[JumpThreading] Fix opcode bonus in getJumpThreadDuplicationCost()

geoffberry · geoffberry · commit 43dc285915f5 · 2015-12-29T18:10:16.000Z
The code that was meant to adjust the duplication cost based on the terminator opcode was not being executed in cases where the initial threshold was hit inside the loop. Subscribers: mcrosier, llvm-commits Differential Revision: http://reviews.llvm.org/D15536 llvm-svn: 256568
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -286,6 +286,21 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
   // FIXME: THREADING will delete values that are just used to compute the
   // branch, so they shouldn't count against the duplication cost.
 
+  unsigned Bonus = 0;
+  const TerminatorInst *BBTerm = BB->getTerminator();
+  // Threading through a switch statement is particularly profitable.  If this
+  // block ends in a switch, decrease its cost to make it more likely to happen.
+  if (isa<SwitchInst>(BBTerm))
+    Bonus = 6;
+
+  // The same holds for indirect branches, but slightly more so.
+  if (isa<IndirectBrInst>(BBTerm))
+    Bonus = 8;
+
+  // Bump the threshold up so the early exit from the loop doesn't skip the
+  // terminator-based Size adjustment at the end.
+  Threshold += Bonus;
+
   // Sum up the cost of each instruction until we get to the terminator.  Don't
   // include the terminator because the copy won't include it.
   unsigned Size = 0;
@@ -326,16 +341,7 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
     }
   }
 
-  // Threading through a switch statement is particularly profitable.  If this
-  // block ends in a switch, decrease its cost to make it more likely to happen.
-  if (isa<SwitchInst>(I))
-    Size = Size > 6 ? Size-6 : 0;
-
-  // The same holds for indirect branches, but slightly more so.
-  if (isa<IndirectBrInst>(I))
-    Size = Size > 8 ? Size-8 : 0;
-
-  return Size;
+  return Size > Bonus ? Size - Bonus : 0;
 }
 
 /// FindLoopHeaders - We do not want jump threading to turn proper loop
diff --git a/llvm/test/Transforms/JumpThreading/select.ll b/llvm/test/Transforms/JumpThreading/select.ll
@@ -91,6 +91,36 @@ L3:
 }
 
 
+; Jump threading of indirectbr with select as address.  Test increased
+; duplication threshold for cases where indirectbr is being threaded
+; through.
+
+; CHECK-LABEL: @test_indirectbr_thresh(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br i1 %cond, label %L1, label %L3
+; CHECK-NOT: indirectbr
+define void @test_indirectbr_thresh(i1 %cond, i8* %address) nounwind {
+entry:
+  br i1 %cond, label %L0, label %L3
+L0:
+  %indirect.goto.dest = select i1 %cond, i8* blockaddress(@test_indirectbr_thresh, %L1), i8* %address
+  call void @quux()
+  call void @quux()
+  call void @quux()
+  indirectbr i8* %indirect.goto.dest, [label %L1, label %L2, label %L3]
+
+L1:
+  call void @foo()
+  ret void
+L2:
+  call void @bar()
+  ret void
+L3:
+  call void @baz()
+  ret void
+}
+
+
 ; A more complicated case: the condition is a select based on a comparison.
 
 ; CHECK-LABEL: @test_switch_cmp(
