[ARM] Fix handling of zero offsets in LOB instructions.

The BF and WLS/WLSTP instructions have various branch-offset fields
occupying different positions and lengths in the instruction encoding,
and all of them were decoded at disassembly time by the function
DecodeBFLabelOffset() which returned SoftFail if the offset was zero.

In fact, it's perfectly fine and not even a SoftFail for most of those
offset fields to be zero. The only one that can't be zero is the 4-bit
field labelled `boff` in the architecture spec, occupying bits {26-23}
of the BF instruction family. If that one is zero, the encoding
overlaps other instructions (WLS, DLS, LETP, VCTP), so it ought to be
a full Fail.

Fixed by adding an extra template parameter to DecodeBFLabelOffset
which controls whether a zero offset is accepted or rejected. Adjusted
existing tests (only in error messages for bad disassemblies); added
extra tests to demonstrate zero offsets being accepted in all the
right places, and a few demonstrating rejection of zero `boff`.

Reviewers: DavidSpickett, ostannard

Subscribers: javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63864

llvm-svn: 364533

2 files changed, 17 insertions, 16 deletions

diff --git a/llvm/lib/Target/ARM/ARMInstrThumb2.td b/llvm/lib/Target/ARM/ARMInstrThumb2.td
index cad35d3e4f8..1525cff080a 100644
--- a/llvm/lib/Target/ARM/ARMInstrThumb2.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb2.td
@@ -360,18 +360,19 @@ def addrmode_tbh : MemOperand {
 // Define ARMv8.1-M specific addressing modes.
 
 // Label operands for BF/BFL/WLS/DLS/LE
-class BFLabelOp<string signed, string isNeg, string size, string fixup>
+class BFLabelOp<string signed, string isNeg, string zeroPermitted, string size,
+                string fixup>
   : Operand<OtherVT> {
   let EncoderMethod = !strconcat("getBFTargetOpValue<", isNeg, ", ",
                                  fixup, ">");
   let OperandType = "OPERAND_PCREL";
   let DecoderMethod = !strconcat("DecodeBFLabelOperand<", signed, ", ",
-                                 isNeg, ", ", size, ">");
+                                 isNeg, ", ", zeroPermitted, ", ", size, ">");
 }
-def bflabel_u4  : BFLabelOp<"false", "false", "4",  "ARM::fixup_bf_branch">;
-def bflabel_s12 : BFLabelOp<"true",  "false", "12", "ARM::fixup_bfc_target">;
-def bflabel_s16 : BFLabelOp<"true",  "false", "16", "ARM::fixup_bf_target">;
-def bflabel_s18 : BFLabelOp<"true",  "false", "18", "ARM::fixup_bfl_target">;
+def bflabel_u4  : BFLabelOp<"false", "false", "false", "4",  "ARM::fixup_bf_branch">;
+def bflabel_s12 : BFLabelOp<"true",  "false", "true",  "12", "ARM::fixup_bfc_target">;
+def bflabel_s16 : BFLabelOp<"true",  "false", "true",  "16", "ARM::fixup_bf_target">;
+def bflabel_s18 : BFLabelOp<"true",  "false", "true",  "18", "ARM::fixup_bfl_target">;
 
 def wlslabel_u11_asmoperand : AsmOperandClass {
   let Name = "WLSLabel";
@@ -380,7 +381,7 @@ def wlslabel_u11_asmoperand : AsmOperandClass {
   let DiagnosticString =
     "loop end is out of range or not a positive multiple of 2";
 }
-def wlslabel_u11 : BFLabelOp<"false", "false", "11", "ARM::fixup_wls"> {
+def wlslabel_u11 : BFLabelOp<"false", "false", "true",  "11", "ARM::fixup_wls"> {
   let ParserMatchClass = wlslabel_u11_asmoperand;
 }
 def lelabel_u11_asmoperand : AsmOperandClass {
@@ -390,7 +391,7 @@ def lelabel_u11_asmoperand : AsmOperandClass {
   let DiagnosticString =
     "loop start is out of range or not a negative multiple of 2";
 }
-def lelabel_u11 : BFLabelOp<"false", "true",  "11", "ARM::fixup_le"> {
+def lelabel_u11 : BFLabelOp<"false", "true",  "true",  "11", "ARM::fixup_le"> {
   let ParserMatchClass = lelabel_u11_asmoperand;
 }
 
diff --git a/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp b/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
index 548fb10fb3f..9fb2fa65fdd 100644
--- a/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
+++ b/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
@@ -488,7 +488,7 @@ static DecodeStatus DecoderForMRRC2AndMCRR2(MCInst &Inst, unsigned Val,
 static DecodeStatus DecodeForVMRSandVMSR(MCInst &Inst, unsigned Val,
                                          uint64_t Address, const void *Decoder);
 
-template <bool isSigned, bool isNeg, int size>
+template <bool isSigned, bool isNeg, bool zeroPermitted, int size>
 static DecodeStatus DecodeBFLabelOperand(MCInst &Inst, unsigned val,
                                          uint64_t Address, const void *Decoder);
 static DecodeStatus DecodeBFAfterTargetOperand(MCInst &Inst, unsigned val,
@@ -5908,13 +5908,13 @@ static DecodeStatus DecodeForVMRSandVMSR(MCInst &Inst, unsigned Val,
   return S;
 }
 
-template <bool isSigned, bool isNeg, int size>
+template <bool isSigned, bool isNeg, bool zeroPermitted, int size>
 static DecodeStatus DecodeBFLabelOperand(MCInst &Inst, unsigned Val,
                                          uint64_t Address,
                                          const void *Decoder) {
   DecodeStatus S = MCDisassembler::Success;
-  if (Val == 0)
-    S = MCDisassembler::SoftFail;
+  if (Val == 0 && !zeroPermitted)
+    S = MCDisassembler::Fail;
 
   uint64_t DecVal;
   if (isSigned)
@@ -5965,8 +5965,8 @@ static DecodeStatus DecodeLOLoop(MCInst &Inst, unsigned Insn, uint64_t Address,
     Inst.addOperand(MCOperand::createReg(ARM::LR));
     LLVM_FALLTHROUGH;
   case ARM::t2LE:
-    if (!Check(S, DecodeBFLabelOperand<false, true, 11>(Inst, Imm, Address,
-                                                        Decoder)))
+    if (!Check(S, DecodeBFLabelOperand<false, true, true, 11>(
+                   Inst, Imm, Address, Decoder)))
       return MCDisassembler::Fail;
     break;
   case ARM::t2WLS:
@@ -5978,8 +5978,8 @@ static DecodeStatus DecodeLOLoop(MCInst &Inst, unsigned Insn, uint64_t Address,
     if (!Check(S,
                DecoderGPRRegisterClass(Inst, fieldFromInstruction(Insn, 16, 4),
                                        Address, Decoder)) ||
-        !Check(S, DecodeBFLabelOperand<false, false, 11>(Inst, Imm, Address,
-                                                         Decoder)))
+        !Check(S, DecodeBFLabelOperand<false, false, true, 11>(
+                   Inst, Imm, Address, Decoder)))
       return MCDisassembler::Fail;
     break;
   case ARM::t2DLS: