(cherry-pick) [Attributor] Do not optimize away externally_initialized loads. (llvm#128170) (llvm#1078)

Author: Pierre-vh

llvm/lib/Transforms/IPO/Attributor.cpp

@@ -260,11 +260,15 @@ AA::getInitialValueForObj(Attributor &A, const AbstractAttribute &QueryingAA,
     if (!Initializer)
       return nullptr;
   } else {
-    if (!GV->hasLocalLinkage() &&
-        (GV->isInterposable() || !(GV->isConstant() && GV->hasInitializer())))
-      return nullptr;
-    if (!GV->hasInitializer())
-      return UndefValue::get(&Ty);
+    if (!GV->hasLocalLinkage()) {
+      // Externally visible global that's either non-constant,
+      // or a constant with an uncertain initializer.
+      if (!GV->hasDefinitiveInitializer() || !GV->isConstant())
+        return nullptr;
+    }
+
+    // Globals with local linkage are always initialized.
+    assert(!GV->hasLocalLinkage() || GV->hasInitializer());
 
     if (!Initializer)
       Initializer = GV->getInitializer();

llvm/test/Transforms/Attributor/value-simplify.ll

@@ -12,13 +12,15 @@ declare ptr @llvm.call.preallocated.arg(token, i32)
 @ConstPtr = constant i32 0, align 4
 @ConstWeakPtr = weak constant i32 0, align 4
 @ConstWeakODRPtr = weak_odr constant i32 0, align 4
+@ExtInitZeroInit = externally_initialized constant i32 zeroinitializer, align 4
 
 ;.
 ; CHECK: @str = private unnamed_addr addrspace(4) constant [1 x i8] zeroinitializer, align 1
 ; CHECK: @ConstAS3Ptr = addrspace(3) global i32 0, align 4
 ; CHECK: @ConstPtr = constant i32 0, align 4
 ; CHECK: @ConstWeakPtr = weak constant i32 0, align 4
 ; CHECK: @ConstWeakODRPtr = weak_odr constant i32 0, align 4
+; CHECK: @ExtInitZeroInit = externally_initialized constant i32 0, align 4
 ; CHECK: @S = external global %struct.X
 ; CHECK: @g = internal constant { [2 x ptr] } { [2 x ptr] [ptr @f1, ptr @f2] }
 ; CHECK: @x = external global i32
@@ -1652,6 +1654,23 @@ define i32 @readWeakOdrConst() {
   ret i32 %l
 }
 
+define i32 @readExtInitZeroInit() {
+; TUNIT: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; TUNIT-LABEL: define {{[^@]+}}@readExtInitZeroInit
+; TUNIT-SAME: () #[[ATTR2]] {
+; TUNIT-NEXT:    [[L:%.*]] = load i32, ptr @ExtInitZeroInit, align 4
+; TUNIT-NEXT:    ret i32 [[L]]
+;
+; CGSCC: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CGSCC-LABEL: define {{[^@]+}}@readExtInitZeroInit
+; CGSCC-SAME: () #[[ATTR1]] {
+; CGSCC-NEXT:    [[L:%.*]] = load i32, ptr @ExtInitZeroInit, align 4
+; CGSCC-NEXT:    ret i32 [[L]]
+;
+  %l = load i32, ptr @ExtInitZeroInit
+  ret i32 %l
+}
+
 ;.
 ; TUNIT: attributes #[[ATTR0:[0-9]+]] = { nocallback nofree nosync nounwind willreturn }
 ; TUNIT: attributes #[[ATTR1]] = { memory(readwrite, argmem: none) }