Switch-to-lookup tables: set threshold to 3 cases

There has been an old FIXME to find the right cut-off for when it's worth
analyzing and potentially transforming a switch to a lookup table.

The switches always have two or more cases. I could not measure any speed-up
by transforming a switch with two cases. A switch with three cases gets a nice
speed-up, and I couldn't measure any compile-time regression, so I think this
is the right threshold.

In a Clang self-host, this causes 480 new switches to be transformed,
and reduces the final binary size with 8 KB.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199294 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 90f8847bc1c29ce0d27a6329218827d8a0cba488..a30dcf2fe0c1cda00a53492901ad7f28fe261695 100644 (file)
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3684,11 +3684,9 @@ static bool SwitchToLookupTable(SwitchInst *SI,
   // GEP needs a runtime relocation in PIC code. We should just build one big
   // string and lookup indices into that.
 
-  // Ignore the switch if the number of cases is too small.
-  // This is similar to the check when building jump tables in
-  // SelectionDAGBuilder::handleJTSwitchCase.
-  // FIXME: Determine the best cut-off.
-  if (SI->getNumCases() < 4)
+  // Ignore switches with less than three cases. Lookup tables will not make them
+  // faster, so we don't analyze them.
+  if (SI->getNumCases() < 3)
     return false;
 
   // Figure out the corresponding result for each case value and phi node in the

diff --git a/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll b/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
index 4ac02dbdf3397868feebbc929a4b89be57570ec0..b47fa02af22996fdbf07f1471b23f954368bde47 100644 (file)
--- a/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
+++ b/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
@@ -856,3 +856,39 @@ return:
 ; CHECK-NOT: @switch.table
 ; CHECK: switch i32
 }
+
+; We build lookup tables for switches with three or more cases.
+define i32 @threecases(i32 %c) {
+entry:
+  switch i32 %c, label %sw.default [
+    i32 0, label %return
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+  ]
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.default: br label %return
+return:
+  %x = phi i32 [ 3, %sw.default ], [ 5, %sw.bb2 ], [ 7, %sw.bb1 ], [ 9, %entry ]
+  ret i32 %x
+; CHECK-LABEL: @threecases(
+; CHECK-NOT: switch i32
+; CHECK: @switch.table
+}
+
+; We don't build tables for switches with two cases.
+define i32 @twocases(i32 %c) {
+entry:
+  switch i32 %c, label %sw.default [
+    i32 0, label %return
+    i32 1, label %sw.bb1
+  ]
+sw.bb1: br label %return
+sw.default: br label %return
+return:
+  %x = phi i32 [ 3, %sw.default ], [ 7, %sw.bb1 ], [ 9, %entry ]
+  ret i32 %x
+; CHECK-LABEL: @twocases(
+; CHECK: switch i32
+; CHECK-NOT: @switch.table
+}