[PATCH] D15536: [JumpThreading] Fix opcode bonus in getJumpThreadDuplicationCost()

[Geoff Berry via llvm-commits]

gberry updated this revision to Diff 42881.
gberry added a comment.

Updated description


http://reviews.llvm.org/D15536

Files:
  lib/Transforms/Scalar/JumpThreading.cpp
  test/Transforms/JumpThreading/select.ll

Index: test/Transforms/JumpThreading/select.ll
===================================================================
--- test/Transforms/JumpThreading/select.ll
+++ test/Transforms/JumpThreading/select.ll
@@ -91,6 +91,36 @@
 }
 
 
+; 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(
Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -286,6 +286,21 @@
   // 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;
@@ -326,14 +341,7 @@
     }
   }
 
-  // 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;
+  Size = Size > Bonus ? Size - Bonus : 0;
 
   return Size;
 }


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