<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Apr 20, 2008, at 6:52 PM, Gordon Henriksen wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On 2008-04-20, at 21:05, Terence Parr wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div>On Apr 20, 2008, at 5:36 PM, Gordon Henriksen wrote:</div></div></div></blockquote><blockquote type="cite"><br></blockquote><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; ">Since the semispace heap doesn't actually work (it's an example, at best), I suggest you simply copy the stack visitor into your project; it's only a dozen lines of code or so.</span></div></blockquote></div><div><div><br></div>Ok, copying; can't find ShadowStackEntry though. Even make in that dir doesn't work:</div></div></blockquote><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><br></div><div>Please use the version from subversion; this is broken in 2.2 release, unfortunately.</div></div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>ah! ok, looks better now. :)</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; ">how does the gc "shadow-stack" gcroot intrinsic work exactly? I couldn't read the assembly very well. Seems my example above wouldn't work would it unless i create/fill in a shadow stack record?</span></div></blockquote></div></div><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"></blockquote></div><div></div></div></blockquote><div><br></div><div>'gc "shadow-stack"' in the LLVM IR instructs the code generator to automatically maintain the linked list of stack frames. You don't have to do anything to maintain these shadow stack frames except to keep your variables in the llvm.gcroot'd allocas. Essentially, it does this:</div><div><br></div><div> struct ShadowStackEntry {</div><div> ShadowStackLink *next;</div><div> const ShadowStackMetadata *metadata;</div><div> void *roots[0];</div><div> };</div></div></div></blockquote><div><br class="webkit-block-placeholder"></div><div>Ok, bear with me here...</div><div><br class="webkit-block-placeholder"></div>What's the difference between ShadowStackLink and ShadowStackEntry?</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div> <span class="Apple-style-span" style="-webkit-text-stroke-width: -1; "> template <size_t count></span></div><div> struct Roots {</div><div> ShadowStackLink *next;</div><div> const ShadowStackMetadata *metadata;</div><div> void *roots[0];</div><div> };</div><div> </div><div> ShadowStackEntry *shadowStackHead;</div><div> </div><div> // Defined by the code generator.</div><div> const ShadowStackMetadata f_metadata = ...;</div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>Do you mean generated by my front end that emits IR or do you mean the backend? It seems that, since I read the source code and build the symbol table, I would need to build this stack frame type information for LLVM.</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div> <span class="Apple-style-span" style="-webkit-text-stroke-width: -1; "> void f() {</span></div><div> Roots<3> roots;</div><div> roots.next = shadowStackHead;</div><div> roots.metadata = f_metadata;</div><div> roots.roots[0] = NULL;</div><div> roots.roots[1] = NULL;</div><div> roots.roots[2] = NULL;</div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>What are the three roots here? Not sure where anything but the next, metadata are coming from. So the gc "shadow-stack" generates that preamble code? That would make sense</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div> shadowStackHead = (ShadowStackEntry *) &roots;</div><div> </div><div> ... user code ...</div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>here is where my gcroots go then I guess.</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div> <span class="Apple-style-span" style="-webkit-text-stroke-width: -1; "> shadowStackHead = entry.next; // before any exit</span></div><div> return;</div><div> }</div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>Can you tell me where to find ShadowStackMetadata? A search does not reveal it:</div><div><br class="webkit-block-placeholder"></div><div><div>/usr/local/llvm-2.2 $ find . -name 'ShadowStackMetadata*'</div><div><br></div><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>Taking a giant step back, I can build something similar to semispace.c myself so I'm in control of my world, right? i would set up the shadow stack using IR instructions and could avoid gcroot by notifying my collector as I see fit... </div><div><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; "></span></div></div></blockquote><div><br></div><div>That's true; the shadow stack design is explicitly for uncooperative environments, after all.</div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>The compiler plug-in for a GC is like a sophisticated macro that knows how to emit preambles and post ambles for each function that says it uses that particular GC, right? Does it do more than an include such as figuring out which alloca's I have that are pointers? If so, then why do I need to use gcroot instructions to identify roots? Seems like it would be much easier to understand to just have my output templates emit the preamble and so on. Oh, maybe the optimizer remove some stuff in there for what I think is a root is actually not around anymore.</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><div><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; ">When you want to eliminate the shadow stack overhead, you will need to (a.) use a conservative GC or (b.) emit stack frame metadata using the LLVM GC support.</span></div></div></div></blockquote><div><br class="webkit-block-placeholder"></div>Unfortunately, I'm thoroughly confused about who generates what. Who is supposed to generate the meta data types? If I am, that is fine, but I really can't find anything in the documentation that is a simple end to end C code -> IR example. Once I get one together, I'll put it in the book I'm writing. I've spent many hours reading and playing as much as I can, but it is still not clear; 'course I ain't always that bright. ;) Note that the paper by Henderson was extremely clear to me, so it's not the contents, it is the details of using LLVM to do GC.</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; ">Sorry I'm so lost...just trying to figure out what llvm does for me and what I have to do.</span></div></div></blockquote><div><br></div>No problem!</div><div><br></div><div>Generally speaking, LLVM is going to help you find roots on the stack, which is the part that the compiler backend <i>must</i> help with; the rest is your playground. </div></div></blockquote><div><br class="webkit-block-placeholder"></div>Is that because only code generation knows what roots exist after processing the IR?</div><div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>The infrastructure is more suited toward interfacing with an existing GC rather than necessarily making writing a new runtime trivial. (See exception handling for precedent…)<br></div></div></blockquote></div><br><div>Well, writing a new garbage collector seems really straightforward (like to mark and sweep). LLVM will give me the roots and I am free to walk them. The part that I don't understand is who defines what metadata types and how exactly I make use of gcroot and LLVM's support. The concepts are clear, the details seem miles away ;)</div><div><br class="webkit-block-placeholder"></div><div>Thanks for all the help...</div><div><br class="webkit-block-placeholder"></div><div>Has anybody else on the list gotten a trivial GC'd language working I could look at? All go back to the scheme translator again to see what I can learn.</div><div><br class="webkit-block-placeholder"></div><div>Thanks,</div><div>Ter</div></body></html>