[LLVMbugs] [Bug 6881] New: variable incorrectly optimized out

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Apr 20 08:36:37 PDT 2010


           Summary: variable incorrectly optimized out
           Product: clang
           Version: unspecified
          Platform: PC
        OS/Version: FreeBSD
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: rdivacky at freebsd.org
                CC: llvmbugs at cs.uiuc.edu

pes ~/clangbsd$ cat weak.c
extern int _DYNAMIC;
#pragma weak _DYNAMIC

void foo() {
   if (&_DYNAMIC != 0)
      printf("foo bar\n");
pes ~/clangbsd$ clang -c weak.c && nm weak.o
0000000000000000 T foo
                 U printf

pes ~/clangbsd$ gcc -c weak.c && nm weak.o
                 w _DYNAMIC
0000000000000000 T foo
                 U puts

this happens because lib/AST/ExprConstant.cpp:1180 logic is flawed. It does not
work for weak variable which are 0. thus the if() check is optimized away. We
need to check if the variable is weakref-ed before deciding it cannot be 0.

this proof of concept (disabling the opt) makes the test work:

Index: lib/AST/ExprConstant.cpp
--- lib/AST/ExprConstant.cpp    (revision 101893)
+++ lib/AST/ExprConstant.cpp    (working copy)
@@ -1178,7 +1178,7 @@
       // Reject any bases from the normal codepath; we special-case
       // to null.
       if (LHSValue.getLValueBase()) {
-        if (!E->isEqualityOp())
+//        if (!E->isEqualityOp())
           return false;
         if (RHSValue.getLValueBase() || !RHSValue.getLValueOffset().isZero())
           return false;
@@ -1187,7 +1187,7 @@
           return false;
         return Success(bres ^ (E->getOpcode() == BinaryOperator::EQ), E);
       } else if (RHSValue.getLValueBase()) {
-        if (!E->isEqualityOp())
+//        if (!E->isEqualityOp())
           return false;
         if (LHSValue.getLValueBase() || !LHSValue.getLValueOffset().isZero())
           return false;

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