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===========
QBE vs LLVM
===========
Both QBE and LLVM are compiler backends using an SSA
representation. This document will explain why LLVM
does not make QBE a redundant project. Obviously,
everything following is probably biased, because
written by me.
- Scope
-------
QBE is a much smaller scale project with different goals
than LLVM.
* QBE is for amateur language designers.
It does not address all the problems faced when
conceiving an industry-grade language. If you are
toying with some language ideas, using LLVM will
be like hauling your backpack in a truck, but using
QBE will feel more like biking.
* QBE is about the first 70%, not the last 30%.
It attempts to pinpoint, in the extremely vast
compilation literature, the optimizations that get
you 70% of the performance in 10% of the code of
full blown compilers.
For example, copy propagation on SSA form is
implemented in 160 lines of code in QBE!
* QBE is extremely hackable.
First, it is, and will remain, a small project
(less than 8 kloc). Second, it is programmed in
non-fancy C99 without any dependencies. Third,
it is able to dump the IL and debug information in
a uniform format after each pass.
On my Core 2 Duo machine, QBE compiles in half a
second.
- Features
----------
LLVM is definitely more packed with features, but there
are a few things provided in QBE to consider.
* LLVM does NOT provide full C compatibility for you.
In more technical terms, any language that provides
good C compatibility and uses LLVM as a backend
needs to reimplement large chunks of the ABI in
its frontend! This well known issue in the LLVM
community causes a great deal of duplication
and bugs.
Implementing a complete C ABI (with struct arguments
and returns) is incredibly tricky, and not really
a lot of fun. QBE provides you with IL operations
to call in (and be called by) C with no pain.
Moreover the ABI implementation in QBE has been
thoroughly tested by fuzzing and manual tests.
* LLVM IR is more cluttered with memory operations.
Implementing SSA construction is hard. To save its
users from having to implement it, LLVM provides
stack slots. This means that one increment of
a variable `v` will be composed of three LLVM
instructions: one load, one add, and one store.
QBE provides simple non-SSA temporaries, so
incrementing `v` is simply done with one instruction
`%v =w add %v, 1`.
This could seem cosmetic, but dividing the size of
the IL by three makes it easier for the frontend
writers to spot bugs in the generated code.
* LLVM IR is more cluttered with type annotations and
casts.
For the sake of advanced optimizations and
correctness, LLVM has complex IL types. However,
only a few types are really first class and many
operations of source languages require casts to be
compiled.
Because QBE makes a much lighter use of types, the
IL is more readable and shorter. It can of course be
argued back that correctness of QBE is jeoparadized,
but remember that, in practice, the large amount
of casts necessary in LLVM IL is compromizing the
overall effectiveness of the type system.
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