From d5e59e99f4046a421983f76f5b4c44b646a4af9b Mon Sep 17 00:00:00 2001 From: Geza Lore Date: Mon, 31 Oct 2022 10:20:11 +0100 Subject: [PATCH] [ValueTracking] Improve performance of programUndefinedIfUndefOrPoison (NFC) programUndefinedIfUndefOrPoison used to eagerly propagate the fact that a value is poison to the users of the value. The problem is that if the value has a lot of uses (orders of magnitude more than the scanning limit we use in this function), then we spend the bulk of our time in eagerly propagating the poison property, which we will mostly never use later anyway due to the scanning limit. I have a test case (of ~50k lines of machine generated C++), where this results in ~60% of 35s compilation time being spent doing just this eager propagation. This patch changes programUndefinedIfUndefOrPoison to only propagate to instructions actually visited, looking back to see if their operands are poison. This should be equivalent and no functional change is intended, but we regain virtually all of the 60% compilation time spent in this function in my test case (i.e.: a 2.5x total compilation speedup). Differential Revision: https://reviews.llvm.org/D137027 --- llvm/lib/Analysis/ValueTracking.cpp | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-) diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp index 7605f660c0f4..b137cd294f97 100644 --- a/llvm/lib/Analysis/ValueTracking.cpp +++ b/llvm/lib/Analysis/ValueTracking.cpp @@ -5722,11 +5722,6 @@ static bool programUndefinedIfUndefOrPoison(const Value *V, SmallSet Visited; YieldsPoison.insert(V); - auto Propagate = [&](const User *User) { - if (propagatesPoison(cast(User))) - YieldsPoison.insert(User); - }; - for_each(V->users(), Propagate); Visited.insert(BB); while (true) { @@ -5740,9 +5735,16 @@ static bool programUndefinedIfUndefOrPoison(const Value *V, if (!isGuaranteedToTransferExecutionToSuccessor(&I)) return false; - // Mark poison that propagates from I through uses of I. - if (YieldsPoison.count(&I)) - for_each(I.users(), Propagate); + // If this instruction propagates poison, mark it as poison if any of + // its operands are poison + if (propagatesPoison(cast(&I))) { + for (const Value *Op : I.operands()) { + if (YieldsPoison.count(Op)) { + YieldsPoison.insert(&I); + break; + } + } + } } BB = BB->getSingleSuccessor();