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2272906641
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
105 lines
3.6 KiB
LLVM
105 lines
3.6 KiB
LLVM
; RUN: opt < %s -lowerswitch -S | FileCheck %s
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; We have switch on input.
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; On output we should got binary comparison tree. Check that all is fine.
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;CHECK: entry:
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;CHECK-NEXT: br label %NodeBlock19
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;CHECK: NodeBlock19: ; preds = %entry
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;CHECK-NEXT: %Pivot20 = icmp slt i32 %tmp158, 10
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;CHECK-NEXT: br i1 %Pivot20, label %NodeBlock5, label %NodeBlock17
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;CHECK: NodeBlock17: ; preds = %NodeBlock19
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;CHECK-NEXT: %Pivot18 = icmp slt i32 %tmp158, 13
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;CHECK-NEXT: br i1 %Pivot18, label %NodeBlock9, label %NodeBlock15
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;CHECK: NodeBlock15: ; preds = %NodeBlock17
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;CHECK-NEXT: %Pivot16 = icmp slt i32 %tmp158, 14
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;CHECK-NEXT: br i1 %Pivot16, label %bb330, label %NodeBlock13
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;CHECK: NodeBlock13: ; preds = %NodeBlock15
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;CHECK-NEXT: %Pivot14 = icmp slt i32 %tmp158, 15
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;CHECK-NEXT: br i1 %Pivot14, label %bb332, label %LeafBlock11
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;CHECK: LeafBlock11: ; preds = %NodeBlock13
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;CHECK-NEXT: %SwitchLeaf12 = icmp eq i32 %tmp158, 15
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;CHECK-NEXT: br i1 %SwitchLeaf12, label %bb334, label %NewDefault
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;CHECK: NodeBlock9: ; preds = %NodeBlock17
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;CHECK-NEXT: %Pivot10 = icmp slt i32 %tmp158, 11
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;CHECK-NEXT: br i1 %Pivot10, label %bb324, label %NodeBlock7
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;CHECK: NodeBlock7: ; preds = %NodeBlock9
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;CHECK-NEXT: %Pivot8 = icmp slt i32 %tmp158, 12
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;CHECK-NEXT: br i1 %Pivot8, label %bb326, label %bb328
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;CHECK: NodeBlock5: ; preds = %NodeBlock19
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;CHECK-NEXT: %Pivot6 = icmp slt i32 %tmp158, 7
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;CHECK-NEXT: br i1 %Pivot6, label %NodeBlock, label %NodeBlock3
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;CHECK: NodeBlock3: ; preds = %NodeBlock5
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;CHECK-NEXT: %Pivot4 = icmp slt i32 %tmp158, 8
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;CHECK-NEXT: br i1 %Pivot4, label %bb, label %NodeBlock1
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;CHECK: NodeBlock1: ; preds = %NodeBlock3
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;CHECK-NEXT: %Pivot2 = icmp slt i32 %tmp158, 9
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;CHECK-NEXT: br i1 %Pivot2, label %bb338, label %bb322
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;CHECK: NodeBlock: ; preds = %NodeBlock5
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;CHECK-NEXT: %Pivot = icmp slt i32 %tmp158, 0
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;CHECK-NEXT: br i1 %Pivot, label %LeafBlock, label %bb338
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;CHECK: LeafBlock: ; preds = %NodeBlock
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;CHECK-NEXT: %tmp158.off = add i32 %tmp158, 6
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;CHECK-NEXT: %SwitchLeaf = icmp ule i32 %tmp158.off, 4
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;CHECK-NEXT: br i1 %SwitchLeaf, label %bb338, label %NewDefault
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define i32 @main(i32 %tmp158) {
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entry:
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switch i32 %tmp158, label %bb336 [
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i32 -2, label %bb338
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i32 -3, label %bb338
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i32 -4, label %bb338
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i32 -5, label %bb338
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i32 -6, label %bb338
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i32 0, label %bb338
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i32 1, label %bb338
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i32 2, label %bb338
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i32 3, label %bb338
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i32 4, label %bb338
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i32 5, label %bb338
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i32 6, label %bb338
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i32 7, label %bb
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i32 8, label %bb338
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i32 9, label %bb322
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i32 10, label %bb324
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i32 11, label %bb326
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i32 12, label %bb328
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i32 13, label %bb330
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i32 14, label %bb332
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i32 15, label %bb334
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]
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bb:
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ret i32 2
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bb322:
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ret i32 3
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bb324:
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ret i32 4
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bb326:
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ret i32 5
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bb328:
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ret i32 6
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bb330:
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ret i32 7
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bb332:
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ret i32 8
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bb334:
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ret i32 9
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bb336:
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ret i32 10
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bb338:
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ret i32 11
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}
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