[LLVMdev] alloca & store generation

[John Criswell]

Ryan M. Lefever wrote:
> After looking at this problem longer, I believe that there is something 
> wrong with the disassembler.  When I run my transformation and then 
> disassemble the output, I get bytecode that looks like:
>
> -----
> int %main(int %argc, sbyte** %argv) {
> entry:
>          alloca int              ; <int*>:0 [#uses=3]
>          alloca sbyte**          ; <sbyte***>:0 [#uses=3]
>          store int %argc, int* %0
>          store sbyte** %argv, sbyte*** %0
>          call void %F( int* %0, sbyte*** %0, int 1, int 0 )
>   

I believe the above bytecode is correct.  When an instruction is not
given a name, the compiler assigns it a numeric name.  Names are unique
to type (for versions of LLVM <= 1.9), so your first two alloca
instructions are named %0 and could be re-written as follows:

%0 = alloca int
%0 = alloca sbyte **

Hence, the store and call instruction are using the alloca'ed pointers
as intended.

It would probably be nice if the disassembler listed those default
assigned names explicitly.  There might be a good motivation for why it
doesn't, but if there is, I don't know the reason.

-- John T.

> -----
>
> However, if I use llc to generate C code from the bytecode that my 
> transformation produces, it looks like:
>
> -----
> int main(int ltmp_75_3, signed char **ltmp_76_9) {
>    int ltmp_3355_85;    /* Address-exposed local */
>    signed char **ltmp_3356_105;    /* Address-exposed local */
>    ...
>    *(&ltmp_3355_85) = ltmp_75_3;
>    *(&ltmp_3356_105) = ltmp_76_9;
>    initLogging((&ltmp_3355_85), (&ltmp_3356_105), 1, 0);
> -----
>
> The C code is what I intended.  That leaves me to believe that my 
> transformation produced the correct bytecode, but the dissassembler is 
> not properly disassebling the bytecode.
>
> Ryan
>
>
> Ryan M. Lefever wrote:
>   
>> I am writing a transformation that needs to add a call to a function F() 
>> at the beginning of main() with the addresses of argc and argv as 
>> parameters to F().  However, the bytecode file I'm transforming has not 
>> allocated space on the stack for argc and argv.  So, I developed my 
>> transformation to change main() from:
>>
>> -----
>> int main(int %argc, sbyte** %argv){
>> entry:
>>    ...
>>    // some use of %argc and %argv
>>    ...
>> }
>> -----
>>
>> to the following code:
>>
>> -----
>> int main(int %argc, sbyte** %argv){
>> entry:
>>    %argc_addr = alloca int
>>    %argv_addr = alloca sbyte**
>>    store int %argc, int* %argc_addr
>>    store sbyte** %argv, sbyte*** %argv_addr
>>    call void %F ( int %argc_addr, sbyte*** %argv_addr )
>>    %tmp = load int* %argc_addr
>>    %tmp1 = load int* %tmp
>>    ...
>>    // the use of %argc and %argv is transformed to use
>>    // %tmp and %tmp1, respectively
>>    ...
>> }
>> -----
>>
>> However, after adding the alloca and stores, I print out main it looks like:
>>
>> -----
>> int %main(int %argc, sbyte** %argv) {
>> entry:
>>    alloca int              ; <int*>:0 [#uses=1]
>>    alloca sbyte**          ; <sbyte***>:0 [#uses=1]
>>    store int %argc, int* %0
>>    store sbyte** %argv, sbyte*** %0
>>    ...
>> -----
>>
>> I used the following code in my transformation:
>>
>> -----
>>    BasicBlock* eb = M.getMainFunction()->getEntryBlock();
>>    Function::arg_iterator argc_it = mainfun->arg_begin();
>>    Function::arg_iterator argv_it = argc_it;
>>    ++argv_it;
>>    Argument* argc = &*argc_it;
>>    Argument* argv = &*argv_it;
>>    Instruction* insertNewInstsBefore = &eb->front();
>>    AllocaInst* argc_alloca = new AllocaInst(argc->getType(), "",
>> 					   insertNewInstsBefore);
>>    AllocaInst* argv_alloca = new AllocaInst(argv->getType(), "",
>> 					   insertNewInstsBefore);
>>    new StoreInst(argc, argc_alloca, false, insertNewInstsBefore);
>>    new StoreInst(argv, argv_alloca, false, insertNewInstsBefore);
>> -----
>>
>> Why isn't llvm giving a name to the value returned by the allocas and 
>> using it in the store instructions?
>>
>> Regards,
>> Ryan
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>     
>
>