diff --git a/llvm/include/llvm/MC/MCFragment.h b/llvm/include/llvm/MC/MCFragment.h
--- a/llvm/include/llvm/MC/MCFragment.h
+++ b/llvm/include/llvm/MC/MCFragment.h
@@ -69,6 +69,10 @@
   /// Whether fragment is being laid out.
   bool IsBeingLaidOut;
 
+  /// SubsectionNumber - the subsection this fragment belongs to. This is 0 if
+  // the fragment is not in any subsection
+  unsigned SubsectionNumber = 0;
+
 protected:
   bool HasInstructions;
 
@@ -102,6 +106,9 @@
   bool hasInstructions() const { return HasInstructions; }
 
   void dump() const;
+
+  void setSubsectionNumber(unsigned Value) { SubsectionNumber = Value; }
+  unsigned getSubsectionNumber() const { return SubsectionNumber; }
 };
 
 class MCDummyFragment : public MCFragment {
diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -579,12 +579,7 @@
   if (!Asm->getWriter().isSymbolRefDifferenceFullyResolved(*Asm, A, B, InSet))
     return;
 
-  MCFragment *FA = SA.getFragment();
-  MCFragment *FB = SB.getFragment();
-  if (FA == FB && !SA.isVariable() && !SA.isUnset() && !SB.isVariable() &&
-      !SB.isUnset()) {
-    Addend += (SA.getOffset() - SB.getOffset());
-
+  auto FinalizeFolding = [&]() {
     // Pointers to Thumb symbols need to have their low-bit set to allow
     // for interworking.
     if (Asm->isThumbFunc(&SA))
@@ -598,11 +593,18 @@
     // Clear the symbol expr pointers to indicate we have folded these
     // operands.
     A = B = nullptr;
-    return;
-  }
+  };
 
-  if (!Layout)
+  const MCFragment *FA = SA.getFragment();
+  const MCFragment *FB = SB.getFragment();
+  // If both symbols are in the same fragment, return the difference of their
+  // offsets
+  if (FA == FB && !SA.isVariable() && !SA.isUnset() && !SB.isVariable() &&
+      !SB.isUnset()) {
+    Addend += (SA.getOffset() - SB.getOffset());
+    FinalizeFolding();
     return;
+  }
 
   const MCSection &SecA = *FA->getParent();
   const MCSection &SecB = *FB->getParent();
@@ -610,30 +612,45 @@
   if ((&SecA != &SecB) && !Addrs)
     return;
 
-  // One of the symbol involved is part of a fragment being laid out. Quit now
-  // to avoid a self loop.
-  if (!Layout->canGetFragmentOffset(FA) || !Layout->canGetFragmentOffset(FB))
-    return;
+  // When layout is not finalized, our ability to resolve differences between
+  // symbols is limited to specific cases where the fragments between two
+  // symbols (including the fragments the symbols are defined in) are fixed-size
+  // fragments so the difference can be calculated. For example, this is
+  // important when the Subtarget is changed and a new MCDataFragment is created
+  // in the case of foo: instr; .arch_extension ext; instr .if . - foo
+  if (!Layout) {
+    if (SA.isVariable() || SA.isUnset() || SB.isVariable() || SB.isUnset() ||
+        FA->getKind() != MCFragment::FT_Data ||
+        FB->getKind() != MCFragment::FT_Data ||
+        FA->getSubsectionNumber() != FB->getSubsectionNumber())
+      return;
+    int64_t Offset = 0;
+    for (auto FI = FB->getIterator(), FE = SecA.end(); FI != FE; ++FI) {
+      if (&*FI == FA) {
+        Addend += Offset + (SA.getOffset() - SB.getOffset());
+        FinalizeFolding();
+        return;
+      }
 
-  // Eagerly evaluate.
-  Addend += Layout->getSymbolOffset(A->getSymbol()) -
-            Layout->getSymbolOffset(B->getSymbol());
-  if (Addrs && (&SecA != &SecB))
-    Addend += (Addrs->lookup(&SecA) - Addrs->lookup(&SecB));
-
-  // Pointers to Thumb symbols need to have their low-bit set to allow
-  // for interworking.
-  if (Asm->isThumbFunc(&SA))
-    Addend |= 1;
-
-  // If symbol is labeled as micromips, we set low-bit to ensure
-  // correct offset in .gcc_except_table
-  if (Asm->getBackend().isMicroMips(&SA))
-    Addend |= 1;
-
-  // Clear the symbol expr pointers to indicate we have folded these
-  // operands.
-  A = B = nullptr;
+      if (FI->getKind() == MCFragment::FT_Data)
+        Offset += (cast<MCDataFragment>(FI))->getContents().size();
+      else
+        return;
+    }
+  } else {
+    // One of the symbol involved is part of a fragment being laid out. Quit now
+    // to avoid a self loop.
+    if (!Layout->canGetFragmentOffset(FA) || !Layout->canGetFragmentOffset(FB))
+      return;
+
+    // Eagerly evaluate when layout is finazlied
+    Addend += Layout->getSymbolOffset(A->getSymbol()) -
+              Layout->getSymbolOffset(B->getSymbol());
+    if (Addrs && (&SecA != &SecB))
+      Addend += (Addrs->lookup(&SecA) - Addrs->lookup(&SecB));
+
+    FinalizeFolding();
+  }
 }
 
