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

Fixed ELF object writer to die gracefully when an undefined label is encountered in a branch instruction.
See https://bugs.llvm.org/show_bug.cgi?id=41914.

Reviewers: arsenm, rampitec

Differential Revision: https://reviews.llvm.org/D79943

Author: dpreobra

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

@@ -6,8 +6,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "AMDGPUFixupKinds.h"
 #include "AMDGPUMCTargetDesc.h"
 #include "llvm/BinaryFormat/ELF.h"
+#include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCELFObjectWriter.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCFixup.h"
@@ -80,6 +82,15 @@ unsigned AMDGPUELFObjectWriter::getRelocType(MCContext &Ctx,
     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");
 }
 

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

@@ -0,0 +1,4 @@
+//  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