153 lines
7.2 KiB
Text
153 lines
7.2 KiB
Text
.so init
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.nr H1 4
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.NH
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FUTURE WORK
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.NH 2
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A critique of EM
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.PP
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In general, EM fits its purpose quite well. Numerous compilers have been
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written using EM as their intermediate language and it has even become a
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commercial product. A great deal of its success is probably due to its
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simplicity. There are no extravagant instructions but it does have all the
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necessary functions to write a decent compiler.
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.PP
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There are, however, a few functions that come rather close to being
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extravagant. The \*(Silar\*(So function for example \(em used
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to fetch an element from an array \(em does not make it much easier
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to write a frontend, but does make it unnecessary hard to write an
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efficient backend. Other instructions for which it is difficult
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to generate efficient code for are those that permit
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dynamic operators, such as the \*(Silos\*(So. Dynamic operators, however, provide
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significant extra possibilities and can therefore not be disposed of.
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Note that even though the array operations \*(Silar\*(So and \*(Sisar\*(So
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provide dynamic operators, they do not add additional power, since
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they can easily be replaced with a sequence using the \*(Silos\*(So or
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\*(Sists\*(So instructions.
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.PP
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EM code to reference arrays generated by the C frontend can be translated
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very efficiently for almost any processor. However the same operation
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generated by the Modula-2 frontend (which uses the \*(Silar\*(So),
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is much less efficient, although the only difference is that the
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latter performs range checking whereas the former does not.\(dg
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.FS
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\(dg Actually this depends on whether or not explicit range checking in enabled.
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This clearly shows that the current code generators are not optimal and
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often depend on ad-hoc decisions.
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.FE
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Since range checking can also be expressed explicitly in
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EM (\*(Sirck\*(So) there is no need for any of the array operations
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(\*(Siaar\*(So, \*(Silar\*(So and \*(Sisar\*(So).
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.PP
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Besides efficiency of the array-operations themselves, there still is another
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major disadvantage of using these array-operations. In sharp contrast to
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all other EM instructions except the \*(Silos\*(So and the \*(Sists\*(So,
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they allow dynamic operators, so their effect on the stack-pointer can not
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always be
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determined at compile-time. This means that efficient caching of the
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top-of-stack in registers is almost impossible,
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so using these array-operations also effects the
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efficiency of the surrounding code. Now that processors are produced with
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more and more registers it could be very beneficiary to cache the
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top-of-stack, so that the memory/register reference ratio decreases
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to the benefit of the overall performance.
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.PP
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As a final critique, we would also like to discuss the semantics of some of
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the EM instructions. In
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.[ [
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Description of a Machine Architecture
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.]]
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it is said that
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all signed instructions such as the \*(Siadi\*(So, should cause an exception
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on overflow. The unsigned operations such as \*(Siadu\*(So, however,
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should act as modulo operations and therefor not perform overflow checking.
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Since it is very expensive to perform overflow checking in EM,
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we would suggest that the backend takes care of this. For languages which
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do not require overflow checking, a simple message could be generated to
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disable overflow checking in backends. This way all backends could be
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written to fully comply to the official EM definition without any reduction in
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efficiency.\(dd
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.FS
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\(dd Currently many backends do not implement error checks because they
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are too expensive and almost never needed. Some frontends even have
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facilities build in to generate EM-code to force these checks. If this
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trend continues we will end up with a de-facto and a de-jure standard
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both developed by the same people but nonetheless incompatible.
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.FE
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When such messages will be added we would like to suggest
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that they can enforce overflow checks on unsigned, as well as signed arithmetic.
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.PP
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As a conclusion we would like to suggest removal of the array operations from
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EM, or at least discontinuation of there usage in frontends.
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.NH 2
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\*(OQWanted: Procedure call information\*(CQ
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.PP
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The advantage of an intermediate language such as EM is that the backend
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no longer has to know about any 'quirks' of the 'input'-language. The major
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disadvantage, however, is that the backend no longer knows about any 'quirks'
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of the 'input'-language... If the SPARC backend ever has to compete
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with Sun's own C-compiler for example, removal of the array-operations
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will not be enough. The amount of information that is lost during
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the translation to EM is too large to ever generate truly efficient SPARC code.
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.PP
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To write such an efficient backend one needs to know, for example, whether,
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when and what type of parameter is being computed, so the result can be stored
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in the proper place and scratch registers can be reused.
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(On the SPARC processor, for example, it is very beneficiary
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to pass the first six parameters of a procedure call through
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registers instead of using the stack.)
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One way to express such things in EM is to insert extra messages in
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the EM-code. The C statement \*(Sia = f(4, a + b);\*(So for example,
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could be translated to the following EM-code:
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.DS
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.TS
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;
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l1f6 lf6 l.
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lol -4 ! a
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lol -8 ! b
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mes x, 2 ! next instruction will compute 2nd parameter
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adi 4
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mes x, 1 ! next instruction will compute 1st parameter
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loc 4
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cal _f ! call function f
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lfr 4
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stl -4 ! store result in a
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.TE
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.DE
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For a code expander it is important that the \*(Simes\*(So pseudo
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instructions appear \fIbefore\fR
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the EM instruction that computes the parameter, because that way the final
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computation (the \*(Siadi\*(So and \*(Siloc\*(So in the previous example)
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can be translated to machine code that performs the required computation
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and also puts the result in the required place. If it is found to be
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too difficult for the frontend to insert these \*(Simes\*(So instructions
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at the right place the peep-hole optimizer might swap the \*(Simes\*(So and
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the instruction that computes the parameter.
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.PP
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For some architectures, it is also
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possible to generate more efficient code for a procedure when it is a
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so-called leaf-procedure: a procedure that doesn't call other procedures.
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On the SPARC, for example, it is not necessary to rotate the register
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window for a call to a leaf procedure and it is also possible to use
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the global registers for register variables in leaf procedures.
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It will be a little harder to insert useful messages about leaf procedures,
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because just as with register messages, they are only useful to the
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backend when they appear immediately
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after or before the \*(Sipro\*(So pseudo instruction. The frontend,
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however, only knows whether a certain procedure is a leaf-procedure or not
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when it has already generated the entire procedure in EM. Just as with the
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\*(Sipro ? / end n\*(So-dilemma the peep-hole optimizer
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.[ [
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Using Peephole Optimization
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.]]
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might be able to lend a hand
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and help us out by delaying EM-code generation until it has reached the
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end of the procedure.
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.PP
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As with most optimizations, the main problem is that they have to be
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implemented with the \*(Simes\*(So pseudo instruction.
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Because the \*(Simes\*(So instruction can have many different meanings
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depending on its argument,
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it is important that all optimizers recognize and respect them. Addition
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of even a single message will require careful inspection of, and maybe even
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incorporate small changes to each of the optimizers.
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.bp
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