[LLVMdev] GEP vs IntToPtr/PtrToInt

[Arushi Aggarwal]

This code is generated for va_arg.
%6 = getelementptr inbounds %struct.__va_list_tag* %5, i32 0, i32 3 ; <i8**>
[#uses=1]
  %7 = load i8** %6, align 8                      ; <i8*> [#uses=1]
  %8 = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i64 0, i64 0
; <%struct.__va_list_tag*> [#uses=1]
 %9 = getelementptr inbounds %struct.__va_list_tag* %8, i32 0, i32 0 ;
<i32*> [#uses=1]
  %10 = load i32* %9, align 8                     ; <i32> [#uses=1]
  %11 = inttoptr i32 %10 to i8*                   ; <i8*> [#uses=1]
  %12 = ptrtoint i8* %7 to i64                    ; <i64> [#uses=1]
  %13 = ptrtoint i8* %11 to i64                   ; <i64> [#uses=1]
  %14 = add i64 %12, %13                          ; <i64> [#uses=1]
  %15 = inttoptr i64 %14 to i8*                   ; <i8*> [#uses=1]
  store i8* %15, i8** %addr.0, align 8

and I have optimized one inttoptr to a zext.

I guess it is safe in this case?


On Mon, Apr 4, 2011 at 9:10 AM, John Criswell <criswell at illinois.edu> wrote:

> On 4/4/2011 6:45 PM, Eli Friedman wrote:
>
>> On Mon, Apr 4, 2011 at 5:02 PM, Arushi Aggarwal<arushi987 at gmail.com>
>>  wrote:
>>
>>>
>>>  Hi,
>>>> Is it correct to convert,
>>>>   %196 = load i32* %195, align 8                  ;<i32>  [#uses=1]
>>>>   %197 = zext i32 %196 to i64                     ;<i64>  [#uses=1]
>>>>   %198 = ptrtoint i8* %193 to i64                 ;<i64>  [#uses=1]
>>>>   %199 = add i64 %198, %197                       ;<i64>  [#uses=1]
>>>>   %200 = inttoptr i64 %199 to i8*                 ;<i8*>  [#uses=1]
>>>> into
>>>> %200 = getelementptr %193, %196
>>>> Reducing the unnecessary casts of converting to integers and then back?
>>>> Thanks,
>>>> Arushi
>>>>
>>>>  See http://llvm.org/docs/LangRef.html#pointeraliasing ; it's not
>> correct in general.  It is correct if %196 isn't dependent on the
>> address of any memory object, though.
>>
>
> Can you clarify why the transform isn't correct?  Is it because in the
> original code, %200 is based on both the originally cast pointer (%193) and
> the indexed offset from it (%197) while the transformed code is only based
> on %193?
>
> Arushi, is some transform converting a GEP into this ptrtoint/inttoptr
> sequence and thereby adding "based on" relationships that didn't exist
> previously in the code?
>
> -- John T.
>
>  -Eli
>>
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>
>
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