[clang] [llvm] [win][x64] Add support for Windows x64 unwind v3 (PR #200249)

[Mészáros Gergely via cfe-commits]

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@@ -106,6 +106,25 @@ static void EmitAbsDifference(MCStreamer &Streamer, const MCSymbol *LHS,
   Streamer.emitValue(Diff, 1);
 }
 
+/// Emit a 16-bit (2-byte LE) label difference. If the difference is
+/// evaluatable at this point, validate that it fits in [0, UINT16_MAX]
+/// and emit it as a constant; otherwise emit a 16-bit fixup.
+static void EmitAbsDifference16(MCStreamer &Streamer, const MCSymbol *LHS,
+                                const MCSymbol *RHS) {
+  MCContext &Context = Streamer.getContext();
+  const MCExpr *Diff =
+      MCBinaryExpr::createSub(MCSymbolRefExpr::create(LHS, Context),
+                              MCSymbolRefExpr::create(RHS, Context), Context);
+  int64_t Value;
+  if (Diff->evaluateAsAbsolute(
+          Value, static_cast<MCObjectStreamer &>(Streamer).getAssembler())) {
+    if (Value < 0 || Value > UINT16_MAX)
+      report_fatal_error(
+          "Label difference out of 16-bit unsigned range for V3 unwind info");
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Maetveis wrote:

There are a lot of new intances of `report_fatal_error` introduced in this patch, which is a deprecated API:

> /// @deprecated Use `reportFatalInternalError()` or `reportFatalUsageError()`
> /// instead.

https://github.com/llvm/llvm-project/blob/2d046bd62066067a1de8cb65752f1dd9404c4121/llvm/include/llvm/Support/ErrorHandling.h#L61-L79

Places where we have a way to continue should use recoverable errors ([`LLVMContext::diagnose()`](https://github.com/llvm/llvm-project/blob/2d046bd62066067a1de8cb65752f1dd9404c4121/llvm/include/llvm/IR/LLVMContext.h#L274-L284)) or other similar APIs.

Error paths that there is a programming error in LLVM should use `reportFatalInternalError()`.

And finally only places where we have absolutely no way to continue and have to terminate the whole process, but the error condition does not indicate a bug in LLVM should use `reportFatalUsageError`.

https://github.com/llvm/llvm-project/pull/200249