[Codegen][X86] Fix /hotpatch with clang-cl and inline asm

This fixes an edge case where functions starting with inline assembly would
assert while trying to lower that inline asm instruction.

After this commit, for now we always add 2 NOPs without considering the size
of the next inline asm instruction. We might want to revisit this in the
future.

This fixes Unreal Engine 5.3.2 compilation with clang-cl and /HOTPATCH.

Should close llvm#56234

llvm/lib/Target/X86/X86MCInstLower.cpp

@@ -980,8 +980,10 @@ void X86AsmPrinter::LowerPATCHABLE_OP(const MachineInstr &MI,
   SmallString<256> Code;
   unsigned MinSize = MI.getOperand(0).getImm();
 
-  if (NextMI != MI.getParent()->end()) {
+  if (NextMI != MI.getParent()->end() && !NextMI->isInlineAsm()) {
     // Lower the next MachineInstr to find its byte size.
+    // If the next instruction is inline assembly, we skip lowering it for now,
+    // and assume we should always generate NOPs.
     MCInst MCI;
     MCIL.Lower(&*NextMI, MCI);
 

llvm/test/CodeGen/X86/patchable-prologue.ll

@@ -193,3 +193,20 @@ do.body:                                          ; preds = %do.body, %entry
 do.end:                                           ; preds = %do.body
   ret void
 }
+
+
+; Test that inline asm is properly hotpatched. We currently don't examine the
+; asm instruction when printing it, thus we always emit patching NOPs.
+
+; 64: inline_asm:
+; 64-NEXT: # %bb.0:
+; 64-NEXT: xchgw   %ax, %ax                        # encoding: [0x66,0x90]
+; 64-NEXT: #APP
+; 64-NEXT: nop                                     # encoding: [0x90]
+; 64-NEXT: #NO_APP
+
+define dso_local void @inline_asm() "patchable-function"="prologue-short-redirect" {
+entry:
+  call void asm sideeffect "nop", "~{dirflag},~{fpsr},~{flags}"()
+  ret void
+}