<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Sep 26, 2010, at 3:01 PM, Talin wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">So I understand now that calls to llvm.gcroot have to be in the first block. (Might be a good idea to mention this in the GC docs).<div><br></div><div>For each local variable that is created, there are 5 possible steps.</div>
<div><ol><li>Allocate the storage for the variable using alloca().</li><li>Declare the variable as a root via llvm.gcroot.</li><li>Initialize the variable to zero.</li><li>Declare the DWARF debugging info for the variable via llvm.dbg.declare.</li>
<li>Evaluate the initializer expression for the variable and store it to the variable.</li></ol><div>(It might seem that step 3 is redundant, but what if step 5 triggers a garbage collection cycle? We need to insure that the alloca doesn't contain stack junk if this happens, so we need to zero-initialize all roots before any initializers are run. I'm hoping that the optimizer will eliminate the redundant store if the initializer expression turns out to be trivial.)</div>
</div><div><br></div><div>My question is this: Is there any constraint on the order of steps 2, 3 and 4? And does step 4 also have to go in the first block?</div></blockquote><div><br></div>Only requirement for llvm.dbg.declare is that it must be after 1. Debug info handling is completely oblivious of llvm.gcroot so no idea about your other questions.</div><div><br></div><div>-</div><div>Devang</div><div><br><blockquote type="cite"><div><br></div><div>Another related question: What effect does llvm.gcroot have on optimizations? For example, I assume a variable that has been declared as a root can't be converted to a register variable. Are there any other impacts I should know about?</div>
<div><br>-- <br>-- Talin<br>
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