[ARM] [Windows] Error out on branch relocations that require a symbol offset

llvm/lib/Target/ARM/MCTargetDesc/ARMAsmBackend.cpp

@@ -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;
     if (!isInt<25>(Value)) {
       Ctx.reportError(Fixup.getLoc(), "Relocation out of range");
@@ -637,6 +645,14 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
       Ctx.reportError(Fixup.getLoc(), "Relocation out of range");
       return 0;
     }
+    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");
+    }
 
     // The value doesn't encode the low bit (always zero) and is offset by
     // four. The 32-bit immediate value is encoded as
@@ -666,6 +682,14 @@ unsigned ARMAsmBackend::adjustFixupValue(const MCAssembler &Asm,
                          Endian == llvm::endianness::little);
   }
   case ARM::fixup_arm_thumb_blx: {
+    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");
+    }
     // The value doesn't encode the low two bits (always zero) and is offset by
     // four (see fixup_arm_thumb_cp). The 32-bit immediate value is encoded as
     //   imm32 = SignExtend(S:I1:I2:imm10H:imm10L:00)

llvm/test/MC/ARM/Windows/branch-reloc-offset.s

@@ -1,4 +1,5 @@
 // RUN: llvm-mc -triple thumbv7-windows-gnu -filetype obj %s -o - | llvm-objdump -D -r - | FileCheck %s
+// RUN: not llvm-mc -triple thumbv7-windows-gnu -filetype obj --defsym ERR=1 %s -o /dev/null 2>&1 | FileCheck %s --check-prefix=ERR
 
     .text
 main:
@@ -55,3 +56,40 @@ main:
 // CHECK:        e: bf00          nop
 // CHECK: 00000010 <.Lother_target>:
 // CHECK:       10: 4770          bx      lr
+
+.ifdef ERR
+    .section "other2", "xr"
+err:
+    nop
+
+// Test errors, if referencing a symbol with an offset
+
+    b .Lerr_target+4
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+    bl .Lerr_target+4
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+    blx .Lerr_target+4
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+
+// Test errors, if referencing a private label which lacks .def/.scl/.type/.endef, in another
+// section, without an offset. Such symbols are omitted from the output symbol table, so the
+// relocation can't reference them. Such relocations usually are made towards the base of the
+// section plus an offset, but such an offset is not supported with this relocation.
+
+    b .Lerr_target2
+// ERR: [[#@LINE-1]]:5: error: cannot perform a PC-relative fixup with a non-zero symbol offset
+
+    .def .Lerr_target
+    .scl 3
+    .type 32
+    .endef
+.Lerr_target:
+    nop
+    nop
+    bx lr
+
+    .section "other3", "xr"
+    nop
+.Lerr_target2:
+    bx lr
+.endif