LowerSwitch: track bounding range for the condition tree.

When LowerSwitch transforms a switch instruction into a tree of ifs it
is actually performing a binary search into the various case ranges, to
see if the current value falls into one cases range of values.

So, if we have a program with something like this:

switch (a) {
case 0:
  do0();
  break;
case 1:
  do1();
  break;
case 2:
  do2();
  break;
default:
  break;
}

the code produced is something like this:

  if (a < 1) {
    if (a == 0) {
      do0();
    }
  } else {
    if (a < 2) {
      if (a == 1) {
        do1();
      }
    } else {
      if (a == 2) {
        do2();
      }
    }
  }

This code is inefficient because the check (a == 1) to execute do1() is
not needed.

The reason is that because we already checked that (a >= 1) initially by
checking that also  (a < 2) we basically already inferred that (a == 1)
without the need of an extra basic block spawned to check if actually (a
== 1).

The patch addresses this problem by keeping track of already
checked bounds in the LowerSwitch algorithm, so that when the time
arrives to produce a Leaf Block that checks the equality with the case
value / range the algorithm can decide if that block is really needed
depending on the already checked bounds .

For example, the above with "a = 1" would work like this:

the bounds start as LB: NONE , UB: NONE
as (a < 1) is emitted the bounds for the else path become LB: 1 UB:
NONE. This happens because by failing the test (a < 1) we know that the
value "a" cannot be smaller than 1 if we enter the else branch.
After the emitting the check (a < 2) the bounds in the if branch become
LB: 1 UB: 1. This is because by checking that "a" is smaller than 2 then
the upper bound becomes 2 - 1 = 1.

When it is time to emit the leaf block for "case 1:" we notice that 1
can be squeezed exactly in between the LB and UB, which means that if we
arrived to that block there is no need to emit a block that checks if (a
== 1).

Patch by: Marcello Maggioni <hayarms@gmail.com>

llvm-svn: 211038

3 files changed, 107 insertions, 75 deletions

diff --git a/llvm/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll b/llvm/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll
new file mode 100644
index 00000000000..3673c04a8b8
--- /dev/null
+++ b/llvm/test/Transforms/LowerSwitch/2014-06-10-SwitchContiguousOpt.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -lowerswitch -S | FileCheck %s
+; CHECK-NOT: icmp eq i32 %0, 1
+
+define i32 @foo(i32 %a) #0 {
+entry:
+  %retval = alloca i32, align 4
+  %a.addr = alloca i32, align 4
+  store i32 %a, i32* %a.addr, align 4
+  %0 = load i32* %a.addr, align 4
+  switch i32 %0, label %sw.default [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+  ]
+
+sw.bb:
+  ret i32 12
+
+sw.bb1:
+  ret i32 4
+
+sw.bb2:
+  ret i32 2
+
+sw.default:
+  ret i32 9
+}
diff --git a/llvm/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll b/llvm/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll
new file mode 100644
index 00000000000..0f737211f59
--- /dev/null
+++ b/llvm/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll
@@ -0,0 +1,41 @@
+; RUN: opt < %s -lowerswitch -S | FileCheck %s
+; CHECK-NOT: {{.*}}icmp eq{{.*}}
+;
+;int foo(int a) {
+;
+;  switch (a) {
+;  case 0:
+;    return 10;
+;  case 1:
+;    return 3;
+;  default:
+;    __builtin_unreachable();
+;  }
+;
+;}
+
+define i32 @foo(i32 %a) nounwind ssp uwtable {
+  %1 = alloca i32, align 4
+  %2 = alloca i32, align 4
+  store i32 %a, i32* %2, align 4
+  %3 = load i32* %2, align 4
+  switch i32 %3, label %6 [
+    i32 0, label %4
+    i32 1, label %5
+  ]
+
+; <label>:4 
+  store i32 10, i32* %1
+  br label %7
+
+; <label>:5
+  store i32 3, i32* %1
+  br label %7
+
+; <label>:6
+  unreachable
+
+; <label>:7
+  %8 = load i32* %1
+  ret i32 %8
+}
diff --git a/llvm/test/Transforms/LowerSwitch/feature.ll b/llvm/test/Transforms/LowerSwitch/feature.ll
index e85f03ee5c7..09d25f0b06d 100644
--- a/llvm/test/Transforms/LowerSwitch/feature.ll
+++ b/llvm/test/Transforms/LowerSwitch/feature.ll
@@ -3,93 +3,57 @@
 ; We have switch on input.
 ; On output we should got binary comparison tree. Check that all is fine.
 
