[LLVMdev] cannot understand global c++API code

[Alexander Poddey]

Hi David,

Ok, got it, thank you!

When extending the struct to something including doubles, the API code uses
const_ptr_xy_indices.push_back(const_int64_yz).

Can I tell (programmatically) from the Type* if I should use a 32 or 64 bit 
int as ConstantInt* - is it the sizeof the Type* pointer?

Is there a 'shortcut' to programmatically accessing a globals sub-field?
Lets say that I know that the sequence would be '0', '1', '3'. 
Can I circumvent creating the Constant* vector, calling something like 
getGetElementPtr on a global using the integer values?


Another question would be if I could use the Constant * retrieved from
Constant* const_ptr_110 = ConstantExpr::getGetElementPtr(gvar_struct_foo, 
const_ptr_110_indices);

just like I do when using Instruction results as operands to other 
instructions; something like:

Value* gv=cast<Value>(const_ptr_110);
MyInst->setOperand(2, gv); // replace 3rd operand by global's data 


Thank you!
Alexander




David Blaikie wrote:

> On Sat, Mar 7, 2015 at 8:37 AM, Alexander Poddey
> <alexander.poddey at gmx.net> wrote:
> 
>> Hi all,
>>
>> translating the following c code to llvm c++Api code, I can not
>> understand the result.
>> Perhaps someone could explain it to me.
>>
>> c code
>> ---------------------------------
>>
>>  struct stest {
>>     int   age;
>>     float weight;
>>  } foo={44,67.2};
>>
>>
>>
>> int main() {
>>
>> foo.weight=68.2;
>>
>> ...
>> ----------------------------------------
>>
>> API code
>>
>> // this is clear
>> ConstantFP* const_float_102 = ConstantFP::get(mod->getContext(),
>> APFloat(6.820000e+01f));
>>
>>
>> ConstantInt* const_int32_98 = ConstantInt::get(mod->getContext(),
>> APInt(32, StringRef("0"), 10));
>> ConstantInt* const_int32_99 = ConstantInt::get(mod->getContext(),
>> APInt(32, StringRef("1"), 10));
>>
>>
>> std::vector<Constant*> const_ptr_103_indices;
>> const_ptr_103_indices.push_back(const_int32_98);
>> const_ptr_103_indices.push_back(const_int32_99);
>> Constant* const_ptr_103 = ConstantExpr::getGetElementPtr(gvar_struct_foo,
>> const_ptr_103_indices);
>>
>> //the store inst is in principle clear
>> StoreInst* void_119 = new StoreInst(const_float_102, const_ptr_103,
>> false, label_entry_113);
>> void_119->setAlignment(4);
>> ---------------------------------------------
>>
>> comment:
>> I don't get whats the meaning of the two const_int32 (98 & 99 above) and
>> why
>> the const_ptr_103 should relate the foo.weight field of the stest struct.
>>
> 
> http://llvm.org/docs/GetElementPtr.html
> 
> They're indexes into the structure (well, imagine that the pointer to the
> struct is a pointer to an array of structs of length 1 - so the first
> index is 0, to access the first (and only) element of that array, and the
> second index is 1 to access the second element of the structure)
> 
> 
>>
>>
>> Any hint appreciated!
>>
>> Alex
>>
>>
>>
>>
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