Compiler Week 3

Course link

Instead of the manual method described below, there is a productivity extension for controlling video playback rate with keyboard shortcuts: Video Speed Controller

To set the YouTube video playback rate to some particular number, e.g.,

$\pi$, execute the following command in the browser's console; use Command + Option + J or Control + Shift + J to access Chrome's console.

document.querySelector('video').playbackRate = Math.PI

What this actually does is setting the playback rate of the first <video> tag on the page to

$\pi$, which happens to be the main video on any YouTube page.

Many links in this page refer to text fragments which are known to work on Chrome 80 and above. I can't say about other browsers.

Q&A

Classify the following as front-end or back-end components

• Target code optimizer: back-end
• Parser: front-end
• Scanner: front-end
• IR code generator: front/middle-end
• Type checker: front-end

Classify 1-3 as (a) or (b)

(a) a common back-end compiling system (b) a retargetable compiler

1. HP PA-RISC compiler/optimizer: (a)
2. Intel C++ Compiler Classic (ICC): (a) for C/C++ (b) for x86/x64
   • According to the developer's guide, ICC actually stands for Intel C++ Compiler Classic.
   • x86 refers to IA-32 whereas x64 refers to IA-64
3. GNU Compiler Collection (GCC): (b)
   • Nowadays, GCC stands for GNU Compiler Collection; historically, at version 1.0, GCC actually stood for GNU C Compiler.
   • Even in the stricter sense, referring exclusively to the C compiler, GCC is still (b).

How to create the first C compiler on the Itanium machine?

• Write a C compiler in C, compile it on Itanium machines using GCC.
  • This line of reasoning is incoherent. If the first C compiler for Itanium is being developed, GCC wouldn't be able to target Itanium.
• Write a C compiler in Itanium machine code.
  • One doesn't attempt this feat unless one is Jeff Dean.
• Develop a cross-compiler on x86 and use it to compile itself into Itanium instructions.
  • A cross compiler is a compiler that can generate instructions for some CPU architecture different from the one it is running on. E.g., GCC running on an Intel machine can compile sources as ARM binaries. A cross compiler targeting Itanium can compile its source code on x86 to produce its Itanium counterpart which can then be executed on Itanium as a native compiler.
  • This illustrates the process.
• Leave it to Intel/Microsoft, i.e., evil empire.
  • Why not? Why is the professor against this.

How to create the first C compiler on the new Itanium if no cross-compiler is available?

Incremental bootstrapping.

1. Craft a simple assembler, C1, in Itanium machine code, i.e., directly editing a binary file from scratch.
2. Craft a more complex assembler, C2, in Itanium assembly with C1.
3. Craft a simple C compiler, C3, with C2.
4. Craft a more complex C compiler, C4, with C3.
5. Continue step 4 for several iterations until a good enough C compiler, C0, is finally produced.

Then, C0 is what we seek for.

Evaluate 1u > -1 in C/C++

Comparing unsigned int with int is defined behavior in C/C++: int is coerced to unsigned int for comparison. Since -1 has all bits being 1 in 2's complement, 1u > -1 evaluates to false in most cases. In fact, C++20 mandates that all values be represented in 2's complement. Here are more C++ specific details.

• 1u and -1 are integer literals that are of type unsigned int and int, respectively.
• Coercion of mismatched operand types of an arithmetic (comparison) operator is called usual arithmetic conversion. In this case, usual arithmetic conversion is due to the unsigned operand's conversion rank being equal to the conversion rank of the signed operand, thus, requiring the signed operand to be converted to the unsigned operand's type where the conversion rank of any unsigned type is equal to the rank of the corresponding signed type.
• For best practices, consult the Arithmetic section (consists of only 8 points; highly recommended) of C++ Core Guidelines. Honorable mentions:
  • ES.100: Don’t mix signed and unsigned arithmetic.
  • ES.101: Use unsigned types for bit manipulation.
  • ES.102: Use signed types for arithmetic.
  • ES.106: Don’t try to avoid negative values by using unsigned.
• In fact, the C++ standard library devoted a whole class – std::bitset – to bit manipulation.
• For safer integral comparisons, C++20 provides std::cmp_{equal,not_equal,less,greater,less_equal,greater_equal}.
• C++20 also provides safer math constants belonging to the namespace std::numbers.

Which of the following are in the language defined by the AC syntax?

• f b i a a = 0 b = a + 3.2 p b
• p15
  An id must follow print.
• An empty file
• f b a = 1.0i cc = 0 p a
  Note that a = 1.0 is not a syntax error but a semantics error.
• f b i a a = 0 b =-a - 3.2 p b

What is the associativity of op given the grammar expr -> expr op primary? How can the grammar be re-defined so as to invert the associativity of op?

• Left associative.
• expr -> primary op expr

Identify expressions with constant folding potential

• 3 + 5 * b
• a + 5- 4
  Left association doesn't make way for folding 5 - 4.
• a +2 * 5+ c
• 2 + 5+ c