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Hi all, this is Jeehoon Kang, a CS PhD student and a newbie to LLVM. I am wondering why LLVM IR's basic block consists of a list of instructions, rather than a DAG of instruction as in the low level (ISelectionDAG). My gut feeling tells me that LLVM IR in DAG form may admit more optimisations in a systematic manner. This is because data dependence is more explicit in a DAG of instructions than in a list of those. However, I found an exact opposite proposal: removing DAG structure at all ( http://lists.cs.u...

...<jeehoon.kang at sf.snu.ac.kr> > wrote: > > > > this is Jeehoon Kang, a CS PhD student and a newbie to LLVM. > > > > I am wondering why LLVM IR's basic block consists of a list of > instructions, > > rather than a DAG of instruction as in the low level (ISelectionDAG). > > SSA form is implicitly a DAG, defined by the uses relation (registers in > LLVM can be thought of as edges between instructions). It is not *solely* > a DAG, however. For example, in some places the order of two loads matters > - particularly when they are atomic operations -...

...> > > this is Jeehoon Kang, a CS PhD student and a newbie to LLVM. > > > > I am wondering why LLVM IR's basic block consists of a list > of instructions, > > rather than a DAG of instruction as in the low level > (ISelectionDAG). > > SSA form is implicitly a DAG, defined by the uses relation > (registers in LLVM can be thought of as edges between > instructions). It is not *solely* a DAG, however. For > example, in some places the order of two loads matters - > p...