[LLVMbugs] [Bug 2471] New: Illegal instcombine for load of address from select

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=2471

           Summary: Illegal instcombine for load of address from select
           Product: new-bugs
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: sharparrow1 at yahoo.com
                CC: llvmbugs at cs.uiuc.edu


Consider the following testcase:
declare i32 @x(i32*)
define i32 @b(i32* %a, i32* %b) {
entry:
        %tmp1 = load i32* %a            ; <i32> [#uses=1]
        %tmp3 = load i32* %b            ; <i32> [#uses=1]
        %add = add i32 %tmp1, %tmp3             ; <i32> [#uses=1]
        %call = call i32 @x( i32* %a )          ; <i32> [#uses=0]
        %tobool = icmp ne i32 %add, 0           ; <i1> [#uses=1]
        %cond = select i1 %tobool, i32* %b, i32* %a             ; <i32*>
[#uses=1]
        %tmp8 = load i32* %cond         ; <i32> [#uses=1]
        ret i32 %tmp8
}

Instcombine transforms this to the following:
declare i32 @x(i32*)
define i32 @b(i32* %a, i32* %b) {
entry:
        %tmp1 = load i32* %a            ; <i32> [#uses=1]
        %tmp3 = load i32* %b            ; <i32> [#uses=1]
        %call = call i32 @x( i32* %a )          ; <i32> [#uses=0]
        %add = sub i32 0, %tmp3         ; <i32> [#uses=1]
        %tobool = icmp ne i32 %tmp1, %add               ; <i1> [#uses=1]
        %b.val = load i32* %b           ; <i32> [#uses=1]
        %a.val = load i32* %a           ; <i32> [#uses=1]
        %tmp8 = select i1 %tobool, i32 %b.val, i32 %a.val               ; <i32>
[#uses=1]
        ret i32 %tmp8
}

The extra loads instcombine has inserted are dangerous; for example, suppose %a
was allocated by malloc(), and @x calls free() on %a.

isSafeToLoadUnconditionally is being too aggressive here.


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