[LLVMdev] Bug in InstructionCombining.cpp

Casey Carter ccarter at cs.uiuc.edu
Tue Sep 17 10:03:00 PDT 2002

ISSUE: This code:

%bob = type { int }

int %alias() {
    %pbob1 = alloca %bob
    %pbob2 = getelementptr %bob* %pbob1                   ;pbob2 aliases 
    %pbobel = getelementptr %bob* %pbob2, long 0, ubyte 0
    %rval = load int* %pbobel
    ret int %rval

Crashes when run through opt -instcombine.  InstCombiner visits 
instructions in reverse declaration order, but 
InstCombiner::visitGetElementPtr() , when trying to combine %pbobel with 
%pbob2, assumes that aliases like %pbob2 have already been eliminated. 
 The result is a dereference of an invalid iterator, and an attempt to 
replace %pbobel with "getelementptr %bob* %pbob1, ubyte 0".

ACTION: When  combining constant index expressions in InstCombiner, 
treat the source index as 0 if not present.

Casey Carter
Casey at Carter.net
ccarter at uiuc.edu
AIM: cartec69

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