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llvm-6502/test/Transforms/LowerSwitch/2014-06-11-SwitchDefaultUnreachableOpt.ll

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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> git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211038 91177308-0d34-0410-b5e6-96231b3b80d8
2014-06-16 16:55:20 +00:00
; 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
}
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