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