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LowerSwitch: track bounding range for the condition tree.

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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>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211038 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Jim Grosbach
2014-06-16 16:55:20 +00:00
parent 408519765b
commit dc2dc390f6
4 changed files with 208 additions and 101 deletions
Download Patch File Download Diff File Expand all files Collapse all files

114
test/Transforms/LowerSwitch/feature.ll
View File

@@ -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: