 1, i32 2, i32 1, i64 5, i64 13
ret i32* %arrayidx
}
or
define i32* @foo(%struct.ST* %s) {
%t1 = getelementptr %struct.ST, %struct.ST* %s, i32 1 ; yields %struct.ST*:%t1
%t2 = getelementptr %struct.ST, %struct.ST* %t1, i32 0, i32 2 ; yields %struct.RT*:%t2
%t3 = getelementptr %struct.RT, %struct.RT* %t2, i32 0, i32 1 ; yields [10 x [20 x i32]]*:%t3
%t4 = getelementptr [10 x [20 x i32]], [10 x [20 x i32]]* %t3, i32 0, i32 5 ; yields [20 x i32]*:%t4
%t5 = getelementptr [20 x i32], [20 x i32]* %t4, i32 0, i32 13 ; yields i32*:%t5
ret i32* %t5
}
I wonder when the llvm will generate the former one, and when it will generate the later one?
Thank you very much!
Sincerely,
Shulin