diff --git a/llvm/lib/MC/MCAssembler.cpp b/llvm/lib/MC/MCAssembler.cpp
--- a/llvm/lib/MC/MCAssembler.cpp
+++ b/llvm/lib/MC/MCAssembler.cpp
@@ -389,14 +389,15 @@
 void MCAsmLayout::layoutFragment(MCFragment *F) {
   MCFragment *Prev = F->getPrevNode();
 
-  // We should never try to recompute something which is valid.
-  assert(!isFragmentValid(F) && "Attempt to recompute a valid fragment!");
   // We should never try to compute the fragment layout if its predecessor
   // isn't valid.
   assert((!Prev || isFragmentValid(Prev)) &&
          "Attempt to compute fragment before its predecessor!");
 
-  ++stats::FragmentLayouts;
+  // We may need to recompute the offset of valid framents if relaxation changes
+  // the offset of some fragments. PR#45190
+  if (!isFragmentValid(F))
+    ++stats::FragmentLayouts;
 
   // Compute fragment offset and size.
   if (Prev)
@@ -785,9 +786,18 @@
   }
 
   // Layout until everything fits.
-  while (layoutOnce(Layout))
+  bool WasRelaxed = false;
+  while (layoutOnce(Layout)) {
     if (getContext().hadError())
       return;
+    WasRelaxed = true;
+  }
+  // recalculate the offsets of fragments if relaxation happens. PR#45190
+  if (WasRelaxed) {
+    for (MCSection &Sec : *this)
+      for (MCFragment &F : Sec)
+        Layout.layoutFragment(&F);
+  }
 
   DEBUG_WITH_TYPE("mc-dump", {
       errs() << "assembler backend - post-relaxation\n--\n";
diff --git a/llvm/test/MC/X86/relax-offset.s b/llvm/test/MC/X86/relax-offset.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/X86/relax-offset.s
@@ -0,0 +1,15 @@
+# RUN: llvm-mc -filetype=obj -triple=i386 %s -o /dev/null
+
+.macro ALTERNATIVE newinstr
+ .skip 144f-143f,0x0
+ .pushsection .altinstr_replacement,"ax"
+143:
+ \newinstr
+144:
+ .popsection
+.endm
+
+ .section .entry.text, "ax"
+  ALTERNATIVE "jmp .Lint80_keep_stack"
+.Lint80_keep_stack:
+# CHECK-NOT: error: Assertion `OS.tell() - Start == Layout.getSectionAddressSize(Sec)' failed