-;CHECK:      entry:
-;CHECK-NEXT:   br label %NodeBlock37
+;CHECK:     entry:
+;CHECK-NEXT:  br label %NodeBlock19
 
-;CHECK:      NodeBlock37:                                      ; preds = %entry
-;CHECK-NEXT:   %Pivot38 = icmp slt i32 %tmp158, 10
-;CHECK-NEXT:   br i1 %Pivot38, label %NodeBlock13, label %NodeBlock35
+;CHECK:     NodeBlock19:                                      ; preds = %entry
+;CHECK-NEXT:  %Pivot20 = icmp slt i32 %tmp158, 10
+;CHECK-NEXT:  br i1 %Pivot20, label %NodeBlock5, label %NodeBlock17
 
-;CHECK:      NodeBlock35:                                      ; preds = %NodeBlock37
-;CHECK-NEXT:   %Pivot36 = icmp slt i32 %tmp158, 13
-;CHECK-NEXT:   br i1 %Pivot36, label %NodeBlock23, label %NodeBlock33
+;CHECK:     NodeBlock17:                                      ; preds = %NodeBlock19
+;CHECK-NEXT:  %Pivot18 = icmp slt i32 %tmp158, 13
+;CHECK-NEXT:  br i1 %Pivot18, label %NodeBlock9, label %NodeBlock15
 
-;CHECK:      NodeBlock33:                                      ; preds = %NodeBlock35
-;CHECK-NEXT:   %Pivot34 = icmp slt i32 %tmp158, 14
-;CHECK-NEXT:   br i1 %Pivot34, label %LeafBlock25, label %NodeBlock31
+;CHECK:     NodeBlock15:                                      ; preds = %NodeBlock17
+;CHECK-NEXT:  %Pivot16 = icmp slt i32 %tmp158, 14
+;CHECK-NEXT:  br i1 %Pivot16, label %bb330, label %NodeBlock13
 
-;CHECK:      NodeBlock31:                                      ; preds = %NodeBlock33
-;CHECK-NEXT:   %Pivot32 = icmp slt i32 %tmp158, 15
-;CHECK-NEXT:   br i1 %Pivot32, label %LeafBlock27, label %LeafBlock29
+;CHECK:     NodeBlock13:                                      ; preds = %NodeBlock15
+;CHECK-NEXT:  %Pivot14 = icmp slt i32 %tmp158, 15
+;CHECK-NEXT:  br i1 %Pivot14, label %bb332, label %LeafBlock11
 
-;CHECK:      LeafBlock29:                                      ; preds = %NodeBlock31
-;CHECK-NEXT:   %SwitchLeaf30 = icmp eq i32 %tmp158, 15
-;CHECK-NEXT:   br i1 %SwitchLeaf30, label %bb334, label %NewDefault
+;CHECK:     LeafBlock11:                                      ; preds = %NodeBlock13
+;CHECK-NEXT:  %SwitchLeaf12 = icmp eq i32 %tmp158, 15
+;CHECK-NEXT:  br i1 %SwitchLeaf12, label %bb334, label %NewDefault
 
-;CHECK:      LeafBlock27:                                      ; preds = %NodeBlock31
-;CHECK-NEXT:   %SwitchLeaf28 = icmp eq i32 %tmp158, 14
-;CHECK-NEXT:   br i1 %SwitchLeaf28, label %bb332, label %NewDefault
+;CHECK:     NodeBlock9:                                       ; preds = %NodeBlock17
+;CHECK-NEXT:  %Pivot10 = icmp slt i32 %tmp158, 11
+;CHECK-NEXT:  br i1 %Pivot10, label %bb324, label %NodeBlock7
 
-;CHECK:      LeafBlock25:                                      ; preds = %NodeBlock33
-;CHECK-NEXT:   %SwitchLeaf26 = icmp eq i32 %tmp158, 13
-;CHECK-NEXT:   br i1 %SwitchLeaf26, label %bb330, label %NewDefault
+;CHECK:     NodeBlock7:                                       ; preds = %NodeBlock9
+;CHECK-NEXT:  %Pivot8 = icmp slt i32 %tmp158, 12
+;CHECK-NEXT:  br i1 %Pivot8, label %bb326, label %bb328
 
-;CHECK:      NodeBlock23:                                      ; preds = %NodeBlock35
-;CHECK-NEXT:   %Pivot24 = icmp slt i32 %tmp158, 11
-;CHECK-NEXT:   br i1 %Pivot24, label %LeafBlock15, label %NodeBlock21
+;CHECK:     NodeBlock5:                                       ; preds = %NodeBlock19
+;CHECK-NEXT:  %Pivot6 = icmp slt i32 %tmp158, 7
+;CHECK-NEXT:  br i1 %Pivot6, label %NodeBlock, label %NodeBlock3
 
-;CHECK:      NodeBlock21:                                      ; preds = %NodeBlock23
-;CHECK-NEXT:   %Pivot22 = icmp slt i32 %tmp158, 12
-;CHECK-NEXT:   br i1 %Pivot22, label %LeafBlock17, label %LeafBlock19
+;CHECK:     NodeBlock3:                                       ; preds = %NodeBlock5
+;CHECK-NEXT:  %Pivot4 = icmp slt i32 %tmp158, 8
+;CHECK-NEXT:  br i1 %Pivot4, label %bb, label %NodeBlock1
 
