Implement jump tables (#453)

* Add 'jump_table_entry' and 'indirect_jump' instructions.

* Update CodeSink to keep track of code size. Pretty up clif-util's disassembly output.

* Only disassemble the machine portion of output. Pretty print the read-only data after it.

* Update switch frontend code to use new br_table instruction w/ default.

lib/codegen/meta-python/base/formats.py

@@ -50,7 +50,10 @@ Branch = InstructionFormat(VALUE, ebb, VARIABLE_ARGS)
 BranchInt = InstructionFormat(intcc, VALUE, ebb, VARIABLE_ARGS)
 BranchFloat = InstructionFormat(floatcc, VALUE, ebb, VARIABLE_ARGS)
 BranchIcmp = InstructionFormat(intcc, VALUE, VALUE, ebb, VARIABLE_ARGS)
-BranchTable = InstructionFormat(VALUE, entities.jump_table)
+BranchTable = InstructionFormat(VALUE, ebb, entities.jump_table)
+BranchTableEntry = InstructionFormat(VALUE, VALUE, uimm8, entities.jump_table)
+BranchTableBase = InstructionFormat(entities.jump_table)
+IndirectJump = InstructionFormat(VALUE, entities.jump_table)
 
 Call = InstructionFormat(func_ref, VARIABLE_ARGS)
 CallIndirect = InstructionFormat(sig_ref, VALUE, VARIABLE_ARGS)

lib/codegen/meta-python/base/instructions.py

@@ -130,6 +130,8 @@ brff = Instruction(
         ins=(Cond, f, EBB, args), is_branch=True)
 
 x = Operand('x', iB, doc='index into jump table')
+Entry = TypeVar('Entry', 'A scalar integer type', ints=True)
+entry = Operand('entry', Entry, doc='entry of jump table')
 JT = Operand('JT', entities.jump_table)
 br_table = Instruction(
         'br_table', r"""
@@ -137,12 +139,47 @@ br_table = Instruction(
 
         Use ``x`` as an unsigned index into the jump table ``JT``. If a jump
         table entry is found, branch to the corresponding EBB. If no entry was
-        found fall through to the next instruction.
+        found or the index is out-of-bounds, branch to the given default EBB.
 
         Note that this branch instruction can't pass arguments to the targeted
         blocks. Split critical edges as needed to work around this.
         """,
-        ins=(x, JT), is_branch=True)
+        ins=(x, EBB, JT), is_branch=True, is_terminator=True)
+
+Size = Operand('Size', uimm8, 'Size in bytes')
+jump_table_entry = Instruction(
+    'jump_table_entry', r"""
+    Get an entry from a jump table.
+
+    Load a serialized ``entry`` from a jump table ``JT`` at a given index
+    ``addr`` with a specific ``Size``. The retrieved entry may need to be
+    decoded after loading, depending upon the jump table type used.
+
+    Currently, the only type supported is entries which are relative to the
+    base of the jump table.
+    """,
+    ins=(x, addr, Size, JT), outs=entry)
+
+jump_table_base = Instruction(
+    'jump_table_base', r"""
+    Get the absolute base address of a jump table.
+
+    This is used for jump tables wherein the entries are stored relative to
+    the base of jump table. In order to use these, generated code should first
+    load an entry using ``jump_table_entry``, then use this instruction to add
+    the relative base back to it.
+    """,
+    ins=JT, outs=addr)
+
+indirect_jump_table_br = Instruction(
+    'indirect_jump_table_br', r"""
+    Branch indirectly via a jump table entry.
+
+    Unconditionally jump via a jump table entry that was previously loaded
+    with the ``jump_table_entry`` instruction.
+    """,
+    ins=(addr, JT),
+    is_branch=True, is_indirect_branch=True, is_terminator=True)
 
 code = Operand('code', trapcode)
 trap = Instruction(

lib/codegen/meta-python/base/legalize.py

@@ -306,10 +306,10 @@ widen.legalize(
 
 for int_ty in [types.i8, types.i16]:
     widen.legalize(
-        br_table.bind(int_ty)(x, y),
+        br_table.bind(int_ty)(x, y, z),
         Rtl(
             b << uextend.i32(x),
-            br_table(b, y),
+            br_table(b, y, z),
         )
     )
 

lib/codegen/meta-python/base/settings.py

@@ -166,4 +166,13 @@ probestack_size_log2 = NumSetting(
         """,
         default=12)
 