 static bool canFold(const MCAssembler *Asm, const MCSymbolRefExpr *A,
diff --git a/llvm/test/MC/ARM/directive_if_offset.s b/llvm/test/MC/ARM/directive_if_offset.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/ARM/directive_if_offset.s
@@ -0,0 +1,14 @@
+@ RUN: llvm-mc -triple armv7a-linux-gnueabihf %s -filetype=obj -o /dev/null 2>&1 | FileCheck --allow-empty %s
+@ RUN: llvm-mc -triple armv7a-linux-gnueabihf %s -filetype=obj -o %t | llvm-objdump -d %t | FileCheck --check-prefix=CHECK-ASM %s
+
+nop
+// Create a new MCDataFragment due to Subtarget change
+.arch_extension sec
+9997:nop
+.if . - 9997b == 0
+// CHECK-NOT: error: expected absolute expression
+orr r1, r1, #1
+.else
+orr r1, r1, #2
+.endif
+// CHECK-ASM: orr r1, r1, #2
diff --git a/llvm/test/MC/ARM/directive_if_offset_error.s b/llvm/test/MC/ARM/directive_if_offset_error.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/ARM/directive_if_offset_error.s
@@ -0,0 +1,23 @@
+@ RUN: not llvm-mc -filetype=obj -triple arm-linux-gnueabihf %s -o /dev/null 2>&1 | FileCheck %s
+
+9997: nop
+      .align 4
+      nop
+.if . - 9997b == 4
+// CHECK: error: expected absolute expression
+.endif
+
+9997: nop
+      .space 4
+      nop
+.if . - 9997b == 4
+// CHECK: error: expected absolute expression
+.endif
+
+9997:
+      ldr r0,=0x12345678
+      .ltorg
+      nop
+.if . - 9997b == 4
+// CHECK: error: expected absolute expression
+.endif
diff --git a/llvm/test/MC/ARM/thumb2_directive_if_offset_error.s b/llvm/test/MC/ARM/thumb2_directive_if_offset_error.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/ARM/thumb2_directive_if_offset_error.s
@@ -0,0 +1,8 @@
+@ RUN: not llvm-mc -filetype=obj -triple thumbv7a-linux-gnueabihf %s -o /dev/null 2>&1 | FileCheck %s
+
+9997: nop
+      b external
+      nop
+.if . - 9997b == 4
+// CHECK: error: expected absolute expression
+.endif
diff --git a/llvm/test/MC/ELF/subsection.s b/llvm/test/MC/ELF/subsection.s
--- a/llvm/test/MC/ELF/subsection.s
+++ b/llvm/test/MC/ELF/subsection.s
@@ -12,7 +12,7 @@
 // CHECK-NOT: Contents of section .rela.text:
 
 // CHECK: Contents of section .data:
-// CHECK-NEXT: 0000 01030402 74657374
+// CHECK-NEXT: 0000 01010402 74657374
 .data
 l0:
 .byte 1
diff --git a/llvm/test/MC/MachO/reloc-diff.s b/llvm/test/MC/MachO/reloc-diff.s
--- a/llvm/test/MC/MachO/reloc-diff.s
+++ b/llvm/test/MC/MachO/reloc-diff.s
@@ -22,9 +22,5 @@
 // CHECK-NEXT:     0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
 // CHECK-NEXT:     0x8 0 2 n/a GENERIC_RELOC_LOCAL_SECTDIFF 1 0x0
 // CHECK-NEXT:     0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
-// CHECK-NEXT:     0x4 0 2 n/a GENERIC_RELOC_LOCAL_SECTDIFF 1 0x0
-// CHECK-NEXT:     0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
-// CHECK-NEXT:     0x0 0 2 n/a GENERIC_RELOC_SECTDIFF 1 0x0
-// CHECK-NEXT:     0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]