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Survey of instruction selection

June 21st, 2013 1 comment

A well written survey of compiler instruction selection has just become available, the first major survey of this topic in 30 years! The academic outlook of the author is given away by the evaluation “…the technique appears to have had very limited impact as the citation count for the paper is low.” and coverage for the last 10 years does tend to thin out (but that could fill another 100 pages). Whatever your interest in compilers this survey is well worth a read.

Anybody reading a compiler book could be forgiven for thinking that instruction set selection was a minor issue; Gabriel Hjort Blindell counted 160 pages devoted to the topic out of 4,600 pages in seven well known compiler books. In a production compiler it is the parsing and semantics that consume 3% of the code with optimization and code generation making up the other 97%.

A 100 page survey of register allocation is also overdue (20 pages is a bit short).

Instruction set selection is one quarter of code generation, another quarter being register allocation and the remaining half being how these two are woven together (Hjort Blindell lists instruction scheduling as a third component and we could all argue for hours about whether this is another optimization, something that is spread over instruction selection/register allocation or a distinct component).

For a given choice of registers there are algorithms that will select the optimal code and for a given sequence of code there are algorithms that will select the optimal registers to use. Papers covering the optimal selection of both registers and instructions are thin on the ground; this is something of a black art that is picked up by building a production compiler.

Why is Cobol still popular in Japan?

October 28th, 2012 No comments

Rummaging around the web for empirical software engineering data I found a survey of programming language usage in Japan. This survey (based on 505 projects in 24 companies) has Cobol in the number two slot for 2012, a bit higher than I would have expected (it very rarely appears at all in US/UK ‘popularity’ lists):

Language       Projects  
Java             822  28.2%
COBOL            464  15.9%
VB               371  12.7%
C                326  11.2%
Other languages  208   7.1%
C++              189   6.5%
Visual Basic.NET 136   4.7%
Visual C++       105   3.6%
C#               101   3.5%
PL/SQL            57   2.0%
Pro*C             23   0.8%
Excel(VBA)        18   0.6%
Developer2000     17   0.6%
ABAP              15   0.5%
HTML              14   0.5%
Delphi            11   0.4%
PL/I              10   0.3%
Perl              10   0.3%
PowerBuilder       7   0.2%
Shell              7   0.2%
XML                6   0.2%

A quick overview of Cobol for those readers who have never encountered it.

Cobol is a domain specific language ideally suited for business data processing in the 1960/70/80/90s. During this period computer memory was often measured in kilobytes, data came in an unbelievably wide range of different formats, operations on data mostly involved sorting and basic arithmetic, and output data format was/is very important. By “unbelievably wide range” think of lots of point-of-sale vendors deciding how their devices would write data to punch cards/paper tape/magnetic tape, just handling the different encodings that have been used for the plus/minus sign can make the head spin; combine the requirement that programs handle different data formats with tiny computer memory capacity and you get data structure overlays that make C programmers look like rank amateurs, all the real action in Cobol programs occurs in the DATA DIVISION.

So where are we today? Companies use computers to solve a wider range of problems don’t they (so even if Cobol usage stayed the same its percentage usage should be low)? If point-of-sale terminals still produce a wide range of weird and wonderful data formats isn’t it easy enough to write the appropriate libraries to convert (and we have much more storage these days)?

Why might Cobol still so be so popular in Japan (and perhaps elsewhere if anybody over 25 was included in the survey)? Some ideas:

  • Cobol is still the best language to use for business data processing,
  • the sample is not representative of the Japanese software development industry. As a government body perhaps the Information-Technology Promotion Agency primarily deals with large well established companies; the data came from a relatively small number of companies (i.e., 24),
  • the Japanese are known for being conservative and maintaining traditions. Change is almost considered a necessity here in the West, this has led to the use of way too many programming languages in industry (I have previously written about what a mistake it is to invent a new language).

Birth month for compiler writers

February 19th, 2012 No comments

Today is my birthday and an event from a long ago project springs to mind. All four of us from the UK arm of the team were born in February, one person on the same day as me (Happy Birthday Mick, where ever you are). This clustering of birth months led us to the obvious conclusion that the best compiler writers are born in February. Over the years I have retold this story to other compiler writers and found out their birth month. Now I will try and be a bit more scientific and have set up a survey (see below).

What counts as a compiler and what does somebody have to do to be considered a compiler writer (lets stay away from the issue of quality)? I would include software that performs computer language translation to another language (i.e., not just intermediate code or assembler) and static analysis of source provided it involved a lot of semantics (i.e., not working on the GUI that presents the data). I would exclude writing test cases, documentation, project management and maintenance (i.e., only fixing faults and dealing with customer queries).

I would classify a compiler writer as somebody who spent a substantial amount of their time working almost exclusively on writing a compiler. How substantial? Well, I think it ought o be possible to do something useful in about 4 months (I thought about saying 6 months, but decided to be generous.

Please take part, even if you do not consider yourself to be a compiler writer. A control group is always useful (perhaps readers of this blog have a preferred birth month)

I will make the numbers available and discuss them in a future article (probably in March).

What month were you born in?












Have you spent more than 4 months working full time on a compiler?




If anybody else is interested in running a survey, the surveys WordPress plugin allows more than one question to be specified and worked better than the other popular plugins for me (there is one bug that needs to be fixed: show_survey.php, line 51 should be:
$email_body = t("Hi,\nThere is a new result for the survey at %s...\n", $_SERVER['REQUEST_URI']); ).