[llvm] [ARM] [Windows] Error out on branch relocations that require a symbol offset (PR #101906)

[Martin Storsjö via llvm-commits]

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@@ -587,6 +587,14 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
         return 0;
     return 0xffffff & ((Value - 8) >> 2);
   case ARM::fixup_t2_uncondbranch: {
+    if (STI->getTargetTriple().isOSBinFormatCOFF() && !IsResolved &&
+        Value != 4) {
+      // MSVC link.exe and lld do not support this relocation type
+      // with a non-zero offset. ("Value" is offset by 4 at this point.)
+      Ctx.reportError(Fixup.getLoc(),
+                      "cannot perform a PC-relative fixup with a non-zero "
+                      "symbol offset");
+    }
     Value = Value - 4;
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mstorsjo wrote:

Yes, I considered that, but as only 1 case out of 3 have the `Value = Value - 4;` as a separate step, I preferred to keep it consistent, and early upfront in each case.

https://github.com/llvm/llvm-project/pull/101906