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

This adds the same kind of verification for ARM, as was added for AArch64
in 1e7f592. This allows catching issues
at assembly time, instead of having the linker misinterpret the relocations
(as the linker ignores the symbol offset). This verifies that the issue
fixed by 8dd065d really is fixed, and
points out explicitly if the same issue appears elsewhere.

Note that the parameter Value in the adjustFixupValue function is offset by
4 from the value that is stored as immediate in the instructions, so we
compare with 4, when we want to make sure that the written immediate will
be zero.

Author: mstorsjo

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