SimplifyCFG: Turn switches into sub+icmp+branch if possible.

This makes the job of the later optzn passes easier, allowing the vast amount of
icmp transforms to chew on it.

We transform 840 switches in gcc.c, leading to a 16k byte shrink of the resulting
binary on i386-linux.

The testcase from README.txt now compiles into
  decl  %edi
  cmpl  $3, %edi
  sbbl  %eax, %eax
  andl  $1, %eax
  ret

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124724 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index b3bc7498566f4add6789eeac727289d354d837b9..c0a2b760de78955f01fa24dcf905f014f78ceb76 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -2274,51 +2274,3 @@ llc time when it gets inlined, because we can use smaller transfers.  This also
 avoids partial register stalls in some important cases.
 
 //===---------------------------------------------------------------------===//
-
-With PR8575 we're now generating better code for:
-
-static _Bool foo(int x) { return x == 1; }
-static _Bool bar(int x) { return x == 2; }
-static _Bool baz(int x) { return x == 3; }
-
-_Bool quux(int x) {
- return foo(x) || bar(x) || baz(x);
-}
-
-$ clang t.c -S -o - -O3 -mkernel -fomit-frame-pointer 
-_quux:                                  ## @quux
-## BB#0:                                ## %entry
-       decl    %edi
-       cmpl    $3, %edi
-       movb    $1, %al
-       jb      LBB0_2
-## BB#1:                                ## %lor.rhs
-       xorb    %al, %al
-LBB0_2:                                 ## %lor.end
-       movzbl  %al, %eax
-       andl    $1, %eax
-       ret
-
-But this should use a "setcc" instead of materializing a 0/1 value
-the hard way.  This looks like #1: simplifycfg should transform the
-switch into a sub+icmp+branch, and an instcombine hack to replace 
-the PHI with a zext of the branch condition.  Here's the IR today:
-
-define zeroext i1 @quux(i32 %x) nounwind readnone ssp noredzone {
-entry:
-  switch i32 %x, label %lor.rhs [
-    i32 1, label %lor.end
-    i32 2, label %lor.end
-    i32 3, label %lor.end
-  ]
-
-lor.rhs:                                          ; preds = %entry
-  br label %lor.end
-
-lor.end:                                          ; preds = %lor.rhs, %entry, %entry, %entry
-  %0 = phi i1 [ true, %entry ], [ false, %lor.rhs ], [ true, %entry ], [ true, %entry ]
-  ret i1 %0
-}
-
-//===---------------------------------------------------------------------===//
-

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index b9432c2e6e1a638e4455431d5afd421efb203a11..bf753dc05d4374563a16718b9e376902b56da205 100644 (file)
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2237,6 +2237,34 @@ bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) {
   return Changed;
 }
 
+/// TurnSwitchRangeIntoICmp - Turns a switch with that contains only a
+/// integer range comparison into a sub, an icmp and a branch.
+static bool TurnSwitchRangeIntoICmp(SwitchInst *SI) {
+  assert(SI->getNumCases() > 2 && "Degenerate switch?");
+  // We can do this transform if the switch consists of an ascending series
+  // and all cases point to the same destination.
+  for (unsigned I = 2, E = SI->getNumCases(); I != E; ++I)
+    if (SI->getSuccessor(I-1) != SI->getSuccessor(I) ||
+        SI->getCaseValue(I-1)->getValue()+1 != SI->getCaseValue(I)->getValue())
+      return false;
+
+  Constant *Offset = ConstantExpr::getNeg(SI->getCaseValue(1));
+  Constant *NumCases = ConstantInt::get(Offset->getType(), SI->getNumCases()-1);
+
+  Value *Sub = BinaryOperator::CreateAdd(SI->getCondition(), Offset, "off", SI);
+  Value *Cmp = new ICmpInst(SI, ICmpInst::ICMP_ULT, Sub, NumCases, "switch");
+  BranchInst::Create(SI->getSuccessor(1), SI->getDefaultDest(), Cmp, SI);
+
+  // Prune obsolete incoming values off the successor's PHI nodes.
+  for (BasicBlock::iterator BBI = SI->getSuccessor(1)->begin();
+       isa<PHINode>(BBI); ++BBI) {
+    for (unsigned I = 0, E = SI->getNumCases()-2; I != E; ++I)
+      cast<PHINode>(BBI)->removeIncomingValue(SI->getParent());
+  }
+  SI->eraseFromParent();
+
+  return true;
+}
 
 bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) {
   // If this switch is too complex to want to look at, ignore it.
@@ -2260,6 +2288,10 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) {
   if (SI == &*BBI)
     if (FoldValueComparisonIntoPredecessors(SI))
       return SimplifyCFG(BB) | true;
+
+  // Try to transform the switch into an icmp and a branch.
+  if (TurnSwitchRangeIntoICmp(SI))
+    return SimplifyCFG(BB) | true;
   
   return false;
 }

diff --git a/test/Transforms/SimplifyCFG/switch_create.ll b/test/Transforms/SimplifyCFG/switch_create.ll
index 4e199bc85964bdb9f62a15207f0e8e337495c5da..7c153e868222d805d22dc9637f31796dd22ae73c 100644 (file)
--- a/test/Transforms/SimplifyCFG/switch_create.ll
+++ b/test/Transforms/SimplifyCFG/switch_create.ll
@@ -141,14 +141,8 @@ UnifiedReturnBlock:             ; preds = %shortcirc_done.4, %shortcirc_next.4
         ret i1 %UnifiedRetVal
         
 ; CHECK: @test6
-; CHECK:   switch i32 %tmp.2.i, label %shortcirc_next.4 [
-; CHECK:       i32 14, label %UnifiedReturnBlock
-; CHECK:       i32 15, label %UnifiedReturnBlock
-; CHECK:       i32 16, label %UnifiedReturnBlock
-; CHECK:       i32 17, label %UnifiedReturnBlock
-; CHECK:       i32 18, label %UnifiedReturnBlock
-; CHECK:       i32 19, label %UnifiedReturnBlock
-; CHECK:     ]
+; CHECK: %off = add i32 %tmp.2.i, -14
+; CHECK: %switch = icmp ult i32 %off, 6
 }
 
 define void @test7(i8 zeroext %c, i32 %x) nounwind ssp noredzone {
@@ -447,11 +441,8 @@ if.end:
 define zeroext i1 @test16(i32 %x) nounwind {
 entry:
 ; CHECK: @test16
-; CHECK: switch i32 %x, label %lor.rhs [
-; CHECK:   i32 1, label %lor.end
-; CHECK:   i32 2, label %lor.end
-; CHECK:   i32 3, label %lor.end
-; CHECK: ]
+; CHECK: %off = add i32 %x, -1
+; CHECK: %switch = icmp ult i32 %off, 3
   %cmp.i = icmp eq i32 %x, 1
   br i1 %cmp.i, label %lor.end, label %lor.lhs.false
 

diff --git a/test/Transforms/SimplifyCFG/switch_formation.dbg.ll b/test/Transforms/SimplifyCFG/switch_formation.dbg.ll
index 357ffb60e1ef40c4208f4ffcc45341e5d5e46cb7..09bef648ab09008264f01cc6301a26729ebaa6a6 100644 (file)
--- a/test/Transforms/SimplifyCFG/switch_formation.dbg.ll
+++ b/test/Transforms/SimplifyCFG/switch_formation.dbg.ll
@@ -13,15 +13,9 @@
 declare void @llvm.dbg.stoppoint(i32, i32, { }*) nounwind
 
 define i1 @t({ i32, i32 }* %I) {
-; CHECK: t
-; CHECK:  switch i32 %tmp.2.i, label %shortcirc_next.4 [
-; CHECK:    i32 14, label %UnifiedReturnBlock
-; CHECK:    i32 15, label %UnifiedReturnBlock
-; CHECK:    i32 16, label %UnifiedReturnBlock
-; CHECK:    i32 17, label %UnifiedReturnBlock
-; CHECK:    i32 18, label %UnifiedReturnBlock
-; CHECK:    i32 19, label %UnifiedReturnBlock
-; CHECK:  ]
+; CHECK: @t
+; CHECK: %off = add i32 %tmp.2.i, -14
+; CHECK: %switch = icmp ult i32 %off, 6
 entry:
         %tmp.1.i = getelementptr { i32, i32 }* %I, i64 0, i32 1         ; <i32*> [#uses=1]
         %tmp.2.i = load i32* %tmp.1.i           ; <i32> [#uses=6]