[ARM] support symbolic expressions as branch target in b.w

Currently ARM backend validates the range of branch targets before the
layout of fragments is finalized. This causes build failure if symbolic
expressions are used, with the exception of a single symbolic value.
For example, "b.w ." works but "b.w . + 2" currently fails to
assemble. This fixes the issue by delaying this check (in
ARMAsmParser::validateInstruction) of b.w instructions until the symbol
expressions are resolved (in ARMAsmBackend::adjustFixupValue).

Link:
ClangBuiltLinux/linux#1286

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D97568

llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp

@@ -7950,7 +7950,10 @@ bool ARMAsmParser::validateInstruction(MCInst &Inst,
     break;
   case ARM::t2B: {
     int op = (Operands[2]->isImm()) ? 2 : 3;
-    if (!static_cast<ARMOperand &>(*Operands[op]).isSignedOffset<24, 1>())
+    ARMOperand &Operand = static_cast<ARMOperand &>(*Operands[op]);
+    // Delay the checks of symbolic expressions until they are resolved.
+    if (!isa<MCBinaryExpr>(Operand.getImm()) &&
+        !Operand.isSignedOffset<24, 1>())
       return Error(Operands[op]->getStartLoc(), "branch target out of range");
     break;
   }

llvm/test/MC/ARM/thumb2-b.w-target.s

@@ -0,0 +1,12 @@
+// RUN: llvm-mc -triple=thumbv7 -filetype=obj %s | llvm-objdump --triple=thumbv7 -d - | FileCheck %s
+
+.syntax unified
+
+// CHECK-LABEL: start
+// CHECK-NEXT:	b.w	#16777208
+// CHECK-NEXT:  b.w	#2
+start:
+  b.w start - 1f + 0x1000000
+1:
+  b.w . + (2f - 1b + 2)
+2:

llvm/test/MC/ARM/thumb2-branch-ranges.s

@@ -94,3 +94,9 @@ start6:
 // CHECK: [[@LINE+2]]:{{[0-9]}}: error: Relocation out of range
 // CHECK-LABEL: beq.w start6
         beq.w start6
+
+start7:
+// branch to thumb function resolved at assembly time
+// CHECK: [[#@LINE+1]]:{{[0-9]}}: error: Relocation out of range
+        b.w start8 - start7 + 0x1000000
+start8: