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[PATCH v2 02/10] target/hexagon: import README for idef-parser

From: Alessandro Di Federico
Subject: [PATCH v2 02/10] target/hexagon: import README for idef-parser
Date: Thu, 25 Feb 2021 16:18:48 +0100

From: Alessandro Di Federico <ale@rev.ng>

Signed-off-by: Alessandro Di Federico <ale@rev.ng>
 target/hexagon/README                 |   5 +
 target/hexagon/idef-parser/README.rst | 447 ++++++++++++++++++++++++++
 2 files changed, 452 insertions(+)
 create mode 100644 target/hexagon/idef-parser/README.rst

diff --git a/target/hexagon/README b/target/hexagon/README
index b0b2435070..2f2814380c 100644
--- a/target/hexagon/README
+++ b/target/hexagon/README
@@ -23,6 +23,10 @@ Hexagon-specific code are
         encode*.def             Encoding patterns for each instruction
         iclass.def              Instruction class definitions used to determine
                                 legal VLIW slots for each instruction
+    qemu/target/hexagon/idef-parser
+        Parser that, given the high-level definitions of an instruction,
+        produces a C function generating equivalent tiny code instructions.
+        See README.rst.
         Helpers for loading the ELF file and making Linux system calls,
         signals, etc
@@ -43,6 +47,7 @@ header files in <BUILD_DIR>/target/hexagon
         gen_tcg_funcs.py                -> tcg_funcs_generated.c.inc
         gen_tcg_func_table.py           -> tcg_func_table_generated.c.inc
         gen_helper_funcs.py             -> helper_funcs_generated.c.inc
+        gen_idef_parser_funcs.py        -> idef_parser_input.h
 Qemu helper functions have 3 parts
     DEF_HELPER declaration indicates the signature of the helper
diff --git a/target/hexagon/idef-parser/README.rst 
new file mode 100644
index 0000000000..95377cc7e0
--- /dev/null
+++ b/target/hexagon/idef-parser/README.rst
@@ -0,0 +1,447 @@
+Hexagon ISA instruction definitions to tinycode generator compiler
+idef-parser is a small compiler able to translate the Hexagon ISA description
+language into tinycode generator code, that can be easily integrated into QEMU.
+Compilation Example
+To better understand the scope of the idef-parser, we'll explore an applicative
+example. Let's start by one of the simplest Hexagon instruction: the ``add``.
+The ISA description language represents the ``add`` instruction as
+.. code:: c
+   A2_add(RdV, in RsV, in RtV) {
+       { RdV=RsV+RtV;}
+   }
+idef-parser will compile the above code into the following code:
+.. code:: c
+   /* A2_add */
+   void emit_A2_add(DisasContext *ctx, Insn *insn, Packet *pkt, TCGv_i32 RdV,
+                    TCGv_i32 RsV, TCGv_i32 RtV)
+   /*  { RdV=RsV+RtV;} */
+   {
+       tcg_gen_movi_i32(RdV, 0);
+       TCGv_i32 tmp_0 = tcg_temp_new_i32();
+       tcg_gen_add_i32(tmp_0, RsV, RtV);
+       tcg_gen_mov_i32(RdV, tmp_0);
+       tcg_temp_free_i32(tmp_0);
+   }
+The output of the compilation process will be a function, containing the
+tinycode generator code, implementing the correct semantics. That function will
+not access any global variable, because all the accessed data structures will 
+passed explicitly as function parameters. Among the passed parameters we will
+have TCGv (tinycode variables) representing the input and output registers of
+the architecture, integers representing the immediates that come from the code,
+and other data structures which hold information about the disassemblation
+context (``DisasContext`` struct).
+Let's begin by describing the input code. The ``add`` instruction is associated
+with a unique identifier, in this case ``A2_add``, which allows to distinguish
+variants of the same instruction, and expresses the class to which the
+instruction belongs, in this case ``A2`` corresponds to the Hexagon
+``ALU32/ALU`` instruction subclass.
+After the instruction identifier, we have a series of parameters that 
+TCG variables that will be passed to the generated function. Parameters marked
+with ``in`` are already initialized, while the others are output parameters.
+We will leverage this information to infer several information:
+-  Fill in the output function signature with the correct TCGv registers
+-  Fill in the output function signature with the immediate integers
+-  Keep track of which registers, among the declared one, have been
+   initialized
+Let's now observe the actual instruction description code, in this case:
+.. code:: c
+   { RdV=RsV+RtV;}
+This code is composed by a subset of the C syntax, and is the result of the
+application of some macro definitions contained in the ``macros.h`` file.
+This file is used to reduce the complexity of the input language where complex
+variants of similar constructs can be mapped to a unique primitive, so that the
+idef-parser has to handle a lower number of computation primitives.
+As you may notice, the description code modifies the registers which have been
+declared by the declaration statements. In this case all the three registers
+will be declared, ``RsV`` and ``RtV`` will also be read and ``RdV`` will be
+Now let's have a quick look at the generated code, line by line.
+   tcg_gen_movi_i32(RdV, 0);
+This code starts by initializing ``RdV``, since reading from that register
+without initialization will cause a segmentation fault by QEMU.  This is 
+because a declaration of the ``RdV`` register was parsed, but no reading of the
+``RdV`` register was found.
+   TCGv_i32 tmp_0 = tcg_temp_new_i32();
+Then we are declaring a temporary TCGv to hold the result from the sum
+   tcg_gen_add_i32(tmp_0, RsV, RtV);
+Now we are actually generating the sum tinycode operator between the selected
+registers, storing the result in the just declared temporary.
+   tcg_gen_mov_i32(RdV, tmp_0);
+The result of the addition is now stored in the temporary, we move it into the
+correct destination register. This might not seem an efficient code, but QEMU
+will perform some tinycode optimization, reducing the unnecessary copy.
+   tcg_temp_free_i32(tmp_0);
+Finally, we free the temporary we used to hold the addition result.
+Parser Structure
+The idef-parser is built using the ``flex`` and ``bison``.
+``flex`` is used to split the input string into tokens, each described using a
+regular expression. The token description is contained in the
+``idef-parser.lex`` source file. The flex-generated scanner takes care also to
+extract from the input text other meaningful information, e.g., the numerical
+value in case of an immediate constant, and decorates the token with the
+extracted information.
+``bison`` is used to generate the actual parser, starting from the parsing
+description contained in the ``idef-parser.y`` file. The generated parser
+executes the ``main`` function at the end of the ``idef-parser.y`` file, which
+opens input and output files, creates the parsing context, and eventually calls
+the ``yyparse()`` function, which starts the execution of the LALR(1) parser
+(see `Wikipedia <https://en.wikipedia.org/wiki/LALR_parser>`__ for more
+information about LALR parsing techniques). The LALR(1) parser, whenever it has
+to shift a token, calls the ``yylex()`` function, which is defined by the
+flex-generated code, and reads the input file returning the next scanned token.
+The tokens are mapped on the source language grammar, defined in the
+``idef-parser.y`` file to build a unique syntactic tree, according to the
+specified operator precedences and associativity rules.
+The grammar describes the whole file which contains the Hexagon instruction
+descriptions, therefore it starts from the ``input`` nonterminal, which is a
+list of instructions, each instruction is represented by the following grammar
+rule, representing the structure of the input file shown above:
+   instruction : INAME code
+   code        : LBR decls statements decls RBR
+   statements  : statements statement
+               | statement
+   statement   : control_statement
+               | rvalue SEMI
+               | code_block
+               | SEMI
+   code_block  : LBR statements RBR
+               | LBR RBR
+With this initial portion of the grammar we are defining the instruction
+statements, which are enclosed by the declarations. Each statement can be a
+``control_statement``, a code block, which is just a bracket-enclosed list of
+statements, a ``SEMI``, which is a ``nop`` instruction, and an ``rvalue SEMI``.
+``rvalue`` is the nonterminal representing expressions, which are everything
+that could be assigned to a variable. ``rvalue SEMI`` can be a statement on its
+own because the assign statement, just as in the C language, is itself an
+``rvalue``\ s can be registers, immediates, predicates, control registers,
+variables, or any combination of other ``rvalue``\ s through operators. An
+``rvalue`` can be either an immediate or a TCGv, the actual type is determined
+by the ``t_hex_value.type`` field. In case it is an immediate, its combination
+with other immediates can be performed at compile-time (constant folding), only
+the result will be written into the code. If the ``rvalue`` instead is a TCGv,
+the operations performed on it will have to be emitted as tinycode 
+therefore their result will be known only at runtime. An immediate can be 
+into a TCGv through the ``rvalue_materialize`` function, which allocates a
+temporary TCGv and copies the value of the immediate in it. Each temporary
+should be freed after that it is no more used, we usually free both operands of
+each operator, in an SSA fashion.
+``lvalue``\ s instead represents all the variables which can be assigned to a
+value, and are specialized into registers and variables:
+   lvalue        : REG
+                 | VAR
+The effective assignment of ``lvalue``\ s is handled by the ``gen_assign()``
+Automatic Variables
+The input code can contain implicitly declared automatic variables, which are
+initialized with a value and then used. We performed a dedicated handling of
+such variables, because they will be matched by a generic ``VARID`` token, 
+will feature the variable name as a decoration. Each time that the variable is
+found, we have to check if that's the first variable use, in that case we
+declare a new automatic variable in the tinycode, which can be considered at 
+effects as an immediate. Special care is taken to make sure that each variable
+is declared only the first time it is seen. Furthermore the variable might
+inherit some characteristics like the signedness and the bit width, which must
+be propagated from the initialization of the variable to all the further uses 
+the variable.
+The combination of ``rvalue``\ s are handled through the use of the
+``gen_bin_op`` and ``gen_bin_cmp`` helper functions. These two functions handle
+the appropriate compile-time or run-time emission of operations to perform the
+required computation.
+Type System
+idef-parser features a simple type system which is used to correctly implement
+the signedness and bit width of the operations.
+The type of each ``rvalue`` is determined by two attributes: its bit width
+(``unsigned bit_width``) and its signedness (``bool is_unsigned``).
+For each operation, the type of ``rvalue``\ s influence the way in which the
+operands are handled and emitted. For example a right shift between signed
+operators will be an algebraic shift, while one between unsigned operators will
+be a logical shift. If one of the two operands is signed, and the other is
+unsigned, the operation will be signed.
+The bit width also influences the outcome of the operations, in particular 
+the input languages features a fine granularity type system, with types of 8,
+16, 32, 64 (and more for vectorial instructions) bits, the tinycode only
+features 32 and 64 bit widths. We propagate as much as possible the fine
+granularity type, until the value has to be used inside an operation between
+``rvalue``\ s; in that case if one of the two operands is greater than 32 bits
+we promote the whole operation to 64 bit, taking care of properly extending the
+two operands.  Fortunately, the most critical instructions already feature
+explicit casts and zero/sign extensions which are properly propagated down to
+our parser.
+Control Statements
+``control_statement``\ s are all the statements which modify the order of
+execution of the generated code according to input parameters. They are 
+by the following grammar rule:
+   control_statement : frame_check
+                     | cancel_statement
+                     | if_statement
+                     | for_statement
+                     | fpart1_statement
+``if_statement``\ s require the emission of labels and branch instructions 
+effectively perform conditional jumps (``tcg_gen_brcondi``) according to the
+value of an expression. All the predicated instructions, and in general all the
+instructions where there could be alternative values assigned to an ``lvalue``,
+like C-style ternary expressions:
+   rvalue            : rvalue QMARK rvalue COLON rvalue
+Are handled using the conditional move tinycode instruction
+(``tcg_gen_movcond``), which avoids the additional complexity of managing 
+and jumps.
+Instead, regarding the ``for`` loops, exploiting the fact that they always
+iterate on immediate values, therefore their iteration ranges are always known
+at compile time, we implemented those emitting plain C ``for`` loops. This is
+possible because the loops will be executed in the QEMU code, leading to the
+consequential unrolling of the for loop, since the tinycode generator
+instructions will be executed multiple times, and the respective generated
+tinycode will represent the unrolled execution of the loop.
+Parsing Context
+All the helper functions in ``idef-parser.y`` carry two fixed parameters, which
+are the parsing context ``c`` and the ``YYLLOC`` location information. The
+context is explicitly passed to all the functions because the parser we 
+is a reentrant one, meaning that it does not have any global variable, and
+therefore the instruction compilation could easily be parallelized in the
+future. Finally for each rule we propagate information about the location of 
+involved tokens to generate a pretty error reporting, able to highlight the
+portion of the input code which generated each error.
+Developing the idef-parser can lead to two types of errors: compile-time errors
+and parsing errors.
+Compile-time errors in Bison-generated parsers are usually due to conflicts in
+the described grammar. Conflicts forbid the grammar to produce a unique
+derivation tree, thus must be solved (except for the dangling else problem,
+which is marked as expected through the ``%expect 1`` Bison option).
+For solving conflicts you need a basic understanding of `shift-reduce conflicts
+and `reduce-reduce conflicts
+then, if you are using a Bison version > 3.7.1 you can ask Bison to generate
+some counterexamples which highlight ambiguous derivations, passing the
+``-Wcex`` option to Bison. In general shift/reduce conflicts are solved by
+redesigning the grammar in an unambiguous way or by setting the token priority
+correctly, while reduce/reduce conflicts are solved by redesigning the
+interested part of the grammar.
+Run-time errors can be divided between lexing and parsing errors, lexing errors
+are hard to detect, since the ``VAR`` token will catch everything which is not
+catched by other tokens, but easy to fix, because most of the time a simple
+regex editing will be enough.
+idef-parser features a fancy parsing error reporting scheme, which for each
+parsing error reports the fragment of the input text which was involved in the
+parsing rule that generated an error.
+Implementing an instruction goes through several sequential steps, here are 
+suggestions to make each instruction proceed to the next step.
+-  not-emitted
+   Means that the parsing of the input code relative to that instruction 
+   this could be due to a lexical error or to some mismatch between the order 
+   valid tokens and a parser rule. You should check that tokens are correctly
+   identified and mapped, and that there is a rule matching the token sequence
+   that you need to parse.
+-  emitted
+   This instruction class contains all the instruction which are emitted but
+   fail to compile when included in QEMU. The compilation errors are shown by
+   the QEMU building process and will lead to fixing the bug.  Most common
+   errors regard the mismatch of parameters for tinycode generator functions,
+   which boil down to errors in the idef-parser type system.
+-  compiled
+   These instruction generate valid tinycode generator code, which however fail
+   the QEMU or the harness tests, these cases must be handled manually by
+   looking into the failing tests and looking at the generated tinycode
+   generator instruction and at the generated tinycode itself. Tip: handle the
+   failing harness tests first, because they usually feature only a single
+   instruction, thus will require less execution trace navigation. If a
+   multi-threaded test fail, fixing all the other tests will be the easier
+   option, hoping that the multi-threaded one will be indirectly fixed.
+-  tests-passed
+   This is the final goal for each instruction, meaning that the instruction
+   passes the test suite.
+Another approach to fix QEMU system test, where many instructions might fail, 
+to compare the execution trace of your implementation with the reference
+implementations already present in QEMU. To do so you should obtain a QEMU 
+where the instruction pass the test, and run it with the following command:
+   sudo unshare -p sudo -u <USER> bash -c \
+   'env -i <qemu-hexagon full path> -d cpu <TEST>'
+And do the same for your implementation, the generated execution traces will be
+inherently aligned and can be inspected for behavioral differences using the
+``diff`` tool.
+Limitations and Future Development
+The main limitation of the current parser is given by the syntax-driven nature
+of the Bison-generated parsers. This has the severe implication of only being
+able to generate code in the order of evaluation of the various rules, without,
+in any case, being able to backtrack and alter the generated code.
+An example limitation is highlighted by this statement of the input language:
+   { (PsV==0xff) ? (PdV=0xff) : (PdV=0x00); }
+This ternary assignment, when written in this form requires us to emit some
+proper control flow statements, which emit a jump to the first or to the second
+code block, whose implementation is extremely convoluted, because when matching
+the ternary assignment, the code evaluating the two assignments will be already
+Instead we pre-process that statement, making it become:
+   { PdV = ((PsV==0xff)) ? 0xff : 0x00; }
+Which can be easily matched by the following parser rules:
+   statement             | rvalue SEMI
+   rvalue                : rvalue QMARK rvalue COLON rvalue
+                         | rvalue EQ rvalue
+                         | LPAR rvalue RPAR
+                         | assign_statement
+                         | IMM
+   assign_statement      : pre ASSIGN rvalue
+Another example that highlight the limitation of the flex/bison parser can be
+found even in the add operation we already saw:
+   TCGv_i32 tmp_0 = tcg_temp_new_i32();
+   tcg_gen_add_i32(tmp_0, RsV, RtV);
+   tcg_gen_mov_i32(RdV, tmp_0);
+The fact that we cannot directly use ``RdV`` as the destination of the sum is a
+consequence of the syntax-driven nature of the parser. In fact when we parse 
+assignment, the ``rvalue`` token, representing the sum has already been 
+and thus its code emitted and unchangeable. We rely on the fact that QEMU will
+optimize our code reducing the useless move operations and the relative
+A possible improvement of the parser regards the support for vectorial
+instructions and floating point instructions, which will require the extension
+of the scanner, the parser, and a partial re-design of the type system, 
+to build the vectorial semantics over the available vectorial tinycode 
+A more radical improvement will use the parser, not to generate directly the
+tinycode generator code, but to generate an intermediate representation like 
+LLVM IR, which in turn could be compiled using the clang TCG backend. That code
+could be furtherly optimized, overcoming the limitations of the syntax-driven
+parsing and could lead to a more optimized generated code.

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