[llvm-commits] [llvm] r145010 - in /llvm/trunk: lib/CodeGen/MachineBlockPlacement.cpp test/CodeGen/X86/block-placement.ll

[Chandler Carruth]

Author: chandlerc
Date: Sun Nov 20 05:22:06 2011
New Revision: 145010

URL: http://llvm.org/viewvc/llvm-project?rev=145010&view=rev
Log:
The logic for breaking the CFG in the presence of hot successors didn't
properly account for the *global* probability of the edge being taken.
This manifested as a very large number of unconditional branches to
blocks being merged against the CFG even though they weren't
particularly hot within the CFG.

The fix is to check whether the edge being merged is both locally hot
relative to other successors for the source block, and globally hot
compared to other (unmerged) predecessors of the destination block.

This introduces a new crasher on GCC single-source, but it's currently
behind a flag, and Ben has offered to work on the reduction. =]

Modified:
    llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
    llvm/trunk/test/CodeGen/X86/block-placement.ll

Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=145010&r1=145009&r2=145010&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Sun Nov 20 05:22:06 2011
@@ -365,9 +365,35 @@
 
     // Only consider successors which are either "hot", or wouldn't violate
     // any CFG constraints.
-    if (SuccChain.LoopPredecessors != 0 && SuccProb < HotProb) {
-      DEBUG(dbgs() << "    " << getBlockName(*SI) << " -> CFG conflict\n");
-      continue;
+    if (SuccChain.LoopPredecessors != 0) {
+      if (SuccProb < HotProb) {
+        DEBUG(dbgs() << "    " << getBlockName(*SI) << " -> CFG conflict\n");
+        continue;
+      }
+
+      // Make sure that a hot successor doesn't have a globally more important
+      // predecessor.
+      BlockFrequency CandidateEdgeFreq
+        = MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
+      bool BadCFGConflict = false;
+      for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
+                                            PE = (*SI)->pred_end();
+           PI != PE; ++PI) {
+        if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
+            BlockToChain[*PI] == &Chain)
+          continue;
+        BlockFrequency PredEdgeFreq
+          = MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
+        if (PredEdgeFreq >= CandidateEdgeFreq) {
+          BadCFGConflict = true;
+          break;
+        }
+      }
+      if (BadCFGConflict) {
+        DEBUG(dbgs() << "    " << getBlockName(*SI)
+                               << " -> non-cold CFG conflict\n");
+        continue;
+      }
     }
 
     DEBUG(dbgs() << "    " << getBlockName(*SI) << " -> " << SuccProb

Modified: llvm/trunk/test/CodeGen/X86/block-placement.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/block-placement.ll?rev=145010&r1=145009&r2=145010&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/block-placement.ll (original)
+++ llvm/trunk/test/CodeGen/X86/block-placement.ll Sun Nov 20 05:22:06 2011
@@ -439,3 +439,39 @@
 }
 
 !1 = metadata !{metadata !"branch_weights", i32 1000, i32 1}
+
+declare i32 @f()
+declare i32 @g()
+declare i32 @h(i32 %x)
+
+define i32 @test_global_cfg_break_profitability() {
+; Check that our metrics for the profitability of a CFG break are global rather
+; than local. A successor may be very hot, but if the current block isn't, it
+; doesn't matter. Within this test the 'then' block is slightly warmer than the
+; 'else' block, but not nearly enough to merit merging it with the exit block
+; even though the probability of 'then' branching to the 'exit' block is very
+; high.
+; CHECK: test_global_cfg_break_profitability
+; CHECK: calll f
+; CHECK: calll g
+; CHECK: calll h
+; CHECK: ret
+
+entry:
+  br i1 undef, label %then, label %else, !prof !2
+
+then:
+  %then.result = call i32 @f()
+  br label %exit
+
+else:
+  %else.result = call i32 @g()
+  br label %exit
+
+exit:
+  %result = phi i32 [ %then.result, %then ], [ %else.result, %else ]
+  %result2 = call i32 @h(i32 %result)
+  ret i32 %result
+}
+
+!2 = metadata !{metadata !"branch_weights", i32 3, i32 1}