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Languages and Compilers (SProg og Oversættere). Bent Thomsen Department of Computer Science Aalborg University. With acknowledgement to Norm Hutchinson who’s slides this lecture is based on. is expressed in the source language. is expressed in the target language. is expressed in the - PowerPoint PPT Presentation
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Languages and Compilers(SProg og Oversættere)
Bent Thomsen
Department of Computer Science
Aalborg University
With acknowledgement to Norm Hutchinson who’s slides this lecture is based on.
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Terminology
Translatorinput output
source program object program
is expressed in thesource language
is expressed in theimplementation language
is expressed in thetarget language
Q: Which programming languages play a role in this picture?
A: All of them!
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Tombstone Diagrams
What are they?– diagrams consisting out of a set of “puzzle pieces” we can use
to reason about language processors and programs
– different kinds of pieces
– combination rules (not all diagrams are “well formed”)
M
Machine implemented in hardware
S -> T
L
Translator implemented in L
ML
Language interpreter in L
Program P implemented in L
LP
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Tombstone diagrams: Combination rules
SP P
TS -> T
M
M
LP
S -> T
MWRONG!
OK!OK!
OK!MMP
OK!
M
LP
WRONG!
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Tetrisx86C
Tetris
Compilation
x86
Example: Compilation of C programs on an x86 machine
C -> x86
x86x86
Tetris
x86
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TetrisPPCC
Tetris
Cross compilation
x86
Example: A C “cross compiler” from x86 to PPC
C -> PPC
x86
A cross compiler is a compiler which runs on one machine (the host machine) but emits code for another machine (the target machine).
Host ≠ Target
Q: Are cross compilers useful? Why would/could we use them?
PPCTetris
PPC
download
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Tetrisx86
TetrisJVMJava
Tetris
Two Stage Compilation
x86
Java->JVM
x86
A two-stage translator is a composition of two translators. The output of the first translator is provided as input to the second translator.
x86
JVM->x86
x86
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x86
Java->x86
Compiling a Compiler
Observation: A compiler is a program! Therefore it can be provided as input to a language processor.Example: compiling a compiler.
Java->x86
C
x86
C -> x86
x86
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Interpreters
An interpreter is a language processor implemented in software, i.e. as a program.
Terminology: abstract (or virtual) machine versus real machine
Example: The Java Virtual Machine
JVMx86
x86
JVMTetris
Q: Why are abstract machines useful?
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Interpreters
Q: Why are abstract machines useful?
1) Abstract machines provide better platform independence
JVMx86
x86 PPC
JVMTetris
JVMPPC
JVMTetris
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Interpreters
Q: Why are abstract machines useful?
2) Abstract machines are useful for testing and debugging.
Example: Testing the “Ultima” processor using hardware emulation
Ultimax86
x86
UltimaUltima
P
UltimaP
Functional equivalence
Note: we don’t have to implement Ultima emulator in x86 we can use a high-level language and compile it.
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Interpreters versus Compilers
Q: What are the tradeoffs between compilation and interpretation?
Compilers typically offer more advantages when – programs are deployed in a production setting– programs are “repetitive”– the instructions of the programming language are complex
Interpreters typically are a better choice when– we are in a development/testing/debugging stage– programs are run once and then discarded – the instructions of the language are simple – the execution speed is overshadowed by other factors
• e.g. on a web server where communications costs are much higher than execution speed
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Interpretive Compilers
Why?A tradeoff between fast(er) compilation and a reasonable runtime performance.
How?Use an “intermediate language”• more high-level than machine code => easier to compile to• more low-level than source language => easy to implement as an
interpreter
Example: A “Java Development Kit” for machine M
Java->JVM
M
JVMM
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PJVMJava
P
Interpretive Compilers
Example: Here is how we use our “Java Development Kit” to run a Java program P
Java->JVM
M JVMMM
JVMP
M
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Portable Compilers
Example: Two different “Java Development Kits”
Java->JVM
JVM
JVMM
Kit 2:
Java->JVM
M
JVMM
Kit 1:
Q: Which one is “more portable”?
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Portable Compilers
In the previous example we have seen that portability is not an “all or nothing” kind of deal.
It is useful to talk about a “degree of portability” as the percentage of code that needs to be re-written when moving to a dissimilar machine.
In practice 100% portability is as good as impossible.
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Example: a “portable” compiler kit
Java->JVM
Java
JVMJava
Java->JVM
JVM
Q: Suppose we want to run this kit on some machine M. How could we go about realizing that goal? (with the least amount of effort)
Portable Compiler Kit:
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Example: a “portable” compiler kit
Java->JVM
Java
JVMJava
Java->JVM
JVM
Q: Suppose we want to run this kit on some machine M. How could we go about realizing that goal? (with the least amount of effort)
JVMJava
JVMC
reimplement
C->M
M
JVMM
M
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Example: a “portable” compiler kit
Java->JVM
Java
JVMJava
Java->JVM
JVM
JVMM
This is what we have now:
Now, how do we run our Tetris program?
TetrisJVMJava
Tetris
M
Java->JVM
JVMJVM
M
JVMTetris
JVMM
M
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Bootstrapping
Java->JVM
Java
JVMJava
Java->JVM
JVM
Remember our “portable compiler kit”:
We haven’t used this yet!
