[llvm] [MC,CodeGen] Update .prefalign for symbol-based preferred alignment (PR #184032)

[Alexis Engelke via llvm-commits]

================
@@ -3472,13 +3472,54 @@ bool AsmParser::parseDirectivePrefAlign() {
   int64_t Alignment;
   if (checkForValidSection() || parseAbsoluteExpression(Alignment))
     return true;
-  if (parseEOL())
-    return true;
 
   if (!isPowerOf2_64(Alignment))
     return Error(AlignmentLoc, "alignment must be a power of 2");
-  getStreamer().emitPrefAlign(Align(Alignment));
 
+  // Parse end symbol: .prefalign N, sym
+  SMLoc SymLoc = getLexer().getLoc();
+  if (!getLexer().is(AsmToken::Comma))
+    return Error(SymLoc, "expected ',' and end symbol");
+  Lex();
+  StringRef Name;
+  SymLoc = getLexer().getLoc();
+  if (parseIdentifier(Name))
+    return Error(SymLoc, "expected symbol name");
+  MCSymbol *End = getContext().getOrCreateSymbol(Name);
+
+  // Parse fill operand: integer byte [0, 255] or "nop".
+  SMLoc FillLoc = getLexer().getLoc();
+  if (!getLexer().is(AsmToken::Comma))
+    return Error(FillLoc, "expected ',' followed by 'nops' or fill byte");
----------------
aengelke wrote:

```suggestion
    return Error(FillLoc, "expected ',' followed by 'nop' or fill byte");
```

https://github.com/llvm/llvm-project/pull/184032