<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Aug 30, 2017, at 1:43 PM, Matthias Braun <<a href="mailto:matze@braunis.de" class="">matze@braunis.de</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">That means you cannot use the code from RegisterPressure.{cpp|h} to compute this. The other liveness analysis we have in llvm codegen is LiveIntervals (LiveItnervalAnalysis) which gives you a list of liveness segments of a given vreg (the same representation is used in most linear scan allocators even though LLVM is not using a linear scan approach any more). This representation is not optimized for register pressure queries though: If you want to know how many variables are alive at a certain point in the program you have to check all virtual registers to see whether that point is contained in the liverange of that variable.</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">To make this efficient you probably need some form of precomputation over the whole function.</div></div></blockquote></div><br class=""><div class="">The code in RegisterPressure.cpp is meant to work with LiveIntervals. Those queries only see within a block but are meant to be “seeded” with live-through information. That could be done be directly calling `addLiveRegs`. Alternately you can record live-through pressure separately via `initLiveThru`. It’s just that the MachineScheduler does not bother initializing the live-through information.</div><div class=""><br class=""></div><div class="">As Matthias said, actually determining live-through information requires a separate global liveness analysis, because LiveIntervals doesn’t tell you “what’s live at this point”.</div><div class=""><br class=""></div><div class="">-Andy</div></body></html>