[LLVMbugs] [Bug 16821] New: Infinite loops in generated assembly when using -D_FORTIFY_SOURCE=2 and -O1

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=16821

            Bug ID: 16821
           Summary: Infinite loops in generated assembly when using
                    -D_FORTIFY_SOURCE=2 and -O1
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zandobersek at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 10999
  --> http://llvm.org/bugs/attachment.cgi?id=10999&action=edit
Test case

Attached is a reduced and simplified test case (originating from WebKit) that
loops infinitely when compiled compiled with Clang and specifying both
-D_FORTIFY_SOURCE=2 and -O1 on the command line.

Breaking the loop under GDB stops in the randomValue function and disassembly
sure enough shows the problem:
-----
Program received signal SIGINT, Interrupt.
0x0000000000400620 in randomValue(unsigned char*, unsigned long) ()
(gdb) disassemble 
Dump of assembler code for function _Z11randomValuePhm:
   0x0000000000400600 <+0>:    push   %rax
   0x0000000000400601 <+1>:    mov    $0x4006f4,%edi
   0x0000000000400606 <+6>:    xor    %esi,%esi
   0x0000000000400608 <+8>:    xor    %edx,%edx
   0x000000000040060a <+10>:    xor    %eax,%eax
   0x000000000040060c <+12>:    callq  0x4004f0 <open at plt>
   0x0000000000400611 <+17>:    test   %eax,%eax
   0x0000000000400613 <+19>:    js     0x400622 <_Z11randomValuePhm+34>
   0x0000000000400615 <+21>:    data32 nopw %cs:0x0(%rax,%rax,1)
=> 0x0000000000400620 <+32>:    jmp    0x400620 <_Z11randomValuePhm+32>
   0x0000000000400622 <+34>:    callq  0x4005f0 <WTFCrash>
End of assembler dump.
-----

The clang command used:
clang++ -D_FORTIFY_SOURCE=2 -O2 -stdlib=libstdc++ -o test test.cpp

Notes:
- setting _FORTIFY_SOURCE to 0 or disabling any optimizations fixes the
problem,
- the problem persists when specifying any other non-null level of
optimization,
- I'm using Clang 3.4 (trunk) on Ubuntu 13.04, hence also the libstdc++ choice,
- I've experienced the same problem with older versions of Clang, from at least
3.0 onwards.

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