[ARM] Remove condition that could never be true

From Arm v8 Architecture Reference Manual F5.1.84 LDREXD
The ldrexd instruction in Arm state has the following conditions:

t = UInt(Rt); t2 = t + 1; n = UInt(Rn);
if Rt<0> == '1' || t2 == 15 || n == 15 then UNPREDICTABLE;

In when Rt is odd or if Rt is 14 (making t2 15).

In the implementation when the pair is the UNPREDICTABLE R14_R15 we
would ideally return SOFT_FAIL. We can't because there is no R14_R15
value for us to return so we fail early returning FAIL.

The early return for registers outside the bounds of the table means
the check for Rt == 14 (0xE) redundant which causes a static analyzer
to flag the condition as never being true.

To fix the warning I've removed the check and replaced with a comment
explaining the difference with the specification.

Fixes pr41660

Differential Revision: https://reviews.llvm.org/D77463

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp b/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
index 6938814..54ff0d9 100644
--- a/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
+++ b/llvm/lib/Target/ARM/Disassembler/ARMDisassembler.cpp
@@ -1225,10 +1225,12 @@ static DecodeStatus DecodeGPRPairRegisterClass(MCInst &Inst, unsigned RegNo,
                                    uint64_t Address, const void *Decoder) {
   DecodeStatus S = MCDisassembler::Success;
 
+  // According to the Arm ARM RegNo = 14 is undefined, but we return fail
+  // rather than SoftFail as there is no GPRPair table entry for index 7.
   if (RegNo > 13)
     return MCDisassembler::Fail;
 
-  if ((RegNo & 1) || RegNo == 0xe)
+  if (RegNo & 1)
      S = MCDisassembler::SoftFail;
 
   unsigned RegisterPair = GPRPairDecoderTable[RegNo/2];