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32 lines
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32 lines
1.5 KiB
Plaintext
Date: Fri, 6 Jul 2001 16:56:56 -0500
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From: Vikram S. Adve <vadve@cs.uiuc.edu>
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To: Chris Lattner <lattner@cs.uiuc.edu>
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Subject: lowering the IR
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BTW, I do think that we should consider lowering the IR as you said. I
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didn't get time to raise it today, but it comes up with the SPARC
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move-conditional instruction. I don't think we want to put that in the core
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VM -- it is a little too specialized. But without a corresponding
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conditional move instruction in the VM, it is pretty difficult to maintain a
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close mapping between VM and machine code. Other architectures may have
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other such instructions.
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What I was going to suggest was that for a particular processor, we define
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additional VM instructions that match some of the unusual opcodes on the
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processor but have VM semantics otherwise, i.e., all operands are in SSA
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form and typed. This means that we can re-generate core VM code from the
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more specialized code any time we want (so that portability is not lost).
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Typically, a static compiler like gcc would generate just the core VM, which
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is relatively portable. Anyone (an offline tool, the linker, etc., or even
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the static compiler itself if it chooses) can transform that into more
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specialized target-specific VM code for a particular architecture. If the
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linker does it, it can do it after all machine-independent optimizations.
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This would be the most convenient, but not necessary.
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The main benefit of lowering will be that we will be able to retain a close
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mapping between VM and machine code.
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--Vikram
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