When people are first confronted with the GEP instruction, they tend to relate it to known concepts from other programming paradigms, most notably C array indexing and field selection. However, GEP is a little different and - this leads to the following questions, all of which are answered in the + this leads to the following questions; all of which are answered in the following sections.
The confusion with the first index usually arises from thinking about the GetElementPtr instruction as if it was a C index operator. They aren't the same. For example, when we write, in "C":
-- AType* Foo; - ... - X = &Foo->F;+ +
+AType *Foo; +... +X = &Foo->F; ++
it is natural to think that there is only one index, the selection of the field F. However, in this example, Foo is a pointer. That pointer must be indexed explicitly in LLVM. C, on the other hand, indexs @@ -85,8 +90,13 @@ code, you would provide the GEP instruction with two index operands. The first operand indexes through the pointer; the second operand indexes the field F of the structure, just as if you wrote:
-- X = &Foo[0].F;+ +
+X = &Foo[0].F; ++
Sometimes this question gets rephrased as:
@@ -96,19 +106,23 @@ the GEP instruction as an operand without any need for accessing memory. It must, therefore be indexed and requires an index operand. Consider this example: -Why is it okay to index through the first pointer, but subsequent pointers won't be dereferenced?
- struct munger_struct {
- int f1;
- int f2;
- };
- void munge(struct munger_struct *P)
- {
- P[0].f1 = P[1].f1 + P[2].f2;
- }
- ...
- munger_struct Array[3];
- ...
- munge(Array);
+
+
+struct munger_struct {
+ int f1;
+ int f2;
+};
+void munge(struct munger_struct *P) {
+ P[0].f1 = P[1].f1 + P[2].f2;
+}
+...
+munger_struct Array[3];
+...
+munge(Array);
+
+In this "C" example, the front end compiler (llvm-gcc) will generate three GEP instructions for the three indices through "P" in the assignment statement. The function argument P will be the first operand of each @@ -117,44 +131,58 @@ struct munger_struct type, for either the f1 or f2 field. So, in LLVM assembly the munge function looks like:
-
- void %munge(%struct.munger_struct* %P) {
- entry:
- %tmp = getelementptr %struct.munger_struct* %P, int 1, uint 0
- %tmp = load int* %tmp
- %tmp6 = getelementptr %struct.munger_struct* %P, int 2, uint 1
- %tmp7 = load int* %tmp6
- %tmp8 = add int %tmp7, %tmp
- %tmp9 = getelementptr %struct.munger_struct* %P, int 0, uint 0
- store int %tmp8, int* %tmp9
- ret void
- }
+
+
+void %munge(%struct.munger_struct* %P) {
+entry:
+ %tmp = getelementptr %struct.munger_struct* %P, i32 1, i32 0
+ %tmp = load i32* %tmp
+ %tmp6 = getelementptr %struct.munger_struct* %P, i32 2, i32 1
+ %tmp7 = load i32* %tmp6
+ %tmp8 = add i32 %tmp7, %tmp
+ %tmp9 = getelementptr %struct.munger_struct* %P, i32 0, i32 0
+ store i32 %tmp8, i32* %tmp9
+ ret void
+}
+
+In each case the first operand is the pointer through which the GEP instruction starts. The same is true whether the first operand is an argument, allocated memory, or a global variable.
To make this clear, let's consider a more obtuse example:
-- %MyVar = unintialized global int - ... - %idx1 = getelementptr int* %MyVar, long 0 - %idx2 = getelementptr int* %MyVar, long 1 - %idx3 = getelementptr int* %MyVar, long 2+ +
+%MyVar = unintialized global i32 +... +%idx1 = getelementptr i32* %MyVar, i64 0 +%idx2 = getelementptr i32* %MyVar, i64 1 +%idx3 = getelementptr i32* %MyVar, i64 2 ++
These GEP instructions are simply making address computations from the base address of MyVar. They compute, as follows (using C syntax):
-Since the type int is known to be four bytes long, the indices + +
+idx1 = (char*) &MyVar + 0 +idx2 = (char*) &MyVar + 4 +idx3 = (char*) &MyVar + 8 ++
Since the type i32 is known to be four bytes long, the indices 0, 1 and 2 translate into memory offsets of 0, 4, and 8, respectively. No memory is accessed to make these computations because the address of %MyVar is passed directly to the GEP instructions.
