[ARM] [Assembler] Support negative immediates for A32, T32 and T16

Summary:
To support negative immediates for certain arithmetic instructions, the
instruction is converted to the inverse instruction with a negated (or inverted)
immediate. For example, "ADD r0, r1, #FFFFFFFF" cannot be encoded as an ADD
instruction.  However, "SUB r0, r1, #1" is equivalent.

These conversions are different from instruction aliases.  An alias maps
several assembler instructions onto one encoding.  A conversion, however, maps
an *invalid* instruction--e.g. with an immediate that cannot be represented in
the encoding--to a different (but equivalent) instruction.

Several instructions with negative immediates were being converted already, but
this was not systematically tested, nor did it cover all instructions.

This patch implements all possible substitutions for ARM, Thumb1 and
Thumb2 assembler and adds tests.  It also adds a feature flag
(-mattr=+no-neg-immediates) to turn these substitutions off.  This is
helpful for users who want their code to assemble to exactly what they
wrote.

Reviewers: t.p.northover, rovka, samparker, javed.absar, peter.smith, rengolin

Reviewed By: javed.absar

Subscribers: aadg, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D30571

llvm-svn: 298380

1 files changed, 30 insertions, 2 deletions

diff --git a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
index 1a6f8e763e2..233a5c7fee1 100644
--- a/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/llvm/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -1245,6 +1245,20 @@ public:
     return ARM_AM::getSOImmVal(Value) == -1 &&
       ARM_AM::getSOImmVal(-Value) != -1;
   }
+  bool isThumbModImmNeg1_7() const {
+    if (!isImm()) return false;
+    const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(getImm());
+    if (!CE) return false;
+    int32_t Value = -(int32_t)CE->getValue();
+    return 0 < Value && Value < 8;
+  }
+  bool isThumbModImmNeg8_255() const {
+    if (!isImm()) return false;
+    const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(getImm());
+    if (!CE) return false;
+    int32_t Value = -(int32_t)CE->getValue();
+    return 7 < Value && Value < 256;
+  }
   bool isConstantPoolImm() const { return Kind == k_ConstantPoolImmediate; }
   bool isBitfield() const { return Kind == k_BitfieldDescriptor; }
   bool isPostIdxRegShifted() const { return Kind == k_PostIndexRegister; }
@@ -2035,6 +2049,20 @@ public:
     Inst.addOperand(MCOperand::createImm(Enc));
   }
 
+  void addThumbModImmNeg8_255Operands(MCInst &Inst, unsigned N) const {
+    assert(N == 1 && "Invalid number of operands!");
+    const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(getImm());
+    uint32_t Val = -CE->getValue();
+    Inst.addOperand(MCOperand::createImm(Val));
+  }
+
+  void addThumbModImmNeg1_7Operands(MCInst &Inst, unsigned N) const {
+    assert(N == 1 && "Invalid number of operands!");
+    const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(getImm());
+    uint32_t Val = -CE->getValue();
+    Inst.addOperand(MCOperand::createImm(Val));
+  }
+
   void addBitfieldOperands(MCInst &Inst, unsigned N) const {
     assert(N == 1 && "Invalid number of operands!");
     // Munge the lsb/width into a bitfield mask.
@@ -2141,7 +2169,7 @@ public:
     // The operand is actually a t2_so_imm, but we have its bitwise
     // negation in the assembly source, so twiddle it here.
     const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(getImm());
-    Inst.addOperand(MCOperand::createImm(~CE->getValue()));
+    Inst.addOperand(MCOperand::createImm(~(uint32_t)CE->getValue()));
   }
 
   void addT2SOImmNegOperands(MCInst &Inst, unsigned N) const {
@@ -2149,7 +2177,7 @@ public:
     // The operand is actually a t2_so_imm, but we have its
     // negation in the assembly source, so twiddle it here.
     const MCConstantExpr *CE = dyn_cast<MCConstantExpr>(getImm());
-    Inst.addOperand(MCOperand::createImm(-CE->getValue()));
+    Inst.addOperand(MCOperand::createImm(-(uint32_t)CE->getValue()));
   }
 
   void addImm0_4095NegOperands(MCInst &Inst, unsigned N) const {