[LLVMbugs] [Bug 15967] New: basicaa claims no alias when there is

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=15967

            Bug ID: 15967
           Summary: basicaa claims no alias when there is
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Global Analyses
          Assignee: unassignedbugs at nondot.org
          Reporter: moo321 at mailinator.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

test.s:

target datalayout = "e"

%struct.foo = type { i32, i32 }

define i32 @main() {
  %t = alloca %struct.foo, align 4
  %1 = getelementptr inbounds %struct.foo* %t, i32 0, i32 0
  store i32 1, i32* %1, align 4
  %2 = getelementptr inbounds %struct.foo* %t, i64 1
  %3 = bitcast %struct.foo* %2 to i8*
  %4 = getelementptr inbounds i8* %3, i32 -1
  store i8 0, i8* %4
  %5 = getelementptr inbounds i8* %4, i32 -1
  store i8 0, i8* %5
  %6 = getelementptr inbounds i8* %5, i32 -1
  store i8 0, i8* %6
  %7 = getelementptr inbounds i8* %6, i32 -1
  store i8 0, i8* %7
  %8 = getelementptr inbounds i8* %7, i32 -1
  store i8 0, i8* %8
  %9 = getelementptr inbounds i8* %8, i32 -1
  store i8 0, i8* %9
  %10 = getelementptr inbounds i8* %9, i32 -1
  store i8 0, i8* %10
  %11 = getelementptr inbounds i8* %10, i32 -1
  store i8 0, i8* %11
  %12 = load i32* %1, align 4
  ret i32 %12
}


This is the source program:

struct foo {
        int a;
        int fill;
};

int main() {
        foo t;
        t.a = 1;

        char*p = (char*)(&t+1);
        for (int i=0;i<sizeof(t);++i) *--p = 0;

        return t.a;
}

Running 'opt test.s -S -basicaa -gvn' transforms the ret to 'ret i32 1'.

'clang -O2 test.cpp -S' produces:

define i32 @main() nounwind uwtable {
  ret i32 1
}

The program is supposed to return 0.

The error seems to be in lib/Analysis/BasicAliasAnalysis.cpp in the function
BasicAliasAnalysis::aliasGEP.
It seems to figure that
  %11 = getelementptr inbounds i8* %10, i32 -1
and
  %t = alloca %struct.foo, align 4
cannot possibly alias each other, because %11 is derived from 
  %2 = getelementptr inbounds %struct.foo* %t, i64 1
and those can not alias each other, or something along those lines.

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