[llvm] e8b83f7 - [RDA] Only store most recent reaching def from predecessors (NFCI)

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2020-04-06T18:39:09+02:00
New Revision: e8b83f7ddc5d0c4ac89f18e024562d0e70bcc40c

URL: https://github.com/llvm/llvm-project/commit/e8b83f7ddc5d0c4ac89f18e024562d0e70bcc40c
DIFF: https://github.com/llvm/llvm-project/commit/e8b83f7ddc5d0c4ac89f18e024562d0e70bcc40c.diff

LOG: [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

Added: 
    

Modified: 
    llvm/lib/CodeGen/ReachingDefAnalysis.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/ReachingDefAnalysis.cpp b/llvm/lib/CodeGen/ReachingDefAnalysis.cpp
index 869529724b11..a143869c72c9 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"));