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dc2dc390f6
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
42 lines
636 B
LLVM
42 lines
636 B
LLVM
; RUN: opt < %s -lowerswitch -S | FileCheck %s
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; CHECK-NOT: {{.*}}icmp eq{{.*}}
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;
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;int foo(int a) {
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;
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; switch (a) {
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; case 0:
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; return 10;
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; case 1:
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; return 3;
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; default:
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; __builtin_unreachable();
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; }
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;
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;}
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define i32 @foo(i32 %a) nounwind ssp uwtable {
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%1 = alloca i32, align 4
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%2 = alloca i32, align 4
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store i32 %a, i32* %2, align 4
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%3 = load i32* %2, align 4
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switch i32 %3, label %6 [
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i32 0, label %4
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i32 1, label %5
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]
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; <label>:4
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store i32 10, i32* %1
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br label %7
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; <label>:5
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store i32 3, i32* %1
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br label %7
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; <label>:6
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unreachable
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; <label>:7
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%8 = load i32* %1
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ret i32 %8
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}
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