Fix PR31896.

[eugenis]

Summary

Address of an alias of a global with offset is incorrectly lowered as an address of the global (i.e. ignoring offset).

This is fixing a problem introduced in D23195 and reported in PR31896.

I'm not really sure this is a correct fix, but
(a) I see no good reason for the chunk of the code that I'm removing to exist
(b) removing it does not break any tests and fixes PR31896

[eugenis]

ping

[olista01]

I don't think this is correct for ROPI/RWPI. We need to look through the alias to check if the underlying object is read-only or read-write, as that affects how it should be addressed.

[eugenis]

In D29740#677198, @olista01 wrote:

I don't think this is correct for ROPI/RWPI. We need to look through the alias to check if the underlying object is read-only or read-write, as that affects how it should be addressed.

something like this?

[olista01]

LGTM

[eugenis]

fixed the test

[eugenis]

Committed in r295762

lib/Target/ARM/ARMISelLowering.cpp

if (!AFI->getGlobalsPromotedToConstantPool().count(GVar)) {
     AFI->markGlobalAsPromotedToConstantPool(GVar);
     AFI->setPromotedConstpoolIncrease(AFI->getPromotedConstpoolIncrease() +
                                       PaddedSize - 4);
   }
   ++NumConstpoolPromoted;
   return DAG.getNode(ARMISD::Wrapper, dl, MVT::i32, CPAddr);
 }
 
+static bool isReadOnly(const GlobalValue *GV) {
+  if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(GV))
+    GV = GA->getBaseObject();
+  return (isa<GlobalVariable>(GV) && cast<GlobalVariable>(GV)->isConstant()) ||
+         isa<Function>(GV);
+}
+
 SDValue ARMTargetLowering::LowerGlobalAddressELF(SDValue Op,
                                                  SelectionDAG &DAG) const {
   EVT PtrVT = getPointerTy(DAG.getDataLayout());
   SDLoc dl(Op);
   const GlobalValue *GV = cast<GlobalAddressSDNode>(Op)->getGlobal();
   const TargetMachine &TM = getTargetMachine();
-  if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(GV))
-    GV = GA->getBaseObject();
-  bool IsRO =
-      (isa<GlobalVariable>(GV) && cast<GlobalVariable>(GV)->isConstant()) ||
-      isa<Function>(GV);
+  bool IsRO = isReadOnly(GV);
 
   // promoteToConstantPool only if not generating XO text section
   if (TM.shouldAssumeDSOLocal(*GV->getParent(), GV) && !Subtarget->genExecuteOnly())
     if (SDValue V = promoteToConstantPool(GV, DAG, PtrVT, dl))
       return V;
 
   if (isPositionIndependent()) {
     bool UseGOT_PREL = !TM.shouldAssumeDSOLocal(*GV->getParent(), GV);

test/CodeGen/ARM/alias_store.ll

+; RUN: llc < %s -mtriple=arm-linux-gnueabi | FileCheck %s
+
+@X = constant {i8, i8 } { i8 0, i8 0 }
+@XA = alias i8, getelementptr inbounds ({ i8, i8 }, {i8, i8}* @X, i32 0, i32 1)
+
+define void @f(i8** %p) align 2 {
+entry:
+  store i8* @XA, i8 **%p, align 4
+  ret void
+}
+
+; CHECK: f:
+; CHECK: ldr r1, [[L:.*]]
+; CHECK: [[L]]:
+; CHECK-NEXT: .long XA
+; CHECK: XA = X+1