Java->JVM
Java
Same language! Q: What can we do with a compiler written in itself? Is that useful at all?
JVMM
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Bootstrapping
Java->JVM
Java
Same language!
Q: What can we do with a compiler written in itself? Is that useful at all?
• By implementing the compiler in (a subset of) its own language, we become less dependent on the target platform => more portable implementation.
• But… “chicken and egg problem”? How do to get around that?=> BOOTSTRAPPING: requires some work to make the first “egg”.
There are many possible variations on how to bootstrap a compiler written in its own language.
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Bootstrapping an Interpretive Compiler to Generate M code
Java->JVM
Java
JVMJava
Java->JVM
JVM
Our “portable compiler kit”:
PMJava
P
Goal we want to get a “completely native” Java compiler on machine M
Java->M
M
JVMM
M
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PM
PMJava
P
Bootstrapping an Interpretive Compiler to Generate M code
Idea: we will build a two-stage Java -> M compiler.
PJVM
M
Java->JVM
MM
JVM->M
M
We will make this by compiling
To get this we implement
JVM->M
JavaJava->JVM
JVM and compile it
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Bootstrapping an Interpretive Compiler to Generate M code
Step 1: implement
JVM->M
Java JVM
JVM->MJava->JVM
JVMJVM
M
M
JVM->M
Java
Step 2: compile it
Step 3: compile this
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Bootstrapping an Interpretive Compiler to Generate M code
Step 3: “Self compile” the JVM (in JVM) compiler
M
JVM->M
JVMM
M
JVM->M
JVM JVM->M
JVM
This is the second stage of our compiler!
Step 4: use this to compile the Java compiler
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Bootstrapping an Interpretive Compiler to Generate M code
Step 4: Compile the Java->JVM compiler into machine code
M
Java->JVM
M
Java->JVM
JVM JVM->M
M
The first stage of our compiler!
We are DONE!
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Full Bootstrap
A full bootstrap is necessary when we are building a new compiler from scratch.
Example:We want to implement an Ada compiler for machine M. We don’t currently have access to any Ada compiler (not on M, nor on any other machine).
Idea: Ada is very large, we will implement the compiler in a subset of Ada and bootstrap it from a subset of Ada compiler in another language. (e.g. C)
Ada-S ->M
C
v1Step 1: build a compiler for Ada-S in another language
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Full Bootstrap
Ada-S ->M
C
v1
Step 1a: build a compiler (v1) for Ada-S in another language.
Ada-S ->M
C
v1
M
Ada-S->Mv1
Step 1b: Compile v1 compiler on M
M
C->M
MThis compiler can be used for bootstrapping on machine M but we do not want to rely on it permanently!
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Full Bootstrap
Ada-S ->M
Ada-S
v2
Step 2a: Implement v2 of Ada-S compiler in Ada-S
Ada-S ->M
Ada-S
v2
M
M
Ada-S->Mv2
Step 2b: Compile v2 compiler with v1 compiler
Ada-S ->M
M
v1
Q: Is it hard to rewrite the compiler in Ada-S?
We are now no longer dependent on the availability of a C compiler!
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Full Bootstrap
Step 3a: Build a full Ada compiler in Ada-S
Ada->M
Ada-S
v3
M
M
Ada->Mv3
Ada-S ->M
M
v2
Step 3b: Compile with v2 compiler
Ada->M
Ada-S
v3
From this point on we can maintain the compiler in Ada. Subsequent versions v4,v5,... of the compiler in Ada and compile each with the the previous version.
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Half Bootstrap
We discussed full bootstrap which is required when we have no access to a compiler for our language at all.
Q: What if we have access to an compiler for our language on a different machine HM but want to develop one for TM ?
Ada->HM
HM
We have:
Ada->TM
TM
We want:
Idea: We can use cross compilation from HM to TM to bootstrap the TM compiler.
Ada->HM
Ada
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HM
Ada->TM
Half Bootstrap
Idea: We can use cross compilation from HM to M to bootstrap the M compiler.
Step 1: Implement Ada->TM compiler in Ada
Ada->TM
Ada
Step 2: Compile on HM
Ada->TM
Ada Ada->HM
HM
HM
Cross compiler: running on HM but
emits TM code
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TM
Ada->TM
Half Bootstrap
Step 3: Cross compile our TM compiler.
Ada->TM
Ada Ada->TM
HM
HM
DONE!
From now on we can develop subsequent versions of the compiler completely on TM
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Bootstrapping to Improve Efficiency
The efficiency of programs and compilers:Efficiency of programs:
- memory usage- runtime
Efficiency of compilers: - Efficiency of the compiler itself- Efficiency of the emitted code
Idea: We start from a simple compiler (generating inefficient code) and develop more sophisticated version of it. We can then use bootstrapping to improve performance of the compiler.
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Bootstrapping to Improve Efficiency
We have:Ada->Mslow
Ada
Ada-> Mslow
Mslow
We implement:Ada->Mfast
Ada
Ada->Mfast
Ada
M
Ada->Mfast
Mslow
Step 1
Ada-> Mslow
Mslow
Step 2 Ada->Mfast
Ada
M
Ada->Mfast
MfastAda-> Mfast
Mslow
Fast compiler that emits fast code!