[LLVMbugs] [Bug 19720] New: processor feature detection confused in VM

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Sun May 11 22:53:23 PDT 2014


            Bug ID: 19720
           Summary: processor feature detection confused in VM
           Product: compiler-rt
           Version: 3.2
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: compiler-rt
          Assignee: unassignedbugs at nondot.org
          Reporter: vtjnash at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

I am running an Ubuntu VM on Ubuntu 12.04 on a mobile core2duo. The default
install detected my CPU model as most closely matching n270 (atom), but did not
enable the MOVBE instruction (since I don't have it). However, LLVM appears to
ignore the lack of MOVBE in my cpuid, and assumes that I have it because of the
atom processor declaration.

this snippet from lib/Target/X86/X86.td (from master) I believe to be at fault:
// Atom.
def : ProcessorModel<"atom", AtomModel,
                     [ProcIntelAtom, FeatureSSSE3, FeatureCMPXCHG16B,
                      FeatureMOVBE, FeatureSlowBTMem, FeatureLeaForSP,

/proc/cpuinfo file, as seen by the client:
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 28
model name      : Intel(R) Atom(TM) CPU N270   @ 1.60GHz
stepping        : 2
microcode       : 0x1
cpu MHz         : 1995.163
cache size      : 4096 KB
fdiv_bug        : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 5
wp              : yes
flags           : fpu vme de tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat
pse36 clflush mmx fxsr sse sse2 ss nx constant_tsc pni vmx ssse3 cx16
hypervisor lahf_lm
bogomips        : 3990.32
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 0 bits virtual
power management:

(and duplicated for processor 1)

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