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121 lines
4.6 KiB
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121 lines
4.6 KiB
Plaintext
Ok, here are my comments and suggestions about the LLVM instruction set.
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We should discuss some now, but can discuss many of them later, when we
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revisit synchronization, type inference, and other issues.
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(We have discussed some of the comments already.)
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o We should consider eliminating the type annotation in cases where it is
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essentially obvious from the instruction type, e.g., in br, it is obvious
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that the first arg. should be a bool and the other args should be labels:
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br bool <cond>, label <iftrue>, label <iffalse>
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I think your point was that making all types explicit improves clarity
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and readability. I agree to some extent, but it also comes at the cost
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of verbosity. And when the types are obvious from people's experience
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(e.g., in the br instruction), it doesn't seem to help as much.
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o On reflection, I really like your idea of having the two different switch
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types (even though they encode implementation techniques rather than
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semantics). It should simplify building the CFG and my guess is it could
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enable some significant optimizations, though we should think about which.
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o In the lookup-indirect form of the switch, is there a reason not to make
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the val-type uint? Most HLL switch statements (including Java and C++)
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require that anyway. And it would also make the val-type uniform
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in the two forms of the switch.
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I did see the switch-on-bool examples and, while cute, we can just use
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the branch instructions in that particular case.
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o I agree with your comment that we don't need 'neg'.
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o There's a trade-off with the cast instruction:
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+ it avoids having to define all the upcasts and downcasts that are
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valid for the operands of each instruction (you probably have thought
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of other benefits also)
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- it could make the bytecode significantly larger because there could
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be a lot of cast operations
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o Making the second arg. to 'shl' a ubyte seems good enough to me.
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255 positions seems adequate for several generations of machines
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and is more compact than uint.
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o I still have some major concerns about including malloc and free in the
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language (either as builtin functions or instructions). LLVM must be
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able to represent code from many different languages. Languages such as
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C, C++ Java and Fortran 90 would not be able to use our malloc anyway
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because each of them will want to provide a library implementation of it.
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This gets even worse when code from different languages is linked
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into a single executable (which is fairly common in large apps).
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Having a single malloc would just not suffice, and instead would simply
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complicate the picture further because it adds an extra variant in
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addition to the one each language provides.
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Instead, providing a default library version of malloc and free
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(and perhaps a malloc_gc with garbage collection instead of free)
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would make a good implementation available to anyone who wants it.
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I don't recall all your arguments in favor so let's discuss this again,
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and soon.
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o 'alloca' on the other hand sounds like a good idea, and the
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implementation seems fairly language-independent so it doesn't have the
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problems with malloc listed above.
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o About indirect call:
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Your option #2 sounded good to me. I'm not sure I understand your
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concern about an explicit 'icall' instruction?
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o A pair of important synchronization instr'ns to think about:
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load-linked
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store-conditional
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o Other classes of instructions that are valuable for pipeline performance:
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conditional-move
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predicated instructions
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o I believe tail calls are relatively easy to identify; do you know why
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.NET has a tailcall instruction?
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o I agree that we need a static data space. Otherwise, emulating global
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data gets unnecessarily complex.
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o About explicit parallelism:
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We once talked about adding a symbolic thread-id field to each
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instruction. (It could be optional so single-threaded codes are
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not penalized.) This could map well to multi-threaded architectures
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while providing easy ILP for single-threaded onces. But it is probably
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too radical an idea to include in a base version of LLVM. Instead, it
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could a great topic for a separate study.
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What is the semantics of the IA64 stop bit?
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o And finally, another thought about the syntax for arrays :-)
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Although this syntax:
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array <dimension-list> of <type>
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is verbose, it will be used only in the human-readable assembly code so
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size should not matter. I think we should consider it because I find it
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to be the clearest syntax. It could even make arrays of function
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pointers somewhat readable.
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