[LLVMdev] GEP vs IntToPtr/PtrToInt
criswell at illinois.edu
Mon Apr 4 07:10:36 PDT 2011
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:
>>> 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]
>>> %200 = getelementptr %193, %196
>>> Reducing the unnecessary casts of converting to integers and then back?
> 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.
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