[LLVMbugs] [Bug 1657] New: licm promotes store to null pointer

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1657

           Summary: licm promotes store to null pointer
           Product: libraries
           Version: 2.0
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: jay.foad at antixlabs.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=1120)
 --> (http://llvm.org/bugs/attachment.cgi?id=1120)
bitcode for test case

I'm using LLVM 2.0 on cygwin.

The attached bitcode file pre.o contains a store to a null pointer:

        %tmp1.i = getelementptr %struct.decision* null, i32 0, i32 0    ; <i8*>
[#uses=1]
        store i8 0, i8* %tmp1.i

but in practice this is never executed, because it's preceded by a conditional
branch (where the condition is always true) which branches over it.

If I run this through "opt -f -o post.o pre.o -licm -debug", the resulting
bitcode file contains a load from a null pointer and a store to a null pointer,
both of which are unconditionally executed:

entry:
        ...
        %tmp1.i = getelementptr %struct.decision* null, i32 0, i32 0    ; <i8*>
[#uses=2]
        %tmp1.i.promoted = load i8* %tmp1.i             ; <i8> [#uses=1]

...

clear_modes.exit:               ; preds = %blah.i
        store i8 %tmp1.i.tmp.0, i8* %tmp1.i
        call void @exit( i32 0 )
        unreachable

This has broken my program: it's now going to trap, when it wouldn't have
before I ran the LICM pass on it.

Here's the debug output from LICM:

LICM hoisting to entry:         %tmp3.i = icmp eq %struct.decision* null, null;
<i1> [#uses=1]
LICM hoisting to entry:         %tmp1.i = getelementptr %struct.decision* null,
i32 0, i32 0            ; <i8*> [#uses=1]
LICM: Promoting value:  %tmp1.i = getelementptr %struct.decision* null, i32 0,
i32 0            ; <i8*> [#uses=1]


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