+#
+# Jump table options.
+#
+jump_tables_enabled = BoolSetting(
+        """
+        Enable the use of jump tables in generated machine code.
+        """,
+        default=True)
+
 group.close(globals())

lib/codegen/meta-python/cdsl/instructions.py

@@ -89,6 +89,7 @@ class Instruction(object):
     :param constraints: Tuple of instruction-specific TypeConstraints.
     :param is_terminator: This is a terminator instruction.
     :param is_branch: This is a branch instruction.
+    :param is_indirect_branch: This is an indirect branch instruction.
     :param is_call: This is a call instruction.
     :param is_return: This is a return instruction.
     :param can_trap: This instruction can trap.
@@ -103,6 +104,8 @@ class Instruction(object):
     ATTRIBS = {
             'is_terminator': 'True for instructions that terminate the EBB.',
             'is_branch': 'True for all branch or jump instructions.',
+            'is_indirect_branch':
+            'True for all indirect branch or jump instructions.',
             'is_call': 'Is this a call instruction?',
             'is_return': 'Is this a return instruction?',
             'can_load': 'Can this instruction read from memory?',

lib/codegen/meta-python/cdsl/isa.py

@@ -469,7 +469,7 @@ class Encoding(object):
                 "Format {} must match recipe: {}".format(
                     self.inst.format, recipe.format))
 
-        if self.inst.is_branch:
+        if self.inst.is_branch and not self.inst.is_indirect_branch:
             assert recipe.branch_range, (
                     'Recipe {} for {} must have a branch_range'
                     .format(recipe, self.inst.name))

lib/codegen/meta-python/isa/x86/encodings.py

@@ -507,6 +507,17 @@ enc_both(base.brz.b1, r.t8jccd_abcd, 0x84)
 enc_both(base.brnz.b1, r.t8jccb_abcd, 0x75)
 enc_both(base.brnz.b1, r.t8jccd_abcd, 0x85)
 
+#
+# Jump tables
+#
+X86_64.enc(base.jump_table_entry.i64.any.any, *r.jt_entry.rex(0x63, w=1))
+X86_32.enc(base.jump_table_entry.i32.any.any, *r.jt_entry(0x8b))
+
+X86_64.enc(base.jump_table_base.i64, *r.jt_base.rex(0x8d, w=1))
+X86_32.enc(base.jump_table_base.i32, *r.jt_base(0x8d))
+
+enc_x86_64(base.indirect_jump_table_br.i64, r.indirect_jmp, 0xff, rrr=4)
+X86_32.enc(base.indirect_jump_table_br.i32, *r.indirect_jmp(0xff, rrr=4))
 #
 # Trap as ud2
 #

lib/codegen/meta-python/isa/x86/recipes.py

@@ -14,6 +14,7 @@ from base.formats import IntCompare, IntCompareImm, FloatCompare
 from base.formats import IntCond, FloatCond
 from base.formats import IntSelect, IntCondTrap, FloatCondTrap
 from base.formats import Jump, Branch, BranchInt, BranchFloat
+from base.formats import BranchTableEntry, BranchTableBase, IndirectJump
 from base.formats import Ternary, FuncAddr, UnaryGlobalValue
 from base.formats import RegMove, RegSpill, RegFill, CopySpecial
 from base.formats import LoadComplex, StoreComplex
@@ -276,6 +277,18 @@ def floatccs(iform):
     return Or(*(IsEqual(iform.cond, cc) for cc in supported_floatccs))
 
 
+def valid_scale(iform):
+    # type: (InstructionFormat) -> PredNode
+    """
+    Return an instruction predicate that checks if `iform.imm` is a valid
+    `scale` for a SIB byte.
+    """
+    return Or(IsEqual(iform.imm, 1),
+              IsEqual(iform.imm, 2),
+              IsEqual(iform.imm, 4),
+              IsEqual(iform.imm, 8))
+
+
 # A null unary instruction that takes a GPR register. Can be used for identity
 # copies and no-op conversions.
 null = EncRecipe('null', Unary, size=0, ins=GPR, outs=0, emit='')
@@ -1473,6 +1486,38 @@ brfd = TailRecipe(
         disp4(destination, func, sink);
         ''')
 
+indirect_jmp = TailRecipe(
+        'indirect_jmp', IndirectJump, size=1, ins=GPR, outs=(),
+        clobbers_flags=False,
+        emit='''
+        PUT_OP(bits, rex1(in_reg0), sink);
+        modrm_r_bits(in_reg0, bits, sink);
+        ''')
+
+jt_entry = TailRecipe(
+        'jt_entry', BranchTableEntry, size=2,
+        ins=(GPR_DEREF_SAFE, GPR_ZERO_DEREF_SAFE),
+        outs=(GPR),
+        clobbers_flags=False,
+        instp=valid_scale(BranchTableEntry),
+        emit='''
+        PUT_OP(bits, rex3(in_reg1, out_reg0, in_reg0), sink);
+        modrm_sib(out_reg0, sink);
+        sib(imm.trailing_zeros() as u8, in_reg0, in_reg1, sink);
+        ''')
+
+jt_base = TailRecipe(
+        'jt_base', BranchTableBase, size=5, ins=(), outs=(GPR),
+        clobbers_flags=False,
+        emit='''
+        // No reloc is needed here as the jump table is emitted directly after
+        // the function body.
+        PUT_OP(bits, rex2(0, out_reg0), sink);
+        modrm_riprel(out_reg0, sink);
+
+        jt_disp4(table, func, sink);
+        ''')
+
 #
 # Test flags and set a register.
 #