The obtuse part of this example is in the cases of %idx2 and %idx3. They result in the computation of addresses that point to memory past the end of the %MyVar global, which is only one - int long, not three ints long. While this is legal in LLVM, + i32 long, not three i32s long. While this is legal in LLVM, it is inadvisable because any load or store with the pointer that results from these GEP instructions would produce undefined results.
@@ -168,30 +196,36 @@Quick answer: there are no superfluous indices.
This question arises most often when the GEP instruction is applied to a global variable which is always a pointer type. For example, consider - this:
- %MyStruct = uninitialized global { float*, int }
- ...
- %idx = getelementptr { float*, int }* %MyStruct, long 0, ubyte 1
- The GEP above yields an int* by indexing the int typed + this:
+ +
+%MyStruct = uninitialized global { float*, i32 }
+...
+%idx = getelementptr { float*, i32 }* %MyStruct, i64 0, i32 1
+
+The GEP above yields an i32* by indexing the i32 typed field of the structure %MyStruct. When people first look at it, they - wonder why the long 0 index is needed. However, a closer inspection + wonder why the i64 0 index is needed. However, a closer inspection of how globals and GEPs work reveals the need. Becoming aware of the following facts will dispell the confusion:
- %MyVar = uninitialized global { [40 x int ]* }
- ...
- %idx = getelementptr { [40 x int]* }* %MyVar, long 0, ubyte 0, long 0, long 17
+
+
+%MyVar = uninitialized global { [40 x i32 ]* }
+...
+%idx = getelementptr { [40 x i32]* }* %MyVar, i64 0, i32 0, i64 0, i64 17
+
+In this example, we have a global variable, %MyVar that is a pointer to a structure containing a pointer to an array of 40 ints. The GEP instruction seems to be accessing the 18th integer of the structure's @@ -218,20 +257,30 @@ GEP instruction never accesses memory, it is illegal.
In order to access the 18th integer in the array, you would need to do the following:
-
- %idx = getelementptr { [40 x int]* }* %, long 0, ubyte 0
- %arr = load [40 x int]** %idx
- %idx = getelementptr [40 x int]* %arr, long 0, long 17
+
+
+%idx = getelementptr { [40 x i32]* }* %, i64 0, i32 0
+%arr = load [40 x i32]** %idx
+%idx = getelementptr [40 x i32]* %arr, i64 0, i64 17
+
+In this case, we have to load the pointer in the structure with a load instruction before we can index into the array. If the example was changed to:
-
- %MyVar = uninitialized global { [40 x int ] }
- ...
- %idx = getelementptr { [40 x int] }*, long 0, ubyte 0, long 17
+
+
+%MyVar = uninitialized global { [40 x i32 ] }
+...
+%idx = getelementptr { [40 x i32] }*, i64 0, i32 0, i64 17
+
+then everything works fine. In this case, the structure does not contain a pointer and the GEP instruction can index through the global variable, - into the first field of the structure and access the 18th int in the + into the first field of the structure and access the 18th i32 in the array there.
@@ -244,15 +293,20 @@If you look at the first indices in these GEP instructions you find that they are different (0 and 1), therefore the address computation diverges with that index. Consider this example:
-
- %MyVar = global { [10 x int ] }
- %idx1 = getlementptr { [10 x int ] }* %MyVar, long 0, ubyte 0, long 1
- %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1
+
+
+%MyVar = global { [10 x i32 ] }
+%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1
+%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
+
+In this example, idx1 computes the address of the second integer in the array that is in the structure in %MyVar, that is MyVar+4. The - type of idx1 is int*. However, idx2 computes the + type of idx1 is i32*. However, idx2 computes the address of the next structure after %MyVar. The type of - idx2 is { [10 x int] }* and its value is equivalent + idx2 is { [10 x i32] }* and its value is equivalent to MyVar + 40 because it indexes past the ten 4-byte integers in MyVar. Obviously, in such a situation, the pointers don't alias.
@@ -267,13 +321,18 @@These two GEP instructions will compute the same address because indexing through the 0th element does not change the address. However, it does change the type. Consider this example:
-
- %MyVar = global { [10 x int ] }
- %idx1 = getlementptr { [10 x int ] }* %MyVar, long 1, ubyte 0, long 0
- %idx2 = getlementptr { [10 x int ] }* %MyVar, long 1
+
+
+%MyVar = global { [10 x i32 ] }
+%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0
+%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1
+
+In this example, the value of %idx1 is %MyVar+40 and - its type is int*. The value of %idx2 is also - MyVar+40 but its type is { [10 x int] }*.
+ its type is i32*. The value of %idx2 is also + MyVar+40 but its type is { [10 x i32] }*.