-;CHECK:      LeafBlock19:                                      ; preds = %NodeBlock21
-;CHECK-NEXT:   %SwitchLeaf20 = icmp eq i32 %tmp158, 12
-;CHECK-NEXT:   br i1 %SwitchLeaf20, label %bb328, label %NewDefault
+;CHECK:     NodeBlock1:                                       ; preds = %NodeBlock3
+;CHECK-NEXT:  %Pivot2 = icmp slt i32 %tmp158, 9
+;CHECK-NEXT:  br i1 %Pivot2, label %bb338, label %bb322
 
-;CHECK:      LeafBlock17:                                      ; preds = %NodeBlock21
-;CHECK-NEXT:   %SwitchLeaf18 = icmp eq i32 %tmp158, 11
-;CHECK-NEXT:   br i1 %SwitchLeaf18, label %bb326, label %NewDefault
+;CHECK:     NodeBlock:                                        ; preds = %NodeBlock5
+;CHECK-NEXT:  %Pivot = icmp slt i32 %tmp158, 0
+;CHECK-NEXT:  br i1 %Pivot, label %LeafBlock, label %bb338
 
-;CHECK:      LeafBlock15:                                      ; preds = %NodeBlock23
-;CHECK-NEXT:   %SwitchLeaf16 = icmp eq i32 %tmp158, 10
-;CHECK-NEXT:   br i1 %SwitchLeaf16, label %bb324, label %NewDefault
-
-;CHECK:      NodeBlock13:                                      ; preds = %NodeBlock37
-;CHECK-NEXT:   %Pivot14 = icmp slt i32 %tmp158, 7
-;CHECK-NEXT:   br i1 %Pivot14, label %NodeBlock, label %NodeBlock11
-
-;CHECK:      NodeBlock11:                                      ; preds = %NodeBlock13
-;CHECK-NEXT:   %Pivot12 = icmp slt i32 %tmp158, 8
-;CHECK-NEXT:   br i1 %Pivot12, label %LeafBlock3, label %NodeBlock9
-
-;CHECK:      NodeBlock9:                                       ; preds = %NodeBlock11
-;CHECK-NEXT:   %Pivot10 = icmp slt i32 %tmp158, 9
-;CHECK-NEXT:   br i1 %Pivot10, label %LeafBlock5, label %LeafBlock7
-
-;CHECK:      LeafBlock7:                                       ; preds = %NodeBlock9
-;CHECK-NEXT:   %SwitchLeaf8 = icmp eq i32 %tmp158, 9
-;CHECK-NEXT:   br i1 %SwitchLeaf8, label %bb322, label %NewDefault
-
-;CHECK:      LeafBlock5:                                       ; preds = %NodeBlock9
-;CHECK-NEXT:   %SwitchLeaf6 = icmp eq i32 %tmp158, 8
-;CHECK-NEXT:   br i1 %SwitchLeaf6, label %bb338, label %NewDefault
-
-;CHECK:      LeafBlock3:                                       ; preds = %NodeBlock11
-;CHECK-NEXT:   %SwitchLeaf4 = icmp eq i32 %tmp158, 7
-;CHECK-NEXT:   br i1 %SwitchLeaf4, label %bb, label %NewDefault
-
-;CHECK:      NodeBlock:                                        ; preds = %NodeBlock13
-;CHECK-NEXT:   %Pivot = icmp slt i32 %tmp158, 0
-;CHECK-NEXT:   br i1 %Pivot, label %LeafBlock, label %LeafBlock1
-
-;CHECK:      LeafBlock1:                                       ; preds = %NodeBlock
-;CHECK-NEXT:   %SwitchLeaf2 = icmp ule i32 %tmp158, 6
-;CHECK-NEXT:   br i1 %SwitchLeaf2, label %bb338, label %NewDefault
-
-;CHECK:      LeafBlock:                                        ; preds = %NodeBlock
-;CHECK-NEXT:   %tmp158.off = add i32 %tmp158, 6
-;CHECK-NEXT:   %SwitchLeaf = icmp ule i32 %tmp158.off, 4
-;CHECK-NEXT:   br i1 %SwitchLeaf, label %bb338, label %NewDefault
+;CHECK:     LeafBlock:                                        ; preds = %NodeBlock
+;CHECK-NEXT:  %tmp158.off = add i32 %tmp158, 6
+;CHECK-NEXT:  %SwitchLeaf = icmp ule i32 %tmp158.off, 4
+;CHECK-NEXT:  br i1 %SwitchLeaf, label %bb338, label %NewDefault
 
 define i32 @main(i32 %tmp158) {
 entry: