[LLVMbugs] [Bug 9357] New: clang doesn't DCE dead switch cases, take 2

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9357

           Summary: clang doesn't DCE dead switch cases, take 2
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: pageexec at freemail.hu
                CC: llvmbugs at cs.uiuc.edu, pageexec at freemail.hu
            Blocks: 4068


cloning from bug 9322 comment 9 per Ted's request.

in linux/kernel/signal.c:has_pending_signals there's the following construct
(linux/include/linux/signal.h has similar):

 102 »·······switch (_NSIG_WORDS) {
 103 »·······default:
 104 »·······»·······for (i = _NSIG_WORDS, ready = 0; --i >= 0 ;)
 105 »·······»·······»·······ready |= signal->sig[i] &~ blocked->sig[i];
 106 »·······»·······break;
 107
 108 »·······case 4: ready  = signal->sig[3] &~ blocked->sig[3];
 109 »·······»·······ready |= signal->sig[2] &~ blocked->sig[2];
 110 »·······»·······ready |= signal->sig[1] &~ blocked->sig[1];
 111 »·······»·······ready |= signal->sig[0] &~ blocked->sig[0];
 112 »·······»·······break;
 113
 114 »·······case 2: ready  = signal->sig[1] &~ blocked->sig[1];
 115 »·······»·······ready |= signal->sig[0] &~ blocked->sig[0];
 116 »·······»·······break;
 117
 118 »·······case 1: ready  = signal->sig[0] &~ blocked->sig[0];
 119 »·······}

on i386 _NSIG_WORDS is #defined to 2 with ->sig[] having 2 elements as well.
clang r126742 generates the following warning on this:

kernel/signal.c:108:19: warning: array index of '3' indexes past the end of an
array (that contains 2 elements) [-Warray-bounds]
        case 4: ready  = signal->sig[3] &~ blocked->sig[3];

and ditto for line 109.

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