[AMDGPU][MC] Corrected branch relocation handling to detect undefined labels

[dp]

Summary

Fixed ELF object writer to die gracefully when an undefined label is encountered in a branch instruction.

See bug 41914.

[rampitec]

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp

13

Includes shall be sorted alphabetically.

[foad]

Typo in summary "udefined".

[dp]

Corrected a list of included files; fixed a typo in the title.

[rampitec]

LGTM

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp

 //===- AMDGPUELFObjectWriter.cpp - AMDGPU ELF Writer ----------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPUMCTargetDesc.h"
+#include "MCTargetDesc/AMDGPUFixupKinds.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/MC/MCELFObjectWriter.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
 #include "llvm/MC/MCObjectWriter.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/MC/MCValue.h"
 #include "llvm/Support/ErrorHandling.h"
 
 using namespace llvm;
case FK_PCRel_4:
     return ELF::R_AMDGPU_REL32;
   case FK_Data_4:
   case FK_SecRel_4:
     return ELF::R_AMDGPU_ABS32;
   case FK_Data_8:
     return ELF::R_AMDGPU_ABS64;
   }
 
+  if (Fixup.getTargetKind() == AMDGPU::fixup_si_sopp_br) {
+    const auto *SymA = Target.getSymA();
+    assert(SymA);
+
+    Ctx.reportError(Fixup.getLoc(),
+                    Twine("undefined label '") + SymA->getSymbol().getName() + "'");
+    return ELF::R_AMDGPU_NONE;
+  }
+
   llvm_unreachable("unhandled relocation type");
 }
 
 std::unique_ptr<MCObjectTargetWriter>
 llvm::createAMDGPUELFObjectWriter(bool Is64Bit, uint8_t OSABI,
                                   bool HasRelocationAddend,
                                   uint8_t ABIVersion) {
   return std::make_unique<AMDGPUELFObjectWriter>(Is64Bit, OSABI,
                                                   HasRelocationAddend,
                                                   ABIVersion);
 }

llvm/test/MC/AMDGPU/labels-branch-err.s

+//  RUN: not llvm-mc -arch=amdgcn -filetype=obj -o /dev/null %s 2>&1 | FileCheck -check-prefix=ERROR %s
+//  ERROR: error: undefined label 'undef_label'
+
+s_branch undef_label