Fix for pr18931: Crash using integrated assembler with immediate arithmetic

Fix description:
Expressions like 'cmp r0, #(l1 - l2) >> 3' could not be evaluated on asm parsing stage,
since it is impossible to resolve labels on this stage. In the end of stage we still have
expression (MCExpr).
Then, when we want to encode it, we expect it to be an immediate, but it still an expression.
Patch introduces a Fixup (MCFixup instance), that is processed after main encoding stage.

llvm-svn: 204899

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

@@ -271,7 +271,23 @@ public:
   unsigned getSOImmOpValue(const MCInst &MI, unsigned Op,
                            SmallVectorImpl<MCFixup> &Fixups,
                            const MCSubtargetInfo &STI) const {
-    unsigned SoImm = MI.getOperand(Op).getImm();
+
+    const MCOperand &MO = MI.getOperand(Op);
+
+    // We expect MO to be an immediate or an expression,
+    // if it is an immediate - that's fine, just encode the value.
+    // Otherwise - create a Fixup.
+    if (MO.isExpr()) {
+      const MCExpr *Expr = MO.getExpr();
+      // In instruction code this value always encoded as lowest 12 bits,
+      // so we don't have to perform any specific adjustments and
+      // can use just 2-bytes fixup.
+      MCFixupKind Kind = MCFixupKind(FK_Data_2);
+      Fixups.push_back(MCFixup::Create(0, Expr, Kind, MI.getLoc()));
+      return 0;
+    }
+
+    unsigned SoImm = MO.getImm();
     int SoImmVal = ARM_AM::getSOImmVal(SoImm);
     assert(SoImmVal != -1 && "Not a valid so_imm value!");
 

llvm/test/MC/ARM/cmp-immediate-fixup.s

@@ -0,0 +1,9 @@
+// PR18931
+// RUN: llvm-mc < %s -triple=arm-linux-gnueabi -filetype=obj -o - \
+// RUN: | llvm-objdump --disassemble -arch=arm - | FileCheck %s
+
+    .text
+// CHECK: cmp r2, #1
+    cmp r2, #(l2 - l1 + 4) >> 2
+l1:
+l2: