From e8b83f7ddc5d0c4ac89f18e024562d0e70bcc40c Mon Sep 17 00:00:00 2001 From: Nikita Popov Date: Sun, 5 Apr 2020 00:46:20 +0200 Subject: [PATCH] [RDA] Only store most recent reaching def from predecessors (NFCI) When entering a basic block, RDA inserts reaching definitions coming from predecessor blocks (which will be negative numbers) in a rather peculiar way. If you have incoming reaching definitions -4, -3, -2, -1, it will insert those. If you have incoming reaching definitions -1, -2, -3, -4, it will insert -1, -1, -1, -1, as the max is taken at each step. That's probably not what was intended... However, RDA only actually cares about the most recent reaching definition from a predecessor (to calculate clearance), so this ends up working fine as far as behavior is concerned. It does waste memory on unnecessary reaching definitions though. This patch changes the implementation to first compute the most recent reaching definition in one loop, and then insert only that one in a separate loop. Differential Revision: https://reviews.llvm.org/D77508 --- llvm/lib/CodeGen/ReachingDefAnalysis.cpp | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/llvm/lib/CodeGen/ReachingDefAnalysis.cpp b/llvm/lib/CodeGen/ReachingDefAnalysis.cpp index 8695297..a143869 100644 --- a/llvm/lib/CodeGen/ReachingDefAnalysis.cpp +++ b/llvm/lib/CodeGen/ReachingDefAnalysis.cpp @@ -83,14 +83,16 @@ void ReachingDefAnalysis::enterBasicBlock( if (Incoming.empty()) continue; - for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit) { - // Use the most recent predecessor def for each register. + // Find the most recent reaching definition from a predecessor. + for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit) LiveRegs[Unit] = std::max(LiveRegs[Unit], Incoming[Unit]); - if ((LiveRegs[Unit] != ReachingDefDefaultVal)) - MBBReachingDefs[MBBNumber][Unit].push_back(LiveRegs[Unit]); - } } + // Insert the most recent reaching definition we found. + for (unsigned Unit = 0; Unit != NumRegUnits; ++Unit) + if (LiveRegs[Unit] != ReachingDefDefaultVal) + MBBReachingDefs[MBBNumber][Unit].push_back(LiveRegs[Unit]); + LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << (!TraversedMBB.IsDone ? ": incomplete\n" : ": all preds known\n")); -- 2.7.4