Chris and Evan noticed that this check was compleatly fubared. I was

checking that there was a from a global instead of a load from the stub
for a global, which is the one that's safe to hoist.

Consider this program:

volatile char G[100];
int B(char *F, int N) {
  for (; N > 0; --N)
    F[N] = G[N];
}

In static mode, we shouldn't be hoisting the load from G:

$ llc -relocation-model=static -o - a.bc -march=x86 -machine-licm

LBB1_1: # bb.preheader
        leal    -1(%eax), %edx
        testl   %edx, %edx
        movl    $1, %edx
        cmovns  %eax, %edx
        xorl    %esi, %esi
LBB1_2: # bb
        movb    _G(%eax), %bl
        movb    %bl, (%ecx,%eax)

llvm-svn: 45626

llvm/lib/Target/X86/X86InstrInfo.cpp

@@ -171,12 +171,15 @@ bool X86InstrInfo::isReallySideEffectFree(MachineInstr *MI) const {
   case X86::MOV32rm:
     if (MI->getOperand(1).isRegister()) {
       unsigned Reg = MI->getOperand(1).getReg();
+      const X86Subtarget &ST = TM.getSubtarget<X86Subtarget>();
 
       // Loads from global addresses which aren't redefined in the function are
       // side effect free.
       if (Reg != 0 && MRegisterInfo::isVirtualRegister(Reg) &&
           MI->getOperand(2).isImm() && MI->getOperand(3).isReg() &&
-          MI->getOperand(4).isGlobal() && MI->getOperand(2).getImm() == 1 &&
+          MI->getOperand(4).isGlobal() &&
+          ST.GVRequiresExtraLoad(MI->getOperand(4).getGlobal(), TM, false) &&
+          MI->getOperand(2).getImm() == 1 &&
           MI->getOperand(3).getReg() == 0)
         return true;
     }