[LLVMbugs] [Bug 13656] New: Optimizers wrongly think a GEP of global without inbounds can't point into a different global

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=13656

             Bug #: 13656
           Summary: Optimizers wrongly think a GEP of global without
                    inbounds can't point into a different global
           Product: new-bugs
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: baldrick at free.fr
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


This testcase was inspired by the fact that pointsToConstantGlobal in
InstCombineLoadStoreAlloca.cpp forgets to check "inbounds" on the GEP.
However my attempts to get a testcase failed because all kinds of other
parts of instcombine and constant folding also forget to check inbounds,
and would do wrong transforms before pointsToConstantGlobal could get
anywhere near the IR!  So here is a simple testcase showing the issue:
it loads the value of a non-constant global @B, but instcombine is fooled
into thinking it is loading from another, constant, global @A thanks to a
tricky GEP.

; RUN: opt < %s -instcombine -S | FileCheck %s
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"

@A = constant i8 0
@B = global i8 0

define i8 @foo() {
; CHECK: @foo
; Load @B while sneakily giving the appearance of loading part of @A.
  %val = load i8* getelementptr (i8* @A, i64 sub (i64 ptrtoint (i8* @B to i64),
i64 ptrtoint (i8* @A to i64)))
  ret i8 %val
; CHECK-NOT: ret i8 0
}

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